CODEREVIEW
GUIDE

2.0

RELEASE

Project leaders: Larry Conklin and Gary Robinson
Creative Commons (CC) Attribution
Free Version at: https://www.owasp.org

1

Introduction

1

Foreword

Acknowledgements

Introduction

How To Use The Code Review Guide

3

5

6

8

2
Secure Code Review

9

Methodology

20

3Technical Reference For Secure Code Review

Appendix

A1 Injection

43

Code Review Do’s And Dont’s

192

A2 Broken Authentication And Session Management

58

Code Review Checklist

196

A3 Cross-Site Scripting (XSS)70

Threat Modeling Example

200

A4 Insecure Direct Object Reference

77

Code Crawling206

A5 Security Misconfiguration

82

A6 Sensitive Data Exposure

117

A7 Missing Function Level Access Control

133

A8 Cross-Site Request Forgery (CSRF)

139

A9 Using Components With Know Vulnerabilities

146

A10 Unvalidated Redirects And Forwards

149

4
HTML5

154

Same Origin Policy

158

Reviewing Logging Code160

Error Handling

163

Reviewing Security Alerts

175

Review For Active Defence

178

Race Conditions

181

Buffer Overruns

183

Client Side JavaScript

188

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Code Review Guide Foreword - By Eoin Keary1

FOREWORD

By Eoin Keary,Long Serving OWASP Global Board Member
The OWASP Code Review guide was originally born from the
OWASP Testing Guide. Initially code review was covered in the
Testing Guide, as it seemed like a good idea at the time. However, the topic of security code review is too big and evolved into
its own stand-alone guide.
I started the Code Review Project in 2006. This current edition
was started in April 2013 via the OWASP Project Reboot initiative and a grant from the United States Department of Homeland Security.
The OWASP Code Review team consists of a small, but talented,
group of volunteers who should really get out more often. The
volunteers have experience and a drive for the best practices
in secure code review in a variety of organizations, from small
start-ups to some of the largest software development organizations in the world.
It is common knowledge that more secure software can be produced and developed in a more cost effective way when bugs
are detected early on in the systems development lifecycle. Organizations with a proper code review function integrated into
the software development lifecycle (SDLC) produced remarkably better code from a security standpoint. To put it simply “We
can’t hack ourselves secure”. Attackers have more time to findvulnerabilities on a system than the time allocated to a defender. Hacking our way secure amounts to an uneven battlefield,
asymmetric warfare, and a losing battle.
By necessity, this guide does not cover all programming languages. It mainly focuses on C#/.NET and Java, but includes C/
C++, PHP and other languages where possible. However, thetechniques advocated in the book can be easily adapted to almost any code environment. Fortunately (or unfortunately), thesecurity flaws in web applications are remarkably consistentacross programming languages.
Eoin Keary, June 2017

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Acknowledgements

APPRECIATION TO UNITED STATES DEPARTMENT OFHOMELAND SECURITYOWASP community and Code Review Guide project leaders wish to expresses its deep appreciation to United States Department of Homeland Security for helping make this bookpossible by funds provided to OWASP thru a grant. OWASP continues be to the preeminentorganization for free unbiased/unfretted application security.
We have seen a disturbing rise in threats and attacks on community institutions thru application vulnerabilities, only by joining forces, and with unfretted information can we help
turn back the tide these threats. The world now runs on software and that software needs
to be trust worthy. Our deepest appreciation and thanks to DHS for helping and in sharing
in this goal.

FEEDBACK
If you have any feedback for the OWASP Code Review team, and/or find any mistakes or
improvements in this Code Review Guide please contact us at:
owasp-codereview-project@owasp.org

Acknowledgements

ACKNOWLEDGEMENTS
Project Leaders
Larry Conklin

Gary Robinson

VERSION 2.0, 2017
Content Contributors
Larry Conklin
Gary Robinson
Johanna Curiel
Eoin Keary
Islam Azeddine Mennouchi
Abbas Naderi
Carlos Pantelides

Michael Hidalgo
Reviewers
Alison Shubert
Fernando Galves
Sytze van Koningsveld
Carolyn Cohen
Helen Gao
Jan Masztal

David Li
Lawrence J Timmins
Kwok Cheng
Ken Prole
David D’Amico
Robert Ferris
Lenny Halseth
Kenneth F. Belva

VERSION 1.0, 2007
Project Leader
Eoin KearyContent Contributors
Jenelle Chapman
Andrew van der Stock
Paolo Perego
David Lowry
David Rook
Dinis Cruz
Jeff Williams

Reviewers
Jeff Williams
Rahin Jina

5

6

Introduction - Contents

INTRODUCTION
Welcome to the second edition of the OWASP Code Review Guide Project. The second edition brings the
successful OWASP Code Review Guide up to date with current threats and countermeasures. This version also includes new content reflecting the OWASP communities’ experiences of secure code review
best practices.

CONTENTSThe Second Edition of the Code Review Guide has been developed to advise software developers andmanagement on the best practices in secure code review, and how it can be used within a secure software development life-cycle (S-SDLC). The guide begins with sections that introduce the reader tosecure code review and how it can be introduced into a company’s S-SDLC. It then concentrates on
specific technical subjects and provides examples of what a reviewer should look for when reviewing
technical code. Specifically the guide covers:
Overview
This section introduces the reader to secure code review and the advantages it can bring to a development organization. It gives an overview of secure code review techniques and describes how code
review compares other techniques for analyzing secure code.
Methodology
The methodology section goes into more detail on how to integrate secure review techniques into development organizations S-SDLC and how the personnel reviewing the code can ensure they have thecorrect context to conduct an effective review. Topics include applying risk based intelligence to security code reviews, using threat modelling to understand the application being reviewed, and understanding how external business drivers can affect the need for secure code review.

How to use

HOW TO USE THE CODE REVIEW GUIDEThe contents and the structure of the book have been carefully designed. Further, all the contributed chapters have been judiciously edited and integrated into a unifying framework that provides uniformity in structure and style.
This book is written to satisfy three different perspectives.
1. Management teams who wish to understand the reasons of why code reviews are needed and why they are included in bestpractices in developing secure enterprise software for todays organizations. Senior management should thoroughly read sections one and two of this book. Management needs to consider the following items if doing secure coding is going to be part ofthe organizations software development lifecycle:• Does organization project estimation allot time for code reviews?
• Does management have the capability to track the relevant metrics of code review and static analysis for each project andprogrammer?
• Management needs to decide when in the project life cycle will that code reviews should be done in the project lifecycle andwhat changes to existing projects require review of previously completed code reviews.
2. Software leads who want to give manfully feedback to peers in code review with ample empirical artifacts as what to look forin helping create secure enterprise software for their organizations. They should consider:•As a peer code reviewer, to use this book you first decided on the type of code review do you want to accomplish. Lets spend a
few minutes going over each type of code review to help in deciding how this book can be assistance to you.
• API/design code reviews. Use this book to understand how architecture designs can lead to security vulnerabilities. Also if the
API is a third party API what security controls are in place in the code to prevent security vulnerabilities.
• Maintainability code reviews. These types of code reviews are more towards the organizations internal best coding practices.
This book does cover code metrics, which can help the code reviewer, better understand what code to look at for security vulnerabilities if a section of code is overly complex.
• Integration code reviews. Again these types of code reviews are more towards the organizations internal coding policies. Is
the code being integrated into the project fully vetted by IT management and approved? Many security vulnerabilities are now
being implemented by using open source libraries whichh may bring in dependencies that are not secure.
• Testing code reviews. Agile and Test Driven design where programmer creates unit tests to prove code methods works as the
programmer intended. This code is not a guide for testing software. The code reviewer may want to pay attention to unit test
cases to make sure all methods have appropriate exceptions; code fails in a safe way. If possible each security control in code has
the appropriate unit test cases.
3. Secure code reviewer who wants an updated guide on how secure code reviews are integrated in to the organizations secure
software development lifecycle. This book will also work as a reference guide for the code review as code is in the review process.
This book provides a complete source of information needed by the code reviewer. It should be read first as a story about code
reviews and seconds as a desktop reference guide.

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2

Secure Code Review

SECURE CODE REVIEWTechnical Reference For Secure Code Review
Here the guide drills down into common vulnerabilities and technical controls, including XSS, SQL injection,
session tracking, authentication, authorization, logging, and information leakage, giving code examples invarious languages to guide the reviewer.
This section can be used to learn the important aspects of the various controls, and as an on-the-job reference
when conducting secure code reviews.
We start with the OWASP Top 10 issues, describing technical aspects to consider for each of these issues. We
then move onto other common application security issues not specific to the OWASP Top 10
Secure code review is probably the single-most effective technique for identifying security bugs early in thesystem development lifecycle. When used together with automated and manual penetration testing, code
review can significantly increase the cost effectiveness of an application security verification effort.
This guide does not prescribe a process for performing a security code review. Rather, it provides guidance on
how the effort should be structured and executed. The guide also focuses on the mechanics of reviewing codefor certain vulnerabilities.
Manual secure code review provides insight into the “real risk” associated with insecure code. This contextual,white-box approach is the single most important value. A human reviewer can understand the relevance ofa bug or vulnerability in code. Context requires human understanding of what is being assessed. With appropriate context we can make a serious risk estimate that accounts for both the likelihood of attack and thebusiness impact of a breach. Correct categorization of vulnerabilities helps with priority of remediation andfixing the right things as opposed to wasting time fixing everything.
5.1 Why Does Code Have Vulnerabilities?
MITRE has catalogued circa 1000 different kinds of software weaknesses in the CWE project. These are alldifferent ways that software developers can make mistakes that lead to insecurity. Every one of these weaknesses is subtle and many are seriously tricky. Software developers are not taught about these weaknesses inschool and most do not receive any on the job training about these problems.
These problems have become so important in recent years because we continue to increase connectivity
and add technologies and protocols at an extremely fast rate. The ability to invent technology has seriously
outstripped the ability to secure it. Many of the technologies in use today simply have not received enough(or any) security scrutiny.
There are many reasons why businesses are not spending the appropriate amount of time on security. Ultimately, these reasons stem from an underlying problem in the software market. Because software is essentially a black box, it is extremely difficult for a customer to tell the difference between good code and insecure
code. Without this visibility vendors are not encouraged to spend extra effort to produce secure code. Nevertheless, information security experts frequently get pushback when they advocate for security code review,
with the following (unjustified) excuses for not putting more effort into security:“We never get hacked (that I know of ), we don’t need security”

910

Secure Code Review“We have a firewall that protects our applications”“We trust our employees not to attack our applications”
Over the last 10 years, the team involved with the OWASP Code Review Project has performed thousands ofapplication reviews, and found that every non-trivial application has had security vulnerabilities. If code has
not been reviewed for security holes, the likelihood that the application has problems is virtually 100%.
Still, there are many organizations that choose not to know about the security of their code. To them, consider
Rumsfeld’s cryptic explanation of what we actually know:“...we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns-- the ones we don’t know we don’t know.”- Donald Rumsfeld
If informed decisions are being made based on a measurement of risk in the enterprise, which will be fully
supported. However, if risks are not being understood, the company is not being duly diligent, and is being
irresponsible both to shareholders and customers.
5.2 What is Secure Code Review?
Code review aims to identify security flaws in the application related to its features and design, along with theexact root causes. With the increasing complexity of applications and the advent of new technologies, thetraditional way of testing may fail to detect all the security flaws present in the applications. One must understand the code of the application, external components, and configurations to have a better chance of finding
the flaws. Such a deep dive into the application code also helps in determining exact mitigation techniquesthat can be used to avert the security flaws.
It is the process of auditing the source code of an application to verify that the proper security and logicalcontrols are present, that they work as intended, and that they have been invoked in the right places. Codereview is a way of helping ensure that the application has been developed so as to be “self-defending” in its
given environment.
Secure code review allows a company to assure application developers are following secure development
techniques. A general rule of thumb is that a penetration test should not discover any additional application
vulnerabilities relating to the developed code after the application has undergone a proper security code
review. At the least very few issues should be discovered.
All security code reviews are a combination of human effort and technology support. At one end of the spectrum is an inexperienced person with a text editor. At the other end of the scale is an expert security team withadvanced static analysis (SAST) tools. Unfortunately, it takes a fairly serious level of expertise to use the current
application security tools effectively. They also don’t understand dynamic data flow or business logic. SAST
tools are great for coverage and setting a minimum baseline.
Tools can be used to perform this task but they always need human verification. They do not understandcontext, which is the keystone of security code review. Tools are good at assessing large amounts of code andpointing out possible issues, but a person needs to verify every result to determine if it is a real issue, if it isactually exploitable, and calculate the risk to the enterprise. Human reviewers are also necessary to fill in forthe significant blind spots, which automated tools, simply cannot check.

Secure Code Review

5.3 What is the difference between Code Review and Secure Code Review?
The Capability Maturity Model (CMM) is a widely recognized process model for measuring the development
processes of a software development organization. It ranges from ‘level 1’ where development processes are
ad hoc, unstable and not repeatable, to ‘level 5’ where the development processes are well organized, documented and continuously improving. It is assumed that a company’s development processes would start out
at level 1 when starting out (a.k.a start-up mode) and will become more defined, repeatable and generally
professional as the organization matures and improves. Introducing the ability to perform code reviews (notethis is not dealing with secure code review yet) comes in when an organization has reached level 2 (Repeatable) or level 3 (Defined).
Secure Code Review is an enhancement to the standard code review practice where the structure of the review process places security considerations, such as company security standards, at the forefront of the decision-making. Many of these decisions will be explained in this document and attempt to ensure that thereview process can adequately cover security risks in the code base, for example ensuring high risk code isreviewed in more depth, ensuring reviewers have the correct security context when reviewing the code, ensuring reviewers have the necessary skills and secure coding knowledge to effectively evaluate the code.
5.4 Determining the Scale of a Secure Source Code Review?
The level of secure source code review will vary depending on the business or regulatory needs of the software, thesize of the software development organization writing the applications and the skills of the personnel. Similar toother aspects of software development such as performance, scalability and maintainability, security is a measure of
maturity in an application. Security is one of the non-functional requirements that should be built into every serious
application or tool that is used for commercial or governmental purposes.
If the development environment consists of one person programming as a hobby and writing a program to track
their weekly shopping in visual basic (CMM level 1), it is unlikely that that programmer will use all of the advice
within this document to perform extensive levels of secure code review. On the other extreme, a large organization
with thousands of developers writing hundreds of applications will (if they wish to be successful) take security very
seriously, just like they would take performance and scalability seriously.
Not every development organization has the necessity, or resources, to follow and implement all of the topics inthis document, but all organizations should be able to begin to write their development processes in a way that
can accommodate the processes and technical advice most important to them. Those processes should thenbe extensible to accommodate more of the secure code review considerations as the organization develops andmatures.
In a start-up consisting of 3 people in a darkened room, there will not be a ‘code review team’ to send the code to,instead it’ll be the bloke in the corner who read a secure coding book once and now uses it to prop up his monitor.
In a medium sized company there might be 400 developers, some with security as an interest or specialty, however
the organizations processes might give the same amount of time to review a 3 line CSS change as it gives to aredesign of the flagship products authentication code. Here the challenge is to increase the workforce’s secure
coding knowledge (in general) and improve the processes through things like threat modelling and secure code
review.
For some larger companies with many thousands of developers, the need for security in the S-SDLC is at its greatest,
but process efficiency has an impact on the bottom line. Take an example of a large company with 5,000 developers.
If a change is introduced to the process that results in each developer taking an extra 15 minutes a week to perform
a task, suddenly that’s 1,250 hours extra each week for the company as a whole This results in a need for an extra 30
full time developers each year just to stay on track (assuming a 40 hour week). The challenge here is to ensure thesecurity changes to the lifecycle are efficient and do not impede the developers from performing their task.

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Secure Code Review

Skilling a Workforce for Secure Code Review
There seems to be a catch-22 with the following sentiment; as many code developers are not aware or skilled insecurity, a company should implement peer secure code reviews amongst developers.
How does a workforce introduce the security skills to implement a secure code review methodology? Many
security maturity models (e.g. BSIMM or OpenSAMM) discuss the concept of a core security team, who are skilled
developers and skill security subject matter experts (SMEs). In the early days of a company rolling out a secure code review process, the security SMEs will be central in the higher risk reviews, using their experience andknowledge to point out aspects of the code that could introduce risk.
As well as the core security team a further group of developers with an interest in security can act as team local
security SMEs, taking part in many secure code reviews. These satellites (as BSIMM calls them) will be guided by
the core security team on technical issues, and will help encourage secure coding.
Over time, an organization builds security knowledge within its core, and satellite teams, which in turns spreads
the security knowledge across all developers since most code reviews will have a security SME taking part.
The ‘on-the-job’ training this gives to all developers is very important. Whereas an organization can send their developers on training courses (classroom or CBT) which will introduce them to common security topics and createawareness, no training course can be 100% relevant to a developer’s job. In the secure code review process, eachdeveloper who submits their code will receive security related feedback that is entirely relevant to them, since
the review is of the code they produced.
It must be remembered though, no matter what size the organization, the reason to perform secure code review is to catch more bugs and catch them earlier in the S-SDLC. It is quicker to conduct a secure code review
and find bugs that way, compared to finding the bugs in testing or in production. For the 5,000-person organization, how long will it take to find a bug in testing, investigate, re-code, re-review, re-release and re-test?
What if the code goes to production where project management and support will get involved in tracking theissue and communicating with customers? Maybe 15 minutes a week will seem like a bargain.
5.5 We Can’t Hack Ourselves Secure
Penetration testing is generally a black-box point in time test and should be repeated on each release (orbuild) of the source code to find any regressions. Many continuous integration tools (e.g. Jenkins/Hudson)allow repeatable tests, including automated penetration tests, to be run against a built and installed version
of a product.
As source code changes, the value of the findings of an unmaintained penetration tests degrade with time.
There are also privacy, compliance and stability and availability concerns, which may not covered by penetration testing, but can be covered in code reviews. Data information leakage in a cloud environment for example
may not be discovered, or allowed, via a penetration test. Therefore penetration testing should be seen as animportant tool in the arsenal, but alone it will not ensure product software is secure.
The common methods of identifying vulnerabilities in a software project are:
• Source Code Scanning using automated tools that run against a source code repository or module, finding
string patterns deemed to potentially cause security vulnerabilities.

Secure Code Review• Automated Penetration Testing (black/grey box) through penetrating testing tools automatic scans, where
the tool is installed on the network with the web site being tested, and runs a set of pre-defined tests against
the web site URLs.
• Manual Penetration Testing, again using tools, but with the expertise of a penetration tester performing more
complicated tests.
• Secure Code Review with a security subject matter expert.
It should be noted that no one method will be able to identify all vulnerabilities that a software project might
encounter, however a defense-in-depth approach will reduce the risk of unknown issues being including in
production software.
During a survey at AppSec USA 2015 the respondents rated which security method was the most effective infinding:
1) General security vulnerabilities
2) Privacy issues
3) Business logic bugs4) Compliance issues (such as HIPPA, PCI, etc.)
5) Availability issues
The results are shown in figure 1.

Figure 1: Survey relating detection methods to general vulnerability types
35
30
25
20
15
10
5
0
Vulnerabilities

Privacy

Source Code Scanning Tools
Automated Scan
Manual Pen Test
Manual Code Review

Business Logic

Compliance(HIPPA)

Availability

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Secure Code Review

Figure 2: Survey relating detection methods to OWASP Top 10 vulnerability types
35
30
25
20
15
10
5
0
A1

A2

A3

A4

A5

A6

A7

A8

A9

A10

Source Code Scanning Tool
Automated Scan
Manual Pen Test
Manual Code Review

These surveys show that manual code review should be a component of a company’s secure lifecycle, as inmany cases it is as good, or better, than other methods of detecting security issues.
5.6 Coupling Source Code Review and Penetration Testing
The term “360 review” refers to an approach in which the results of a source code review are used to plan andexecute a penetration test, and the results of the penetration test are, in turn, used to inform additional sourcecode review.

Figure 3: Code Review and Penetration Testing Interactions

CODE
REVIEW

EXPLOITEDVULNERABILITIESSUSPECTED KNOWN
VULNERABILITIESPENETRATIONTESTSecure Code Review

Knowing the internal code structure from the code review, and using that knowledge to form test cases andabuse cases is known as white box testing (also called clear box and glass box testing). This approach can lead toa more productive penetration test, since testing can be focused on suspected or even known vulnerabilities. Using knowledge of the specific frameworks, libraries and languages used in the web application, the penetration
test can concentrate on weaknesses known to exist in those frameworks, libraries and languages.
A white box penetration test can also be used to establish the actual risk posed by a vulnerability discovered
through code review. A vulnerability found during code review may turn out not to be exploitable during penetration test due to the code reviewer(s) not considering a protective measure (input validation, for instance).
While the vulnerability in this case is real, the actual risk may be lower due to the lack of exposure. However there
is still an advantage to adding the penetration test encase the protective measure is changed in the future andtherefore exposes the vulnerability.
While vulnerabilities exploited during a white box penetration test (based on secure code review) are certainly
real, the actual risk of these vulnerabilities should be carefully analyzed. It is unrealistic that an attacker would
be given access to the target web application’s source code and advice from its developers. Thus, the risk thatan outside attacker could exploit the vulnerabilities found by the white box penetration tester is probably lower. However, if the web application organization is concerned with the risk of attackers with inside knowledge(former employees or collusion with current employees or contractors), the real-world risk may be just as high.
The results of the penetration test can then be used to target additional areas for code review. Besides addressing the par-ticular vulnerability exploited in the test, it is a good practice to look for additional places where thatsame class of vulnerability is present, even if not explicitly exploited in test. For instance, if output encoding is
not used in one area of the application and the penetration test exploited that, it is quite possible that output
encoding is also not used elsewhere in the application.
5.7 Implicit Advantages of Code Review to Development Practices
Integrating code review into a company’s development processes can have many benefits which will depend
upon the processes and tools used to perform code reviews, how well that data is backed up, and how those
tools are used. The days of bringing developers into a room and displaying code on a projector, whilst recording
the review results on a printed copy are long gone, today many tools exist to make code review more efficientand to track the review records and decisions. When the code review process is structured correctly, the act ofreviewing code can be efficient and provide educational, auditable and historical benefits to any organization.
This section provides a list of benefits that a code review procedure can add to development organization.
Provides an historical record
If any developer has joined a company, or moved teams within a company, and had to maintain or enhance apiece of code written years ago, one of the biggest frustrations can be the lack of context the new developer hason the old code. Various schools of opinion exist on code documentation, both within the code (comments) andexternal to the code (design and functional documents, wikis, etc.). Opinions range from zero-documentation
tolerance through to near-NASA level documentation, where the size of the documentation far exceeds the sizeof the code module.
Many of the discussions that occur during a code review, if recorded, would provide valuable information (context) to module maintainers and new programmers. From the writer describing the module along with some of
their design decisions, to each reviewers comments, stating why they think one SQL query should be restructured, or an algorithm changed, there is a development story unfolding in front of the reviewers eyes which canbe used by future coders on the module, who are not involved in the review meetings.

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Secure Code Review

Capturing those review discussions in a review tool automatically and storing them for future reference will provide the development organization with a history of the changes on the module which can be queried at a later time by new developers. These discussions can also contain links to any architectural/functional/design/test
specifications, bug or enhancement numbers.
Verification that the change has been tested
When a developer is about to submit code into the repository, how does the company know they have sufficiently tested it? Adding a description of the tests they have run (manually or automated) against the changed codecan give reviewers (and management) confidence that the change will work and not cause any regressions. Also
by declaring the tests the writer has ran against their change, the author is allowing reviewers to review the tests
and suggest further testing that may have been missed by the author.
In a development scenario where automated unit or component testing exists, the coding guidelines can requirethat the developer include those unit/component tests in the code review. This again allows reviewers within thisenvironment to ensure the correct unit/component tests are going to be included in the environment, keeping
the quality of the continuous integration cycles.
Coding education for junior developers
After an employee learns the basics of a language and read a few of the best practices book, how can they getgood on-the-job skills to learn more? Besides buddy coding (which rarely happens and is never cost effective)
and training sessions (brown bag sessions on coding, tech talks, etc.) the design and code decisions discussed
during a code review can be a learning experience for junior developers. Many experienced developers admit tothis being a two way street, where new developers can come in with new ideas or tricks that the older developers
can learn from. Altogether this cross pollination of experience and ideas can only be beneficial to a development
organization.
Familiarization with code base
When a new feature is developed, it is often integrated with the main code base, and here code review can be a
conduit for the wider team to learn about the new feature and how its code will impact the product. This helps
prevent functional duplication where separate teams end up coding the same small piece of functionality.
This also applies for development environments with siloed teams. Here the code review author can reach out toother teams to gain their insight, and allow those other teams to review their modules, and everyone then learnsa bit more about the company’s code base.
Pre-warning of integration clashes
In a busy code base there will be times (especially on core code modules) where multiple developers can write
code affecting the same module. Many people have had the experience of cutting the code and running thetests, only to discover upon submission that some other change has modified the functionality, requiring theauthor to recode and retest some aspects of their change. Spreading the word on upcoming changes via code
reviews gives a greater chance of a developer learning that a change is about to impact their upcoming commit,
and development timelines, etc., can be updated accordingly.

Secure Coding Guidelines Touch Point
Many development environments have coding guidelines which new code must adhere to. Coding guidelines
can take many forms. It’s worth pointing out that security guidelines can be a particularly relevant touch point

Secure Code Review

within a code review, as unfortunately the secure coding issues are understood only by a subset of the development team. Therefore it can be possible to include teams with various technical expertise into the code reviews,
i.e. someone from the security team (or that person in the corner who knows all the security stuff) can be invited
as a technical subject expert to the review to check the code from their particular angle. This is where the OWASP
top 10 guidelines could be enforced.

5.8 Technical Aspects of Secure Code Review
Security code reviews are very specific to the application being reviewed. They may highlight some flaws thatare new or specific to the code implementation of the application, like insecure termination of execution flow,
synchronization errors, etc. These flaws can only be uncovered when we understand the application code flow
and its logic. Thus, security code review is not just about scanning the code for set of unknown insecure code
patterns but it also involves understanding the code implementation of the application and enumerating theflaws specific to it.
The application being reviewed might have been designed with some security controls in place, for example acentralized blacklist, input validation, etc. These security controls must be studied carefully to identify if they
are fool-proof. According to the implementation of the control, the nature of attack or any specific attack vector that can be used to bypass it, must be analyzed. Enumerating the weakness in the existing security control
is another important aspect of the security code reviews.
There are various reasons why security flaws manifest in the application, like a lack of input validation orparameter mishandling. In the process of a code review the exact root cause of flaws are exposed and thecomplete data flow is traced. The term ‘source to sink analysis’ means to determine all possible inputs to theapplication (source) and how they are being processed by it (sink). A sink could be an insecure code pattern
like a dynamic SQL query, a log writer, or a response to a client device.
Consider a scenario where the source is a user input. It flows through the different classes/components of theapplication and finally falls into a concatenated SQL query (a sink) and there is no proper validation being
applied to it in the path. In this case the application will be vulnerable to SQL injection attack, as identified
by the source to sink analysis. Such an analysis helps in understanding, which vulnerable inputs can lead to apossibility of an exploit in the application.
Once a flaw is identified, the reviewer must enumerate all the possible instances present in the application.
This would not be a code review initiated by a code change, this would be a code scan initiated by management based on a flaw being discovered and resources being committed to find if that flaw exists in otherparts of the product. For example, an application can be vulnerable to XSS vulnerability because of use of
un-validated inputs in insecure display methods like scriptlets ‘response.write’ method, etc. in several places.
5.9 Code Reviews and Regulatory Compliance
Many organizations with responsibility for safeguarding the integrity, confidentiality and availability of theirsoftware and data need to meet regulatory compliance. This compliance is usually mandatory rather than avoluntary step taken by the organization.
Compliance regulations include:• PCI (Payment Card Industry) standards• Central bank regulations

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Secure Code Review• Auditing objectives• HIPPA
Compliance is an integral part of software security development life-cycle and code review is an importantpart of compliance as many rules insist on the execution of code reviews in order to comply with certain regulations.
To execute proper code reviews that meet compliance rules it is imperative to use an approved methodology. Compliance requirements such as PCI, specifically requirement 6: “Develop and maintain secure systems”,
while PCI-DSS 3.0, which has been available since November 2013, exposes a series of requirements which
apply to development of software and identifying vulnerabilities in code. The Payment Card Industry Data
Security Standard (PCI-DSS) became a mandatory compliance step for companies processing credit card payments in June 2005. Performing code reviews on custom code has been a requirement since the first versionof the standard.
The PCI standard contains several points relating to secure application development, but this guide will focus
solely on the points, which mandate code reviews. All of the points relating to code reviews can be found inrequirement 6 “Develop and maintain secure systems and applications”.
5.10 PCI-DSS Requirements Related to Code Review
Specifically, requirement 6.3.2 mandates a code review of custom code. Reviewing custom code prior to release to production or customers in order to identify any potential coding vulnerability (using either manualor automated processes) to include at least the following:• Code changes are reviewed by individuals other than the originating code author, and by individuals knowledgeable about code review techniques and secure coding practices.
• Code reviews ensure code is developed according to secure coding guidelines• Appropriate corrections are implemented prior to release.
• Code review results are reviewed and approved by management prior to release.
Requirement 6.5 address common coding vulnerabilities in software-development processes as follows:• Train developers in secure coding techniques, including how to avoid common coding vulnerabilities, andunderstanding how sensitive data is handled in memory.
• Develop applications based on secure coding guidelines.
The PCI Council expanded option one to include internal resources performing code reviews. This added
weight to an internal code review and should provide an additional reason to ensure this process is performed
correctly.
The Payment Application Data Security Standard (PA-DSS) is a set of rules and requirements similar to PCI-DSS.
However, PA-DSS applies especially to software vendors and others who develop payment applications that
store, process, or transmit cardholder data as part of authorization or settlement, where these payment applications are sold, distributed, or licensed to third parties.

Secure Code Review

PA-DSS Requirements Related to Code Review
Requirements regarding code review are also applied since these are derived from PA-DSS in requirement 5(PCI, 2010):
5.2 Develop all payment applications (internal and external, and including web administrative access to product) based on secure coding guidelines.
5.1.4 Review of payment application code prior to release to customers after any significant change, to identify any potential coding vulnerability.
Note: This requirement for code reviews applies to all payment application components (both internal andpublic-facing web applications), as part of the system development life cycle. Code reviews can be conductedby knowledgeable internal personnel or third parties.

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METHODOLOGY
Code review is systematic examination of computer source code and reviews are done in various forms andcan be accomplished in various stages of each organization S-SDLC. This book does not attempt to tell eachorganization how to implement code reviews in their organization but this section does go over in genericterms and methodology of doing code reviews from informal walkthroughs, formal inspections, or Tool-assisted code reviews.
6.1 Factors to Consider when Developing a Code Review Process
When planning to execute a security code review, there are multiple factors to consider since every code
review is unique to its context. In addition to the elements discussed in this section, one must consider any
technical or business related factors (business decisions such as deadlines and resources) that impact theanalysis as these factors and may ultimately decide the course of the code review and the most effective way
to execute it.
Risks
It is impossible to secure everything at 100%, therefore it is essential to prioritize what features and components
must be securely reviewed with a risk based approach. While this project highlights some of the vital areas of design
security peer programmers should review all code being submitted to a repository, not all code will receive the attention and scrutiny of a secure code review.
Purpose & Context
Computer programs have different purposes and consequently the grade of security will vary depending on thefunctionality being implemented. A payment web application will have higher security standards than a promotional website. Stay reminded of what the business wants to protect. In the case of a payment application, data such as
credit cards will have the highest priority however in the case of a promotional website, one of the most important
things to protect would be the connection credentials to the web servers. This is another way to place context into arisk-based approach. Persons conducting the security review should be aware of these priorities.
Lines of CodeAn indicator of the amount of work is the number of lines of code that must be reviewed. IDEs (Integrated Development Environments) such as Visual Studio or Eclipse contain features, which allows the amount of lines
of code to be calculated, or in Unix/Linux there are simple tools like ‘wc’ that can count the lines. Programs
written in object-oriented languages are divided into classes and each class is equivalent to a page of code.
Generally line numbers help pinpoint the exact location of the code that must be corrected and is very usefulwhen reviewing corrections done by a developer (such as the history in a code repository). The more lines ofcode a program contains, the greater the chances that errors are present in the code.
Programming language
Programs written in typed safe languages (such as C# or Java) are less vulnerable to certain security bugs
such as buf- fer overflows than others like C and C++. When executing code review, the kind of language will
determine the types of expected bugs. Typically software houses tend towards a few languages that theirprogrammers are experienced in, however when a decision is made to create new code in a language new tothe developer management must be aware of the increased risk of securely reviewing that code due to thelack of in-house experience. Throughout this guide, sections explain the most common issues surrounding
the specific programming language code to be reviewed, use this as a reference to spot specific security issues
in the code.

Methodology

Resources, Time & Deadlines
As ever, this is a fundamental factor. A proper code review for a complex program will take longer and it willneed higher analysis skills than a simple one. The risks involved if resources are not properly provided are
higher. Make sure that this is clearly assessed when executing a review.
6.2 Integrating Code Reviews in the S-SDLC
Code reviews exist in every formal Secure Software Development Lifecycle (S-SDLC), but code reviews also
vary widely in their level of formality. To confuse the subject more, code reviews vary in purpose and in relation to what the code reviewer is looking for, be it security, compliance, programming style, etc. Throughout
the S-SDLC (XP, Agile, RAD, BSIMM, CMMI, Microsoft ALM) there are points where an application security SME
should to be involved. The idea of integrating secure code reviews into an S-SLDC may sound daunting as
there is another layer of complexity or additional cost and time to an already over budget and time constrained project. However it is proven to be cost effective and provides an additional level of security thatstatic analyzers cannot provide.
In some industries the drive for secure enhancements to a company’s S-SDLC may not be driven purely by
the desire to produce better code, these industries have regulations and laws that demand a level of due carewhen writing software (e.g. the governmental and financial industries) and the fines levelled at a company
who has not attempted to secure their S-SDLC will be far greater than the costs of adding security into thedevelopment lifecycle.
When integrating secure code reviews into the S-SDLC the organization should create standards and policies
that the secure code reviewer should adhere to. This will create the right importance of the task so it is not justlooked at as a project task that just needs to be checked off. Project time also needs to be assigned to the task
so there is enough time to complete the tasks (and for any remedial tasks that come out of the secure code
review). Standards also allow management and security experts (e.g. CISOs, security architects) to direct employees on what secure coding is to be adhered to, and allows the employees to refer to the (standard) whenreview arbitration is necessary.
Code Review Reports
A standard report template will provide enough information to enable the code reviewer to classify and prioritize the software vulnerabilities based on the applications threat model. This report
does not need to be pages in length, it can be document based or incorporated into many automated code review tools. A report should provide the following information:• Date of review.
• Application name, code modules reviewed.
• Developers and code reviewer names.
• Task or feature name, (TFS, GIT, Subversion, trouble ticket, etc.).

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Methodology• A brief sentence(s) to classify and prioritize software vulnerability if any and what if any remedialtasks need to be accomplished or follow up is needed.
• Link to documents related to task/feature, including requirements, design, testing and threat
modeling documents.
• Code Review checklist if used, or link to organization Code Review Checklist. (see Appendix A)• Testing the developer has carried out on the code. Preferably the unit or automated tests themselves can be part of the review submission.
• If any tools such as FxCop, BinScope Binary Analyzer, etc. were used prior to code review.

Today most organizations have modified their S-SDLC process to add agile into their S-SDLC process. Because of thisthe organization is going to need to look at their own internal development practices to best determine where andhow often secure code reviews need to happen. If the project is late and over budget then this increases the chancethat a software fix could cause a secure vulnerability since now the emphasis is on getting the project to deployment
quicker. Code reviews for code in production may find software vulnerabilities but understand that there is a racewith hackers to find the bug and the vulnerable software will remain in production while the remedial fix is being
worked on.
6.3 When to Code Review
Once an organization decides to include code reviews part of their internal code process. The next big question to
ask is to determine what stages of the SDLC will the code be reviewed. This section talks about three possible ways toinclude code reviews. There are three stages be in the SDLC when code can be reviewed:
When code is about to be checked in (pre-commit)
The development organization can state in their process that all code has to be reviewed before the code can besubmitted to the source code repository. This has the disadvantage of slowing the check-in process down, as thereview can take time, however it has many advantages in that below standard code is never placed in the code line,
and management can be confident that (if processes are being followed) the submitted code is at the quality thathas been stipulated.
For example, processes may state that code to be submitted must include links to requirements and design documentation and necessary unit and automated tests. This way the reviewers will have context on the exact code modification being done (due to the documentation) and they will know how the developer has tested the code (due tothe tests). If the peer reviewers do not think the documentation is complete, or the tests are extensive enough, they
can reject the review, not because of the code itself, but because the necessary docs or tests are not complete. In an
environment using CI with automated tests running nightly, the development team as a whole will know the nextday (following check-in) if the submitted code was of enough quality. Also management know that once a bug orfeature is checked in that the developer has finished their task, there’s no “I’ll finish those tests up next week” scenarios which adds risk to the development task.
When code has just been checked into a code base (post-commit)
Here the developer submits their code change, and then uses the code repository change-lists to send the code
diff for review. This has the advantage of being faster for the developer as there’s no review gate to pass before they
check-in their code. The disadvantage is that, in practice, this method can lead to a lesser quality of code. A develop-

Methodology

er will be less inclined to fix smaller issues once the code has been checked in, usually with a mantra of “Well the code
is in now, it’ll do.”. There also a risk of timing, as other developers could write other code fixes into the same module
before the review is done or changes and tests have been written, meaning the developer not only has to implement
the code changes from the peer or security review, but they also have to do so in a way that does not break othersubsequent changes. Suddenly the developer has to re-test the subsequent fixes to ensure no regressions.
Some development organizations using the Agile methodology add a ‘security sprint’ into their processes. During thesecurity sprint the code can be security reviewed, and have security specific test cases (written or automated) added.
When code audits are done
Some organizations have processes to review code at certain intervals (i.e. yearly) or when a vulnerable piece ofcode is suspected of being repeated throughout the code base. Here static code analyzers, or simple string searches
through the code (for specific vulnerability patterns) can speed up the process. This review is not connected to thesubmission of a feature or bug fix, they are triggered by process considerations and are likely to involve the review ofan entire application or code base rather than a review of a single submission.
Who Should Perform Secure Code Reviews
Some organizations assume secure code review can be a job for a security or risk-analysis team member. However all developers need to understand the exposure points of their applications and what threats exist for theirapplications.
Many companies have security teams that do not have members with coding backgrounds, which can makeinteractions with development teams challenging. Because of this development teams are usually skeptical of
security input and guidance. Security teams are usually willing to slow things down to ensure confidentiality
and integrity controls are in place while developers are face with pressure from business units they support tocreate and update code as quickly as possible. Unfortunately the more critical the application to operational or
business needs, the more pressure to deploy the code to production.
It is best to weave secure code reviews into the SDLC processes so that development organizations do not see
security as a hindrance, but as an assistance. As mentioned previously, spreading secure coding SMEs throughout an organization (satellites in BSIMM terminology) allows the secure code review tasks to scale and reachmore development teams. As the process grows, more of the developers gain awareness of secure coding issues(as they have reviews rejected on secure coding grounds) and the frequency of secure coding issues in code
reviews should drop.
6.4 Security Code Review for Agile and Waterfall Development
Today agile development is an umbrella term for a lot of practices that include programming, continuous integration, testing, project management, etc. There are many flavors of agile development, perhaps as many flavors
as there are practitioners. Agile development is a heterogeneous reference framework where the development
team can pick what practices they want to use.
Agile has some practices that could affect how and when code is reviewed, for example agile tries to keep code
review and testing as near as possible to the development phase. It is a common practice to define short development cycles (a.k.a. Iterations or Sprints). At the end of each cycle, all the code should be production quality
code. It can be incomplete, but it must add some value. That affects the review process, as reviewing should becontinuous. From the point of view of secure coding review, it shouldn’t make a difference if the development

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Methodology

organization uses agile or waterfall development practices. Code review is aligned to the code submitted, not theorder of feature development vs testing, or the time patterns assigned to the coding task. In many organizations
the line between waterfall and agile is becoming blurred, with traditional waterfall departments introducing thecontinuous integration (CI) aspects from agile, including nightly builds, automated testing, test driven development, etc.
6.5 A Risk Based Approach to Code Review
A development house will have various degrees of code changes being reviewed, from simple one line bug fixes
in backend scripts that run once a year, to large feature submissions in critical business logic. Typically the intensity of the code review varies based on the perceived risk that the change presents.
In the end, the scale of the code review comes down to the management of resources (skilled persons, company
time, machines, etc.). It would not be scalable to bring in multiple security experts for every code change occurring on a product, the resources of those persons or those teams would not be large enough to handle everychange. Therefore companies can make a call on which changes are important and need to be closely scrutinized, and which ones can be allowed through with minimal inspection. This will allow management to better
size the development cycle, if a change is going to be done in an area which is high risk, management can know
to set aside sufficient time for code review and ensure persons with relevant skills will be available. The processof deciding which changes need which level of code review is based on the risk level of the module the change
is within.
If the review intensity of code changes is based on the risk level of the module being changed, who should
decide the level of risk? Ultimately management is responsible for the output of a company, and thus they are
responsible for the risk associated with products sold by the company. Therefore it is up to management (or persons delegated by management) to create a reproducible measure or framework for deciding the risk associatedwith a code change.
Decisions on the risk of a module or piece of code should be based on solid cost benefit analysis and it would beirresponsible to decide all modules are high risk. Therefore management should meet with persons who havean understanding of the code base and security issues faced by the products, and create a measure of risk forvarious ele-ments of code. Code could be split up into modules, directories, products, etc., each with a risk levelassociated with it.
Various methods exist in the realm of risk analysis to assign risk to entities, and many books have been dedicatedto this type of discussion. The three main techniques for establishing risk are outlined in table 1 below.
Table 1: Options For Establishing Risk
Technique

MethodQuantitative

Bring people together and establish a monetary value on the potential loss associated with the code. Gauge the likelihood that the code could be compromised. Use dollar values produced from these calculations to determine the levelof risk.

Qualitative

Bring people together and discuss opinions on what level of loss is associated with the modules, and opinions on likelihood of compromise. Qualitative does not attempt to nail down monetary associations with the loss, but tends towards
the perception or opinion of associated losses.

DelphiIndependently interview or question people on the losses and compromises of the modules, whilst letting them know
the feedback will be anonymous. The impression here is that the people will give more honest answers to the questions
and will not be swayed by other people’s arguments and answers.

Methodology

Risk is chance of something bad happening and the damage that can be caused if it occurs. The criteria for deciding the risk profile of different code modules will be up to the management team responsible for delivering thechanges, examples are provided in table 2.
Table 2: Common Criteria For Establishing The Risk Profile Of A Code Module
Criteria

Explanation

Ease of exposure

Is the code change in a piece of code directly exposed to the internet? Does an insider use the interface directly?

Value of loss

How much could be lost if the module has a vulnerability introduced? Does the module contain some critical password
hashing mechanism, or a simple change to HTML border on some internal test tool?

Regulatory controls

If a piece of code implements business logic associated with a standard that must be complied with, then these modules can be considered high risk as the penalties for non-conformity can be high.

When levels of risk have been associated with products and modules, then the policies can be created determining what level of code review must be conducted. It could be that code changes in a level one risk module
must be reviewed by 3 persons including a Security Architect, whereas changes in a level 4 risk module only
need a quick one person peer review.
Other options (or criteria) for riskier modules can include demands on automated testing or static analysis, e.g.
code changes in high risk code must include 80% code coverage on static analysis tools, and sufficient automated tests to ensure no regressions occur. These criteria can be demanded and checked as part of the code
review to ensure they are capable of testing the changed code.
Some companies logically split their code into differing repositories, with more sensitive code appearing in a
repository with a limited subset of developers having access. If the code is split in this fashion, then it must be
remembered that only developers with access to the riskier code should be able to conduct reviews of thatcode.
Risk analysis could also be used during the code review to decide how to react to a code change that introduces risk into the product, as in table 3. In a typical risk analysis process, the team needs to decide whether toaccept, transfer, avoid or reduce the risks. When it comes to code reviews it is not possible to transfer the risk
as transferring risk normally means taking out insurance to cover the cost of exposure.
Table 3: Options For Handling Risks Identified In A Code Review
Risk Resolution

Explanation

ReduceThis is the typical resolution path. When a code reviewer finds that the code change introduces risk into an element ofbusiness logic (or simply a bug) the code will be changed to fix the bug or code in a way that reduces the risk.

AcceptWhen the code change introduces a risk in the code but there is no other way to implement the business logic, the codechange can pass code review if the risk is considered acceptable. The risk and any workarounds or mitigating factors
should be documented correctly so that it is not ignored.

Avoid

When the code change introduces a risk that is too great to be accepted, and it is not possible to reduce the risk by implementing a code change, then the team need to consider not performing the change. Ideally this decision should be reached
before the code review stage, but it is entirely possible that factors can arise during code implementation that changes theunderstood risk profile of a code module and prompts management to reconsider if a change should go ahead.

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Methodology

6.6 Code Review Preparation
A security review of the application should uncover common security bugs as well as the issues specific to businesslogic of the application. In order to effectively review a body of code it is important that the reviewers understandthe business purpose of the application and the critical business impacts. The reviewers should understand the attack
surface, identify the different threat agents and their motivations, and how they could potentially attack the application.
For the software developer whose code is being reviewed, performing code review can feel like an audit and developers may find it challenging to not take the feedback personally. A way to approach this is to create an atmosphereof collaboration be-tween the reviewer, the development team, the business representatives, and any other vested
interests. Por-traying the image of an advisor and not a policeman is important to get co-operation from the development team.
The extent to which information gathering occurs will depend on the size of the organization, the skill set of the reviewers, and the criticality/risk of the code being reviewed. A small change to the CSS file in a 20- person start-up will
not result in a full threat model and a separate secure review team. At the same time a new single sign-on authentication module in a multi-billion dollar company will not be secure code reviewed by a person who once read an article
on secure coding. Even within the same organization, high-risk modules or applications may get threat modeled,where the lower risk modules can be reviewed with a lesser emphasis on the reviewer understanding the security
model of the module.
This section will present the basic items the reviewer (or review team) should attempt to understand about the application subjected to a secure code review. This can be used in smaller companies that don’t have the resources tocreate a full security baseline, or on low risk code within larger companies. A later section goes into detail on threat
modeling, which would be used by larger companies on their highest risk code bases.
In an ideal world the reviewer would be involved in the design phase of the application, but this is rarely the case.
However regardless of the size of the code change, the engineer initiating the code review should direct reviewers
to any relevant architecture or design documents. The easiest way to do this is to include a link to the documents (assuming they’re stored in an online document repository) in the initial e-mail, or in the code review tool. The reviewer
can then verify that the key risks have been properly addressed by security controls and that those controls are usedin the right places.
To effectively conduct the review the reviewer should develop familiarity with the following aspects:
Application features and Business Rules
The reviewer should understand all the features currently provided by the application and capture all the businessrestrictions/rules related to them. There is also a case for being mindful of potential future functionality that might beon the roadmap for an application, thereby future-proofing the security decisions made during current code reviews.
What are the consequences of this system failing? Shall the enterprise be affected in any great way if the application
cannot perform its functions as intended?
Context
All security is in context of what we are trying to secure. Recommending military standard security mechanisms on
an application that vends apples would be overkill and out of context. What type of data is being manipulated orprocessed, and what would the damage to the company be if this data was compromised? Context is the “Holy Grail”
of secure code inspection and risk assessment.

Methodology

Sensitive Data
The reviewer should also make a note of the data entities like account numbers and passwords that are sensitive tothe application. The categorizing the data entities based on their sensitivity will help the reviewer to determine theim-pact of any kind of data loss in the application.
User roles and access rights
It is important to understand the type of users allowed to access the application. Is it externally facing or internal to“trusted” users? Generally an application that is accessible only for the internal users of an organization might be
exposed to threats that are different than the one that is available for anyone on the Internet. Hence, knowing theusers of the application and its deployed environment would allow the reviewer to realize the threat agents correctly.
In addition to this, the different privileges levels present in the application must also be understood. It would help
the reviewer to enumerate different security violations/privilege escalation attacks that can be applicable to the application.
Application typeThis refers to understanding whether the application is browser based application, a desktop based standalone application, a web-service, a mobile applications or a hybrid application. Different type of application faces different
kinds of security threats and understanding the type of the application would help the reviewer to look for specificsecurity flaws, determine correct threats agents and highlight necessary controls suitable to the application.
Code
The language(s) used, the features and issues of that language from a security perspective. The issues a programmer
needs to look out for and language best practices from a security and performance perspective.
DesignGenerally web applications have a well-defined code layout if they are developed using MVC design principle. Applications can have their own custom design or they may use some well-known design frameworks like Struts/Springetc. Where are the application properties/configuration parameters stored? How is the business class identified forany feature/URL? What types of classes get executed for any processing any request? (e.g. centralized controller,
command classes, view pages etc.) How is the view rendered to the users for any request?
Company Standards and Guidelines
Many companies will have standards and guidelines dictated by management. This is how the management (ultimately responsible for the organizations information security) control what levels of security are applied to various
functions, and how they should be applied. For example, if the company has a Secure Coding Guidelines document,
reviewers should know and understand the guidelines and apply them during the code review.
6.7 Code Review Discovery and Gathering Information
The reviewers will need certain information about the application in order to be effective. Frequently, this information can be obtained by studying design documents, business requirements, functional specifications, test results,
and the like. However, in most real-world projects, the documentation is significantly out of date and almost neverhas appropriate security information. If the development organization has procedures and templates for architectureand design documents, the reviewer can suggest updates to ensure security is considered (and documented) atthese phases.
If the reviewers are initially unfamiliar with the application, one of the most effective ways to get started is to talkwith the developers and the lead architect for the application. This does not have to be a long meeting, it could be awhiteboard session for the development team to share some basic information about the key security considerations

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Methodology

and controls. A walkthrough of the actual running application is very helpful to give the reviewers a good idea about
how the application is intended to work. Also a brief overview of the structure of the code base and any libraries used
can help the reviewers get started.
If the information about the application cannot be gained in any other way, then the reviewers will have to spendsome time doing reconnaissance and sharing information about how the application appears to work by examining
the code. Preferably this information can then be documented to aid future reviews.
Security code review is not simply about the code structure. It is important to remember the data; the reason that
we review code is to ensure that it adequately protects the information and assets it has been entrusted with, suchas money, intellectual property, trade secrets, or lives. The context of the data with which the application is intended
to process is very important in establishing potential risk. If the application is developed using an inbuilt/well-known
design framework the answers to the most of these questions would be pre-defined. But, in case it is custom then thisinformation will surely aid the review process, mainly in capturing the data flow and internal validations. Knowing
the architecture of the application goes a long way in understanding the security threats that can be applicable tothe application.
A design is a blueprint of an application; it lays a foundation for its development. It illustrates the layout of the application and identifies different application components needed for it. It is a structure that determines execution flowof the application. Most of the application designs are based on a concept of MVC. In such designs different components interact with each other in an ordered sequence to serve any user request. Design review should be an integral
part of secure software development process. Design reviews also help to implementing the security requirements
in a better way.
Collecting all the required information of the proposed design including flow charts, sequence diagrams, class diagrams and requirements documents to understand the objective of the proposed design. The design is thoroughly
studied mainly with respect to the data flow, different application component interactions and data handling. This isachieved through manual analysis and discussions with the design or technical architect’s team. The design and thearchitecture of the application must be understood thoroughly to analyze vulnerable areas that can lead to security
breaches in the application.
After understanding the design, the next phase is to analyze the threats to the design. This involves observing thedesign from an attacker’s perspective and uncovering the backdoors and insecure areas present in it. Table 4 below
Table 4: Example Design Questions During Secure Code Review
Design Area

Questions to consider

Data Flow• Are user inputs used to directly reference business logic?
• Is there potential for data binding flaws?
• Is the execution flow correct in failure cases?

Authentication andaccess control• Does the design implement access control for all resources?
• Are sessions handled correctly?
• What functionality can be accessed without authentication?

Existing security controls• Are there any known weaknesses in third-part security controls• Is the placements of security controls correct?

Architecture• Are connections to external servers secure?
• Are inputs from external sources validated?

Configuration files anddata stores• Is there any sensitive data in configuration files?
• Who has access to configuration or data files?

Methodology

Code Review Checklist
Defining a generic checklist, which the development team can fill out can give reviewers the desired context. The
checklist is a good barometer for the level of security the developers have attempted or thought of. If security
code review becomes a common requirement, then this checklist can be incorporated into a development procedure (e.g. document templates) so that the information is always available to code reviewers. See Appendix A
for a sample code review checklist.
The checklist should cover the most critical security controls and vulnerability areas such as:• Data Validation• Authentication• Session Management• Authorization• Cryptography• Error Handling• Logging• Security Configuration• Network Architecture
highlights some questions that can be asked of the architecture and design to aid secure code reviews.
Every security requirement should be associated with a security control best suited for the design. Here, we would
identify exact changes or additions to be incorporated in the design that are needed to meet any requirement or
mitigate a threat. The list of security requirements and proposed controls can be then discussed with the development teams. The queries of the teams should be addressed and feasibility of incorporating the controls must be
determined. Exceptions, if any must be taken into account and alternate recommendations should be proposed. In
this phase a final agreement on the security controls is achieved. The final design incorporated by the development
teams can be reviewed again and finalized for further development process.
6.8 Static Code Analysis
Static Code Analysis is carried out during the implementation phase of S-SDLC. Static code analysis commonlyrefers to running static code analysis tools that attempt to highlight possible vulnerabilities within the ‘static’(non-running) source code.
Ideally static code analysis tools would automatically find security flaws with few false positives. That means it shouldhave a high degree of confidence that the bugs that it finds are real flaws. However, this ideal is beyond the state of
the art for many types of application security flaws. Thus, such tools frequently serve as aids for an analyst to help
them zero in on security relevant portions of code so they can find flaws more efficiently, rather than a tool that finds
all flaws automatically.
Bugs may exist in the application due to insecure code, design or configuration. Automated analysis can be carried on
the application code to identify bugs through either of the following two options:

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1. Static code scanner scripts based on a pattern search (in-house and open source).
2. Static code analyzers (commercial and open source).
Advantages and disadvantages of source code scanners are shown in tables 5 and 6.
Table 5: Advantages To Using Source Code Scanners
Advantage

Explanation

Reduction in manual
efforts

The type of patterns to be scanned for remains common across applications, computers are better at such scans thanhumans. In this scenario, scanners play a big role is automating the process of searching the vulnerabilities throughlarge codebases.

Find all the instances ofthe vulnerabilities

Scanners are very effective in identifying all the instances of a particular vulnerability with their exact location. This is
helpful for larger code base where tracing for flaws in all the files is difficult.

Source to sink analysis

Some analyzers can trace the code and identify the vulnerabilities through source to sink analysis. They identify possible
inputs to the application and trace them thoroughly throughout the code until they find them to be associated with any
insecure code pattern. Such a source to sink analysis helps the developers in understanding the flaws better as they get
a complete root cause analysis of the flaw

Elaborate reporting
formatScanners provide a detailed report on the observed vulnerabilities with exact code snippets, risk rating and complete
description of the vulnerabilities. This helps the development teams to easily understand the flaws and implement
necessary controls

Though code scanning scripts and open source tools can be efficient at finding insecure code patterns, they often
lack the capability of tracing the data flow. This gap is filled by static code analyzers, which identify the insecure code
patterns by partially (or fully) compiling the code and investigating the execution branches, allowing for source tosink analysis. Static code analyzers and scanners are comprehensive options to complement the process of code
review.
Table 6: Disadvantages To Using Source Code Scanners
Limitation

Explanation

Business logic flaws
remain untouched

The flaws that are related to application’s business logic, transactions, and sensitive data remain untouched by the scanners. The security controls that need to be implemented in the application specific to its features and design are often
not pointed by the scanners. This is considered as the biggest limitation of static code analyzers.

Limited scope

Static code analyzers are often designed for specific frameworks or languages and within that scope they can search
for a certain set of vulnerable patterns. Outside of this scope they fail to address the issues not covered in their searchpattern repository.

Design flaws

Design flaws are not specific to the code structure and static code analyzers focus on the code. A scanner/analyzer will
not spot a design issue when looking at the code, whilst a human can often identify design issues when looking at theirimplementation.

False positives

Not all of the issues flagged by static code analyzers are truly issues, and thus the results from these tools need to beunderstood and triaged by an experienced programmer who understands secure coding. Therefore anyone hopingthat secure code checking can be automated and run at the end of the build will be disappointed, and there is still adeal of manual intervention required with analyzers.

Choosing a static analysis tool
Choosing a static analysis tool is a difficult task since there are a lot of choices. The comparison charts below could
help organization decide which tool is right for them, although this list is not exhaustive.

Methodology

Some of the criteria for choosing a tool are:• Does the tool support the programming language used?
• Is there a preference between commercial or free tools? Usually the commercial tools have more features and are
more reliable than the free ones, whilst their usability might differ.
• What type of analysis is being carried out? Is it security, quality, static or dynamic analysis?
The next step requires that some work is done since it is quite subjective. The best thing to do is to test a few tools to
see if the team is satisfied with different aspects such as the user experience, the reporting of vulnerabilities, the levelof false positives and the customization and the customer support. The choice should not be based on the number offeatures, but on the features needed and how they could be integrated in the S-SDLC. Also, before choosing the tool,
the expertise of the targeted users should be clearly evaluated in order to choose an appropriate tool.
6.9 Application Threat ModelingThreat modeling is an in-depth approach for analyzing the security of an application. It is a structured approach thatenables employees to identify, quantify, and address the security risks associated with an application. Threat modeling is not an approach to reviewing code, but it complements the secure code review process by providing context
and risk analysis of the application.
The inclusion of threat modeling in the S-SDLC can help to ensure that applications are being developed with security built-in from the very beginning. This, combined with the documentation produced as part of the threat modelingprocess, can give the reviewer a greater understanding of the system, allows the reviewer to see where the entry
points to the application are (i.e. the attack surface) and the associated threats with each entry point (i.e. attack vectors).
The concept of threat modeling is not new but there has been a clear mind-set change in recent years. Modern threat
modeling looks at a system from a potential attacker’s perspective, as opposed to a defender’s viewpoint. Many companies have been strong advocates of the process over the past number of years, including Microsoft who has madethreat modeling a core component of their S-SDLC, which they claim to be one of the reasons for the increased security of their products in recent years.
When source code analysis is performed outside the S-SDLC, such as on existing applications, the results of the threat
modeling help in reducing the complexity of the source code analysis by promoting a risk based approach. Instead ofreviewing all source code with equal focus, a reviewer can prioritize the security code review of components whose
threat modeling has ranked with high risk threats.
The threat modeling process can be decomposed into 3 high level steps:
6.9.1. Step 1: Decompose the Application.
The first step in the threat modelling process is concerned with gaining an understanding of the application and how
it interacts with external entities. This involves creating use-cases to understand how the application is used, identifying entry points to see where a potential attacker could interact with the application, identifying assets i.e. items/
areas that the attacker would be interested in, and identifying trust levels which represent the access rights that theapplication will grant to external entities. This information is documented in the threat model document and it is also
used to produce data flow diagrams (DFDs) for the application. The DFDs show the different data paths through thesystem, highlighting the privilege (trust) boundaries.

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Items to consider when decomposing the application include
External Dependencies
External dependencies are items external to the code of the application that may pose a threat to the application.
These items are typically still within the control of the organization, but possibly not within the control of the development team. The first area to look at when investigating external dependencies is how the application will be
deployed in a production environment.
This involves looking at how the application is or is not intended to be run. For example if the application is expected
to be run on a server that has been hardened to the organization’s hardening standard and it is expected to sit behinda firewall, then this information should be documented.
Entry Points
Entry points (aka attack vectors) define the interfaces through which potential attackers can interact with the application or supply it with data. In order for a potential attacker to attack an application, entry points must exist. Entry
points in an application can be layered, for example each web page in a web application may contain multiple entry
points.
Assets
The system must have something that the attacker is interested in; these items/areas of interest are defined as assets.
Assets are essentially threat targets, i.e. they are the reason threats will exist. Assets can be both physical assets andabstract assets. For example, an asset of an application might be a list of clients and their personal information; this isa physical asset. An abstract asset might be the reputation of an organization.
Determining the Attack Surface
The attack surface is determined by analyzing the inputs, data flows and transactions. A major part of actually performing a security code review is performing an analysis of the attack surface. An application takes inputs and produces output of some kind. The first step is to identify all input to the code.
Inputs to the application may include the bullet points below and figure 4 describes an example process for identifying an applications input paths:• Browser input• Cookies• Property files• External processes• Data feeds• Service responses• Flat files• Command line parameters• Environment variables

Methodology

Figure 4: Example process diagram for identifying input paths

TRANSITIONALANALYSISINITIATIONIDENTIFY INPUT PATHSIDENTIFYATTACK
SURFACE

INPUT
PARAMETERS(CONFIG)

INPUT
PARAMETERS(USER)IDENTIFYATTACK
SURFACEIDENTIFY AREAS
OF LATE & DYNAMIC BINDING

INPUTPARAMETERS(CONTROL)IDENTIFYATTACK
SURFACEFOLLOW PATH
EACH PARAMETER THROUGH
CODE

INPUT
PARAMETERS(BLACKEND)

IDENTIFYATTACK
SURFACEIDENTIFY AREAS
OF CONFIG FILE
REFERENCE

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Trust Levels
Trust levels represent the access rights that the application will grant to external entities. The trust levels are cross-referenced with the entry points and assets. This allows a team to define the access rights or privileges required at eachentry point, and those required to interact with each asset.
Data flow analysis
Exploring the attack surface includes dynamic and static data flow analysis. Where and when variables are set andhow the variables are used throughout the workflow, how attributes of objects and parameters might affect otherdata within the program. It determines if the parameters, method calls, and data exchange mechanisms implement
the required security.
Transaction analysis
Transaction analysis is needed to identify and analyze all transactions within the application, along with the relevant
security functions invoked.
The areas that are covered during transaction analysis are:
• Data/Input Validation of data from all untrusted sources• Authentication• Session Management• Authorization• Cryptography (data at rest and in transit)• Error Handling /Information Leakage• Logging /Auditing
Data Flow Diagrams
All of the information collected allows an accurately model the application through the use of Data Flow Diagrams(DFDs). The DFDs will allow the employee to gain a better understanding of the application by providing a visual
representation of how the application processes data. The focus of the DFDs is on how data moves through theapplication and what happens to the data as it moves. DFDs are hierarchical in structure, so they can be used todecompose the application into subsystems. The high level DFD will allow the employee to clarify the scope of theapplication being modelled. The lower level iterations will allow more focus on the specific processes involved whenprocessing specific data.
There are a number of symbols that are used in DFDs for threat modelling, as show in the following table 7 below:
Table 7: Threat Modeling Symbols
Methodology

ELEMENTIMAGE

DESCRIPTIONEXTERNAL ENTITY

The external entity shape is used to represent any entity
outside the application that interacts with the application via
an entry point.

PROCESSThe process shape represents a task that handles data within
the application. The task may process the data or perform anaction based on the data.

Methodology

MULTIPLE PROCESSThe multiple process shape is used to present a collection of
subprocesses. The multiple process can be broken down into
its subprocesses in another DFD.

DATA STORE

The data store shape is used to represent locations wheredata is stored. Data stores do not modify the data, they only
store data.

DATA FLOW

The data flow shape represents data movement within theapplication. The direction of the data movement is represented by the arrow.

PRIVILEGE BOUNDARY

The privilege boundary shape is used to represent thechange of privilege levels as the data flows through theapplication.

DFDs show how data moves logically through the system and allows the identification data entering or leaving the system along with the storage of data and the flow of control through these components. Trust boundaries show any location where the level of trust changes. Process components show where data is processed,
such as web servers, application servers, and database servers. Entry points show where data enters the system (i.e. input fields, methods) and exit points are where it leaves the system (i.e. dynamic output, methods),
respectively. Entry and exit points define a trust boundary.
6.9.2 Step 2: Determine and rank threats
Critical to the identification of threats is using a threat categorization methodology. A threat categorizationsuch as STRIDE can be used, or the Application Security Frame (ASF) that defines threat categories such as Auditing & Logging, Authentication, Authorization, Configuration Management, Data Protection in Storage andTransit, Data Validation and Exception Management.
The goal of the threat categorization is to help identify threats both from the attacker (STRIDE) and the defensive perspective (ASF). DFDs produced in step 1 help to identify the potential threat targets from the attacker’s
perspective, such as data sources, processes, data flows, and interactions with users. These threats can be
identified further as the roots for threat trees; there is one tree for each threat goal.
From the defensive perspective, ASF categorization helps to identify the threats as weaknesses of security
controls for such threats. Common threat-lists with examples can help in the identification of such threats. Useand abuse cases can illustrate how existing protective measures could be bypassed, or where a lack of suchprotection exists.
The determination of the security risk for each threat can be determined using a value-based risk model suchas DREAD or a less subjective qualitative risk model based upon general risk factors (e.g. likelihood and impact).
The first step in the determination of threats is adopting a threat categorization. A threat categorization pro-

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Methodology

vides a set of threat categories with corresponding examples so that threats can be systematically identified
in the application in a structured and repeatable manner.
STRIDE
Threat lists based on the STRIDE model are useful in the identification of threats with regards to the attacker
goals. For example, if the threat scenario is attacking the login, would the attacker brute force the password to
break the authentication? If the threat scenario is to try to elevate privileges to gain another user’s privileges,
would the attacker try to perform forceful browsing?
A threat categorization such as STRIDE is useful in the identification of threats by classifying attacker goals
such as shown in table 8.
Table 8: Explanation Of The Stride Attributes
STRIDE

Explanation

Spoofing“Identity spoofing” is a key risk for applications that have many users but provide a single execution context at the application and database level. In particular, users should not be able to become any other user or assume the attributesof another user.

Tampering

Users can potentially change data delivered to them, return it, and thereby potentially manipulate client-side validation, GET and POST results, cookies, HTTP headers, and so forth. The application should also carefully check data received from the user and validate that it is sane and applicable before storing or using it.

Repudiation

Users may dispute transactions if there is insufficient auditing or recordkeeping of their activity. For example, if a user
says they did not make a financial transfer, and the functionality cannot track his/her activities through the application,then it is extremely likely that the transaction will have to be written off as a loss.

Information Disclosure

Users are rightfully wary of submitting private details to a system. Is possible for an attacker to publicly reveal user dataat large, whether anonymously or as an authorized user?

Denial of Service

Application designers should be aware that their applications may be subject to a denial of service attack. The use of
expensive resources such as large files, complex calculations, heavy-duty searches, or long queries should be reserved
for authenticated and authorized users, and not available to anonymous users.

Elevation of Privilege

If an application provides distinct user and administrative roles, then it is vital to ensure that the user cannot elevate
his/her role to a higher privilege one.

It is vital that all possible attack vectors should be evaluated from the attacker’s point of view. For example, the login
page allows sending authentication credentials, and the input data accepted by an entry point has to validate forpotential malicious input to exploit vulnerabilities such as SQL injection, cross site scripting, and buffer overflows.
Additionally, the data flow passing through that point has to be used to determine the threats to the entry points
to the next components along the flow. If the following components can be regarded critical (e.g. the hold sensitivedata), that entry point can be regarded more critical as well. In an end to end data flow the input data (i.e. username
and password) from a login page, passed on without validation, could be exploited for a SQL injection attack to manipulate a query for breaking the authentication or to modify a table in the database.
Exit points might serve as attack points to the client (e.g. XSS vulnerabilities) as well for the realization of information
disclosure vulnerabilities. In the case of exit points from components handling confidential data (e.g. data accesscomponents), any exit points lacking security controls to protect the confidentiality and integrity can lead to disclosure of such confidential information to an unauthorized user.
In many cases threats enabled by exit points are related to the threats of the corresponding entry point. In the login
example, error messages returned to the user via the exit point might allow for entry point attacks, such as accountMethodology

harvesting (e.g. username not found), or SQL injection (e.g. SQL exception errors). From the defensive perspective,
the identification of threats driven by security control categorization such as ASF allows a threat analyst to focus on
specific issues related to weaknesses (e.g. vulnerabilities) in security controls. Typically the process of threat identification involves going through iterative cycles where initially all the possible threats in the threat list that apply toeach component are evaluated. At the next iteration, threats are further analyzed by exploring the attack paths, theroot causes (e.g. vulnerabilities) for the threat to be exploited, and the necessary mitigation controls (e.g. countermeasures).
Once common threats, vulnerabilities, and attacks are assessed, a more focused threat analysis should take in consideration use and abuse cases. By thoroughly analyzing the use scenarios, weaknesses can be identified that could
lead to the realization of a threat. Abuse cases should be identified as part of the security requirement engineering
activity. These abuse cases can illustrate how existing protective measures could be bypassed, or where a lack of suchprotection exists. Finally, it is possible to bring all of this together by determining the types of threat to each component of the decomposed system. This can be done by repeating the techniques already discussed on a lower level
threat model, again using a threat categorization such as STRIDE or ASF, the use of threat trees to determine how thethreat can be exposed by vulnerability, and use and misuse cases to further validate the lack of a countermeasure to
mitigate the threat.
Microsoft DREAD threat-risk ranking model
In the Microsoft DREAD threat-risk ranking model, the technical risk factors for impact are Damage and Affected Users, while the ease of exploitation factors are Reproducibility, Exploitability and Discoverability. This risk factorization
allows the assignment of values to the different influencing factors of a threat.
To determine the ranking of a threat, the threat analyst has to answer basic questions for each factor of risk, for example:
Table 9: Explanation Of The Dread Attributes
DREAD

Questions

DamageHow big would the damage be if the attack succeeded?
Can an attacker completely take over and manipulate the system?
Can an attacker crash the system?

Reproducibility

How easy is it to reproduce an attack to work?
Can the exploit be automated?

Exploitability

How much time, effort, and expertise is needed to exploit the threat?
Does the attacker need to be authenticated?

Affected Users

If a threat were exploited, what percentage of users would be affected?
Can an attacker gain administrative access to the system?

Discoverability

How easy is it for an attacker to discover this threat?

The impact mainly depends on the damage potential and the extent of the impact, such as the number ofcomponents that are affected by a threat.

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These questions help in the calculation of the overall risk values by assigning qualitative values such as High,
Medium and Low to Likelihood and Impact factors. In this case, using qualitative values, rather than numericones like in the case of the DREAD model, help avoid the ranking becoming overly subjective.
Likelihood
A more generic risk model takes into consideration the Likelihood (e.g. probability of an attack) and the Impact(e.g. damage potential):
Risk = Likelihood x Impact
Note that this is a conceptual formula and is not expected to use actual values for likelihood and impact. The
likelihood or probability is defined by the ease of exploitation, which mainly depends on the type of threat andthe system characteristics, and by the possibility to realize a threat, which is determined by the existence of an
appropriate countermeasure.

6.9.3 Step 3: Determine countermeasures and mitigation.
A lack of protection against a threat might indicate a vulnerability whose risk exposure could be mitigatedwith the implementation of a countermeasure. Such countermeasures can be identified using threat-countermeasure mapping lists. Once a risk ranking is assigned to the threats, it is possible to sort threats from the highest to the lowest risk, and prioritize the mitigation effort, such as by responding to such threats by applying theidentified countermeasures.
The risk mitigation strategy might involve evaluating these threats from the business impact that they poseand establishing countermeasures (or design changes) to reduce the risk.
Other options might include accepting the risk, assuming the business impact is acceptable because of compensating controls, informing the user of the threat, removing the risk posed by the threat completely, or the
least preferable option, that is, to do nothing. If the risk identified is extreme, the functionality or productcould be discontinued, as the risk of something going wrong is greater than the benefit.
The purpose of the countermeasure identification is to determine if there is some kind of protective measure(e.g. security control, policy measures) in place that can prevent each threat previously identified via threat
analysis from being realized. Vulnerabilities are then those threats that have no countermeasures.
Since each of these threats has been categorized either with STRIDE or ASF, it can be possible to find appropriate countermeasures in the application within the given category. Each of the above steps is documented asthey are carried out. The resulting set of documents is the threat model for the application. Detailed examples
of how to carry out threat modeling is given in Appendix B.
Threat Profile
Once threats and corresponding countermeasures are identified it is possible to derive a threat profile with thefollowing criteria:

Methodology

Table 10: Types Of Mitigated Threats
Threat Type

Description

Non-mitigated threats

Threats which have no countermeasures and represent vulnerabilities that can be fully exploited and cause an impact.

Partially mitigatedthreats

Threats partially mitigated by one or more countermeasures, which represent vulnerabilities that can only partially be
exploited and cause a limited impact.

Fully mitigated threats

These threats have appropriate countermeasures in place and do not expose vulnerability and cause impact.

6.10 Metrics and Code Review
Metrics measure the size and complexity of a piece of code. There is a long list of quality and security characteristics that can be considered when reviewing code (such as, but not limited to, correctness, efficiency,
portability, maintainability, reliability and securability). No two-code review sessions will be the same so some
judgment will be needed to decide the best path. Metrics can help decide the scale of a code review.
Metrics can also be recorded relating to the performance of the code reviewers and the accuracy of the review
process, the performance of the code review function, and the efficiency and effectiveness of the code review
function.
The figure 5 describes the use of metrics throughout the code review process.
Some of the options for calculating the size of a review task include:Lines of Code (LOC):
A count of the executable lines of code (commented-out code or blank lines are not counted). This gives arough estimate but is not particularly scientific.
Function Point:
The estimation of software size by measuring functionality. The combination of a number of statements which
perform a specific task, independent of programming language used or development methodology. In an
object orientated language a class could be a functional point.
Defect Density:
The average occurrence of programming faults per Lines of Code (LOC). This gives a high level view of the code
quality but not much more. Fault density on its own does not give rise to a pragmatic metric. Defect density
would cover minor issues as well as major security flaws in the code; all are treated the same way. Security ofcode cannot be judged accurately using defect density alone.
Risk Density:
Similar to defect density, but discovered issues are rated by risk (high, medium & low). In doing this we cangive insight into the quality of the code being developed via a [X Risk / LoC] or [Y Risk / Function Point] value(X&Y being high, medium or low risks) as defined by internal application development policies and standards.
For example:
4 High Risk Defects per 1000 (Lines of Code)2 Medium Risk Defects per 3 Function Points

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Figure 5: Example process diagram for identifying input paths

The Use of Metrics
Throughout The
Code Review
Process

RECOMENDED
CODE TRIAGE
MEETINGCODESUBMITEDFOR SCRNOCODE
RESUBMITEDFOR
RE-REVIEWTREND
ANALYSIS

HAS CONTEXT
OF CODE BEEN
DEFINED

DEFINE CRITERIAPROJECT OR
VULNERABILITY
BASED

YES
STANDARDSPOLICIESPERFORM
REVIEWCOMMUNICATERESULTS TO TEAMGUIDELINESCODE REVIEWDATABASE
RECORD
FINDINGSDEVELOPMETRICSPREVIOUSFINDINGSPERSIST METRICS

Methodology

Cyclomatic complexity (CC):A static analysis metric used to assist in the establishment of risk and stability estimations on an item of code,such as a class, method, or even a complete system. It was defined by Thomas McCabe in the 70’s and it is easy
to calculate and apply, hence its usefulness.
The McCabe cyclomatic complexity metric is designed to indicate a program’s testability, understandability
and maintainability. This is accomplished by measuring the control flow structure, in order to predict the difficulty of understanding, testing, maintaining, etc. Once the control flow structure is understood one can gain arealization of the extent to which the program is likely to contain defects. The cyclomatic complexity metric is
intended to be independent of language and language format that measures the number of linearly independent paths through a program module. It is also the minimum number of paths that should be tested.
By knowing the cyclomatic complexity of the product, one can focus on the module with the highest complexity. This will most likely be one of the paths data will take, thus able to guide one to a potentially high risk
location for vulnerabilities. The higher the complexity the greater potential for more bugs. The more bugs thehigher the probability for more security flaws.
Does cyclomatic complexity reveal security risk? One will not know until after a review of the security postureof the module. The cyclomatic complexity metric provides a risk-based approach on where to begin to review
and analyze the code. Securing an application is a complex task and in many ways complexity an enemy ofsecurity as software complexity can make software bugs hard to detect. Complexity of software increases overtime as the product is updated or maintained.
Cyclomatic complexity can be calculated as:
CC = Number of decisions +1… where a decision would be considered as commands where execution is branched with if/else, switch, case,catch, while, do, templated class calls, etc.,
As the decision count increases, so do the complexity and the number of paths. Complex code leads to lessstability and maintainability.
The more complex the code, the higher risk of defects. A company can establish thresholds for cyclomatic
complexity for a module:
0-10: Stable code, acceptable complexity
11-15: Medium Risk, more complex
16-20: High Risk code, too many decisions for a unit of code.
Modules with a very high cyclomatic complexity are extremely complex and could be refactored into smaller
methods.
Bad Fix Probability:
This is the probability of an error accidentally inserted into a program while trying to fix a previous error,
known in some companies as a regression.
Cyclomatic Complexity: 1 – 10 == Bad Fix Probability: 5%
Cyclomatic Complexity: 20 –30 == Bad Fix Probability: 20%
Cyclomatic Complexity: > 50 == Bad Fix Probability: 40%
Cyclomatic Complexity: Approaching 100 == Bad Fix Probability: 60%
As the complexity of software increase so does the probability to introduce new errors.

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Inspection Rate:
This metric can be used to get a rough idea of the required duration to perform a code review. The inspection
rate is the rate of coverage a code reviewer can cover per unit of time. For example, a rate of 250 lines per hour
could be a baseline. This rate should not be used as part of a measure of review quality, but simply to determine duration of the task.
Defect Detection Rate:
This metric measure the defects found per unit of time. Again, can be used to measure performance of thecode review team, but not to be used as a quality measure. Defect detection rate would normally increase as
the inspection rate (above) decreases.
Re-inspection Defect Rate:
The rate at which upon re-inspection of the code more defects exist, some defects still exist, or other defects
manifest through an attempt to address previously discovered defects (regressions).
6.11 Crawling CodeCrawling code is the practice of scanning a code base of the review target and interface entry points, looking
for key code pointers wherein possible security vulnerability might reside. Certain APIs are related to interfacing to the external world or file IO or user management, which are key areas for an attacker to focus on. In
crawling code we look for APIs relating to these areas. We also need to look for business logic areas which may
cause security issues, but generally these are bespoke methods which have bespoke names and cannot be detected directly, even though we may touch on certain methods due to their relationship with a certain key API.
We also need to look for common issues relating to a specific language; issues that may not be security related
but which may affect the stability/availability of the application in the case of extraordinary circumstances.
Other issues when performing a code review are areas such a simple copyright notice in order to protect one’s
intellectual property. Generally these issues should be part of a companies Coding Guidelines (or Standard),
and should be enforceable during a code review. For example a reviewer can reject a code review because
the code violates something in the Coding Guidelines, regardless of whether or not the code would work in
its current state.
Crawling code can be done manually or in an automated fashion using automated tools. However working
manually is probably not effective, as (as can be seen below) there are plenty of indicators, which can apply toa language. Tools as simple as grep or wingrep can be used. Other tools are available which would search for
keywords relating to a specific programming language. If a team is using a particular review tool that allows
it to specify strings to be highlighted in a review (e.g. Python based review tools using pygments syntax highlighter, or an in-house tool for which the team can change the source code) then they could add the relevant
string indicators from the lists below and have them highlighted to reviewers automatically.
The basis of the code review is to locate and analyze areas of code, which may have application security implications. Assuming the code reviewer has a thorough understanding of the code, what it is intended to do, andthe context in which it is to be used, firstly one needs to sweep the code base for areas of interest.
Appendix C gives practical examples of how to carry out code crawling in the following programming languages:• .Net• Java• ASP• C++/Apache

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A1 - Injection

A1

INJECTION

7.1 Overview
What is Injection?
Injection attacks allow a malicious user to add or inject content and commands into an application in order tomodify its behaviour. These types of attacks are common, widespread, an easy for a hacker to test if a web site
is vulnerable and easy and quick for the attacker to take advantage of. Today they are very common in legacy
applications that haven’t been updated.
7.2 SQL Injection
The most common injection vulnerability is SQL injection. Injection vulnerability is also easy to remediate andprotect against. This vulnerability covers SQL, LDAP, Xpath, OS commands, XML parsers.
Injection vulnerability can lead to…
1. Disclosure/leaking of sensitive information.
2. Data integrity issues. SQL injection may modify data, add new data, or delete data.
3. Elevation of privileges.
4. Gaining access to back-end network.
SQL commands are not protected from the untrusted input. SQL parser is not able to distinguish between
code and data.

String custQuery =SELECT custName, address1 FROM cust_table WHERE custID= ‘“+ request.GetParameter(“id”) + ““

Code

Data

Using string concatenation to generate a SQL statement is very common in legacy applications where developers were not considering security. The issue is this coding technique does not tell the parser which partof the statement is code and which part is data. In situations where user input is concatenated into the SQL
statement, an attacker can modify the SQL statement by adding SQL code to the input data.
1. Untrusted input is acceptable by the application. There are several ways to mitigate injection vulnerability,
whitelisting, regex, etc. The five best ways are. All five should be used together for a defense in depth approach.
1. HtmlEncode all user input.
2. Using static analysis tools. Most static analysis for languages like .Net, Java, python are accurate. However
static analysis can become an issue when injection comes from JavaScript and CSS.
3. Parameterize SQL queries. Use SQL methods provided by the programming language or framework thatparameterize the statements, so that the SQL parser can distinguish between code and data.

A1 - Injection

4. Use Stored Procedures. Stored procedures will generally help the SQL parser differentiate code and data.
However Stored Procedures can be used to build dynamic SQL statements allowing the code and data to become blended together causing the it to become vulnerable to injection.
5. Provide developer training for best practices for secure coding.
Blind SQL Injection
Typically SQL queries return search results that are presented to a user. However, there are cases where SQL
queries are happening behind the scenes that influence how the page is rendered, and unfortunately attackers can still glean information based on the error responses from various UI elements. Blind SQL injection is a
type of attack that asks the database true or false questions and determines the answer based on the applications response.
Effectively the attacker uses SQL queries to determine what error responses are returned for valid SQL, andwhich responses are returned for invalid SQL. Then the attacker can probe; for example check if a table called“user_password_table” exists. Once they have that information, they could use an attack like the one describedabove to maliciously delete the table, or attempt to return information from the table (does the username“john” exist?). Blind SQL injections can also use timings instead of error messages, e.g. if invalid SQL takes 2
seconds to respond, but valid SQL returns in 0.5 seconds, the attacker can use this information.
Parameterized SQL Queries
Parameterized SQL queries (sometimes called prepared statements) allow the SQL query string to be definedin such a way that the client input can’t be treated as part of the SQL syntax.
Take the example in sample 7.1:
Sample 7.11234String query = “SELECT id, firstname, lastname FROM authors WHERE forename = ? and surname = ?”;PreparedStatement pstmt = connection.prepareStatement( query );pstmt.setString( 1, firstname );pstmt.setString( 2, lastname );In this example the string ‘query’ is constructed in a way that it does not rely on any client input, and the‘PreparedStatement’ is constructed from that string. When the client input is to be entered into the SQl, the‘setString’ function is used and the first question mark “?” is replaced by the string value of ‘firstname’, the second question mark is replaced by the value of ‘lastname’. When the ‘setString’ function is called, this function
automatically checks that no SQL syntax is contained within the string value. Most prepared statement APIs
allow you to specify the type that should be entered, e.g. ‘setInt’, or ‘setBinary’, etc.
Safe String Concatenation?
So does this mean you can’t use string concatenation at all in your DB handling code? It is possible to use string
concatenation safely, but it does increase the risk of an error, even without an attacker attempting to inject
SQL syntax into your application.
You should never use string concatenation in combination with the client input value. Take an example where
the existence (not the value) of a client input variable “surname” is used to construct the SQL query of the
prepared statement;

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Sample 7.2
1 String query = “Select id, firstname, lastname FROM authors WHERE forename = ?”;2 if (lastname!= NULL && lastname.length != 0) {3 query += “ and surname = ?”;4 }5 query += “;”;6
7
PreparedStatement pstmt = connection.prepareStatement( query );8
pstmt.setString( 1, firstname);910
if (lastname!= NULL && lastname.length != 0) { pstmt.setString( 2, lastname ); }Here the value of ‘lastname’ is not being used, but the existance of it is being evaluated. However there is still a risk
when the SQL statement is larger and has more complex business logic involved in creating it. Take the following
example where the function will search based on firstname or lastname:
Sample 7.3
1 String query = “select id, firstname, lastname FROM authors”;
2
3 if (( firstname != NULL && firstname.length != 0 ) && ( lastname != NULL && lastname.length != 0 )) {4 query += “WHERE forename = ? AND surname = ?”;5}
6 else if ( firstname != NULL && firstname.length != 0 ) {7 query += “WHERE forename = ?”;8}
9 else if ( lastname!= NULL && lastname.length != 0 ) {10 query += “WHERE surname = ?”;11 }1213 query += “;”;
14
15 PreparedStatement pstmt = connection.prepareStatement( query )This logic will be fine when either firstname, or lastname is given, however if neither were given then the SQL statement would not have any WHERE clause, and the entire table would be returned. This is not an SQL injection (theattacker has done nothing to cause this situation, except not passing two values) however the end result is the same,
information has been leaked from the database, despite the fact that a parameterized query was used.
For this reason, the advice is to avoid using string concatenation to create SQL query strings, even when using parameterized queries, especially if the concatenation involves building any items in the where clause.
Using Flexible Parameterized Statements
Functional requirements often need the SQL query being executed to be flexible based on the user input, e.g. if theend user specifies a time span for their transaction search then this should be used, or they might wish to query basedon either surname or forename, or both. In this case the safe string concatenation above could be used, however from

A1 - Injection

a maintenance point of view this could invite future programmers to misunderstand the difference between safe
concatenation and the unsafe version (using input string values directly).
One option for flexible parameterized statements is to use ‘if’ statements to select the correct query based on the
input values provided, for example:
Sample 7.4
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22

String query;PreparedStatement pstmt;
if ( (firstname!= NULL && firstname.length != 0) &&lastname!= NULL && lastname.length != 0) ) {query = “Select id, firstname, lastname FROM authors WHERE forename = ? and surname = ?”pstmt = connection.prepareStatement( query );pstmt.setString( 1, firstname );pstmt.setString( 2, lastname );}else if (firstname != NULL && firstname.length != 0) {query = “Select id, firstname, lastname FROM authors WHERE forename = ?”;pstmt = connection.prepareStatement( query );pstmt.setString( 1, firstname );}else if (lastname != NULL && lastname.length != 0){
query = “Select id, firstname, lastname FROM authors WHERE surname= ?”;pstmt = connection.prepareStatement( query );pstmt.setString( 1, lastname);}
else{
throw NameNotSpecifiedException(); }PHP SQL Injection
An SQL injection attack consists of injecting SQL query portions in the back-end database system via the clientinterface in the web application. The consequence of a successful exploitation of an SQL injection varies fromjust reading data to modifying data or executing system commands. SQL Injection in PHP remains the number
one attack vector, and also the number one reason for data compromises as shown in sample 7.5.
Example 1 :Sample 7.5
1 <?php
1 $pass=$_GET[“pass”];
2 $con = mysql_connect(‘localhost’, ‘owasp’, ‘abc123’);
3 mysql_select_db(“owasp_php”, $con);
4 $sql=”SELECT card FROM users WHERE password = ‘”.$pass.”’”;
5 $result = mysql_query($sql);
6 ?>

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A1 - Injection

The most common ways to prevent SQL Injection in PHP are using functions such as addslashes() and mysql_
real_escape_string() but those function can always cause SQL Injections in some cases.
Addslashes :You will avoid Sql injection using addslashes() only in the case when you wrap the query string with quotes.
The following example would still be vulnerable
Sample 7.6
12$id = addslashes( $_GET[‘id’] );$query = ‘SELECT title FROM books WHERE id = ‘ . $id;

mysql_real_escape_string():
mysql_real_escape_string() is a little bit more powerful than addslashes() as it calls MySQL’s library function
mysql_real_escape_string, which prepends backslashes to the following characters: \x00, \n, \r, \, ‘, “ and \x1a.
As with addslashes(), mysql_real_escape_string() will only work if the query string is wrapped in quotes. A
string such as the following would still be vulnerable to an SQL injection:
SQL injections occur when input to a web application is not controlled or sanitized before executing to theback-end database.
The attacker tries to exploit this vulnerability by passing SQL commands in her/his input and therefore will
create a undesired response from the database such as providing information that bypasses the authorization
and authentication programmed in the web application. An example of vulnerable java code is shown insample 7.7
Sample 7.7
1 HttpServletRequest request = ...;
2 String userName = request.getParameter(“name”);
3 Connection con = ...
4 String query = “SELECT * FROM Users WHERE name = ‘” + userName + “’”;
5 con.execute(query);An example of a vulnerable java code
The input parameter “name” is passed to the String query without any proper validation or verification. The
query ‘SELECT* FROM users where name” is equal to the string ‘username’ can be easily misused to bypass
something different that just the ‘name’. For example, the attacker can attempt to pass instead in this way accessing all user records and not only the one entitled to the specific user
“ OR 1=1.

A1 - Injection.NET Sql Injection
Framework 1.0 & 2.0 might be more vulnerable to SQL injections than the later versions of .NET. Thanks to theproper implementation and use of design patters already embedded in ASP.NET such as MVC(also dependingon the version), it is possible to create applications free from SQL injections, however, there might be times
where a developer might prefer to use SQL code directly in the code.
Example.
A developer creates a webpage with 3 fields and submit button, to search for employees on fields ‘name’,‘lastname’ and ‘id’
The developer implements a string concatenated SQL statement or stored procedure in the code such as insample 7.8.
Sample 7.8
1 SqlDataAdapter thisCommand = new SqlDataAdapter(2 “SELECT name, lastname FROM employees WHERE ei_id = ‘” +idNumber.Text + “’”, thisConnection);

This code is equivalent to the executed SQL statement in sample 7.9.
Sample 7.9
1 SqlDataAdapter thisCommand = new SqlDataAdapter(2 “SearchEmployeeSP ‘” + idNumber.Text + “’”, thisConnection);

A hacker can then insert the following employee ID via the web interface “123’;DROP TABLE pubs --” and execute the following code:
SELECT name, lastname FROM authors WHERE ei_id = ‘123’; DROP TABLE pubs --’The semicolon “;” provides SQL with a signal that it has reached the end of the sql statement, however, thehacker uses this to continue the statement with the malicious SQL code
; DROP TABLE pubs;
Parameter collections
Parameter collections such as SqlParameterCollection provide type checking and length validation. If you use
a parameters collection, input is treated as a literal value, and SQL Server does not treat it as executable code,and therefore the payload can not be injected.
Using a parameters collection lets you enforce type and length checks. Values outside of the range trigger an
exception. Make sure you handle the exception correctly. Example of the SqlParameterCollection:
Hibernate Query Language (HQL)

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Sample 7.10
1 using (SqlConnection conn = new SqlConnection(connectionString)) {2 DataSet dataObj = new DataSet();3 SqlDataAdapter sqlAdapter = new SqlDataAdapter( “StoredProc”, conn); sqlAdapter.SelectCommand.
CommandType =4 CommandType.StoredProcedure;
5 sqlAdapter.SelectCommand.Parameters.Add(“@usrId”, SqlDbType.VarChar, 15);
6 sqlAdapter.SelectCommand.Parameters[“@usrId “].Value = UID.Text;
Hibernate facilitates the storage and retrieval of Java domain objects via Object/Relational Mapping (ORM).
It is a very common misconception that ORM solutions, like hibernate, are SQL Injection proof. Hibernate allows theuse of “native SQL” and defines a proprietary query language, called HQL (Hibernate Query Language); the former isprone to SQL Injection and the later is prone to HQL (or ORM) injection.
What to Review• Always validate user input by testing type, length, format, and range.
• Test the size and data type of input and enforce appropriate limits.
• Test the content of string variables and accept only expected values. Reject entries that contain binary data, escapesequences, and comment characters.
• When you are working with XML documents, validate all data against its schema as it is entered.
• Never build SQL statements directly from user input.
• Use stored procedures to validate user input, when not using stored procedures use SQL API provided by platform.
i.e. Parameterized Statements.
• Implement multiple layers of validation.
• Never concatenate user input that is not validated. String concatenation is the primary point of entry for script
injection.
You should review all code that calls EXECUTE, EXEC, any SQL calls that can call outside resources or command line.
OWASP References• https://www.owasp.org/index.php/SQL_Injection_Prevention_Cheat_Sheet OWASP SQL Injection Prevention
Cheat Sheet• https://www.owasp.org/index.php/Query_Parameterization_Cheat_Sheet OWASP Query Parameterization
Cheat Sheet• https://www.owasp.org/index.php/Command_Injection OWASP Command Injection Article• https://www.owasp.org/index.php/XXE OWASP XML eXternal Entity (XXE) Reference Article

A1 - Injection• https://www.owasp.org/index.php/ASVS ASVS: Output Encoding/Escaping Requirements (V6)• https://www.owasp.org/index.php/Testing_for_SQL_Injection_(OWASP-DV-005) OWASP Testing Guide: Chapter on SQL Injection TestingExternal References• http://cwe.mitre.org/data/definitions/77.html CWE Entry 77 on Command Injection• http://cwe.mitre.org/data/definitions/89.html CWE Entry 89 on SQL Injection• http://cwe.mitre.org/data/definitions/564.html CWE Entry 564 on Hibernate Injection• Livshits and Lam, 2005 “Finding Security Vulnerabilities in Java Applications with Static Analysis” available athttps://www.usenix.org/legacy/event/sec05/tech/full_papers/livshits/livshits_html/#sec:sqlinjexample• http://www.php.net/manual/en/book.pdo.php PDO• https://technet.microsoft.com/en-us/library/ms161953(v=sql.105).aspx
7.3 JSON (JavaScript Object Notation)
JSON (JavaScript Object Notation) is an open standard format that uses easy to read text to transmit data between a server and web applications. JSON data can be used by a large number of programming Languages
and is becoming the de-facto standard in replacing XML.
JSON main security concern is JSON text dynamically embedded in JavaScript, because of this injection is a very realvulnerability. The vulnerability in the program that may inadvertently to run a malicious script or store the malicious
script to a database. This is a very real possibility when dealing with data retrieved from the Internet.
The code reviewer needs to make sure the JSON is not used with Javascript eval. Make sure JSON.parse(…) is used.
Var parsed_object = eval(“(“ + Jason_text + “)”); // Red flag for the code reviewer.
JSON.parse(text[, reviver]); .. // Much better then using javascript eval function.
Code reviewer should check to make sure the developer is not attempting to reject known bad patterns in text/
string data, Using regex or other devices is fraught with error and makes testing for correctness very hard. Allow only
whitelisted alphanumeric keywords and carefully validated numbers.
Do not allow JSON data to construct dynamic HTML. Always us safe DOM features like innerText or CreateTextNode(…)
Object/Relational Mapping (ORM)
Object/Relation Mapping (ORM) facilitates the storage and retrieval of domain objects via HQL (Hibernate Query
Language) or .NET Entity framework.
It is a very common misconception that ORM solutions, like hibernate are SQL Injection proof. They are not. ORM’s
allow the use of “native SQL”. Thru proprietary query language, called HQL is prone to SQL Injection and the later isprone to HQL (or ORM) injection. Linq is not SQL and because of that is not prone to SQL injection. However using
excutequery or excutecommand via linq causes the program not to use linq protection mechanism and is vulnerability to SQL injection.

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A1 - Injection

Bad Java Code Examples
List results = session.createQuery(“from Items as item where item.id = “ + currentItem.getId()).list();NHibernate is the same as Hibernate except it is an ORM solution for Microsoft .NET platform. NHibernate is also vulnerable to SQL injection if used my dynamic queries.
Bad .Net Code Example
string userName = ctx.GetAuthenticatedUserName();String query = “SELECT * FROM Items WHERE owner = ‘”+ userName + “’ AND itemname = ‘”+ ItemName.Text + “’”;
List items = sess.CreateSQLQuery(query).List()

Code Reviewer Action
Code reviewer needs to make sure any data used in an HQL query uses HQL parameterized queries so that it
would be used as data and not as code. They can also use the Criteria API at https://docs.jboss.org/hibernate/orm/3.3/reference/en/html/querycriteria.html
7.5 Content Security Policy (CSP)
Is a W3C specification offering the possibility to instruct the client browser from which location and/or which
type of resources are allowed to be loaded. To define a loading behavior, the CSP specification use “directive”
where a directive defines a loading behavior for a target resource type. CSP helps to detect and mitigate certain types of attacks, including Cross Site Scripting (XSS) and data injection attacks. These attacks are used foreverything from data theft to site defacement or distribution of malware
Directives can be specified using HTTP response header (a server may send more than one CSP HTTP header
field with a given resource representation and a server may send different CSP header field values with different representations of the same resource or with different resources) or HTML Meta tag, the HTTP headers
below are defined by the specs:• Content-Security-Policy : Defined by W3C Specs as standard header, used by Chrome version 25 and later,
Firefox version 23 and later, Opera version 19 and later.
• X-Content-Security-Policy : Used by Firefox until version 23, and Internet Explorer version 10 (which partially
implements Content Security Policy).
• X-WebKit-CSP : Used by Chrome until version 25
Risk
The risk with CSP can have 2 main sources:• Policies misconfiguration,• Too permissive policies.
What to Review
Code reviewer needs to understand what content security policies were required by application design andhow these policies are tested to ensure they are in use by the application.

A1 - Injection

Useful security-related HTTP headers
In most architectures these headers can be set in web servers configuration without changing actual application’s code. This offers significantly faster and cheaper method for at least partial mitigation of existing issues,
and an additional layer of defense for new applications.
Table 11: Security Related HTTP Headers
Header name

Description

ExampleStrict-Transport-Security
https://tools.ietf.org/
html/rfc6797

HTTP Strict-Transport-Security (HSTS) enforces secure (HTTP over SSL/TLS) connections
to the server. This reduces impact of bugs in web applications leaking session data
through cookies and external links and defends against Man-in-the-middle attacks.
HSTS also disables the ability for user’s to ignore SSL negotiation warnings.

Strict-Transport-Security:
max-age=16070400;
includeSubDomains

X-Frame-Options
https://tools.ietf.org/
html/draft-ietf-websec-xframe-options-01

Provides Click jacking protection. Values: deny - no rendering within a frame, sameorigin- no rendering if origin mismatch, allow-from: DOMAIN - allow rendering if framed byframe loaded from DOMAIN

X-Frame-Options: denyThis header enables the Cross-site scripting (XSS) filter built into most recent web browsers. It’s usually enabled by default anyway, so the role of this header is to re-enable thefilter for this particular website if it was disabled by the user. This header is supported in
IE 8+, and in Chrome (not sure which versions). The anti-XSS filter was added in Chrome
4. Its unknown if that version honored this header.

X-XSS-Protection: 1;
mode=block

The only defined value, “nosniff”, prevents Internet Explorer and Google Chrome from
MIME-sniffing a response away from the declared content-type. This also applies to Google Chrome, when downloading extensions. This reduces exposure to drive-by download attacks and sites serving user uploaded content that, by clever naming, could betreated by MSIE as executable or dynamic HTML files.

X-Content-Type-Options:nosniff

Content Security Policy requires careful tuning and precise definition of the policy. If
enabled, CSP has significant impact on the way browser renders pages (e.g., inline JavaScript disabled by default and must be explicitly allowed in policy). CSP prevents a wide
range of attacks, including Cross-site scripting and other cross-site injections.

Content-Security-Policy:
default-src ‘self’

Like Content-Security-Policy, but only reports. Useful during implementation, tuning
and testing efforts.

Content-Security-Policy-Report-Only: default-src ‘self’; report-uri
http://loghost.example.
com/reports.jsp

Frame-Options
https://tools.ietf.org/
html/draft-ietf-websecframe-options-00
X-XSS-Protection[http://blogs.
msdn.com/b/ie/archive/2008/07/02/
ie8-security-part-iv-thexss-filter.aspx X-XSS-Protection]

X-Content-Type-Options
https://blogs.msdn.
microsoft.com/
ie/2008/09/02/ie8-security-part-vi-beta-2-update/Content-Security-Policy,
X-Content-Security-policy,X-WebKit-CSP
https://www.w3.org/TR/
CSP/
Content-Security-Policy-Report_Only
https://www.w3.org/TR/
CSP/

Note the Spring Security library can assist with these headers, see http://docs.spring.io/spring-security/site/
docs/current/reference/html/headers.html

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References
Apache: http://httpd.apache.org/docs/2.0/mod/mod_headers.html
IIS: http://technet.microsoft.com/pl-pl/library/cc753133(v=ws.10).aspx
7.6 Input Validation
Input validation is one of the most effective technical controls for application security. It can mitigate numerous vulnerabilities including cross-site scripting, various forms of injection, and some buffer overflows. Input
validation is more than checking form field values.
All data from users needs to be considered untrusted. Remember one of the top rules of secure coding is“Don’t trust user input”. Always validate user data with the full knowledge of what your application is trying
to accomplish.
Regular expressions can be used to validate user input, but the more complicated the regular express are themore chance it is not full proof and has errors for corner cases. Regular expressions are also very hard fro QA
to test. Regular expressions may also make it hard for the code reviewer to do a good review of the regularexpressions.
Data Validation
All external input to the system (and between systems/applications) should undergo input validation. The
validation rules are defined by the business requirements for the application. If possible, an exact match validator should be implemented. Exact match only permits data that conforms to an expected value. A “Knowngood” approach (white-list), which is a little weaker, but more flexible, is common. Known good only permits
characters/ASCII ranges defined within a white-list.
Such a range is defined by the business requirements of the input field. The other approaches to data validation are “known bad,” which is a black list of “bad characters”. This approach is not future proof and would needmaintenance. “Encode bad” would be very weak, as it would simply encode characters considered “bad” to aformat, which should not affect the functionality of the application.
Business Validation
Business validation is concerned with business logic. An understanding of the business logic is required prior
to reviewing the code, which performs such logic. Business validation could be used to limit the value range
or a transaction inputted by a user or reject input, which does not make too much business sense. Reviewing
code for business validation can also include rounding errors or floating point issues which may give rise toissues such as integer overflows, which can dramatically damage the bottom line.
Canonicalization
Canonicalization is the process by which various equivalent forms of a name can be resolved to a single standard name, or the “canonical” name.
The most popular encodings are UTF-8, UTF-16, and so on (which are described in detail in RFC 2279). A single
character, such as a period/full-stop (.), may be represented in many different ways: ASCII 2E, Unicode C0 AE,
and many others.
With the myriad ways of encoding user input, a web application’s filters can be easily circumvented if they’re
not carefully built.

A1 - Injection

Bad Example
Sample 7.11
public static void main(String[] args) {File x = new File(“/cmd/” + args[1]);String absPath = x.getAbsolutePath();}Good Example
Sample 7.12
public static void main(String[] args) throws IOException {File x = new File(“/cmd/” + args[1]);String canonicalPath = x.getCanonicalPath();.NET Request Validation
One solution is to use .Net “Request Validation”. Using request validation is a good start on validating user
data and is useful. The downside is too generic and not specific enough to meet all of our requirements toprovide full trust of user data.
You can never use request validation for securing your application against cross-site scripting attacks.
The following example shows how to use a static method in the Uri class to determine whether the Uri provided by a user is valid.
var isValidUri = Uri.IsWellFormedUriString(passedUri, UriKind.Absolute);
However, to sufficiently verify the Uri, you should also check to make sure it specifies http or https. The following example uses instance methods to verify that the Uri is valid.
var uriToVerify = new Uri(passedUri);
var isValidUri = uriToVerify.IsWellFormedOriginalString();var isValidScheme = uriToVerify.Scheme == “http” || uriToVerify.Scheme == “https”;
Before rendering user input as HTML or including user input in a SQL query, encode the values to ensure malicious code is not included.
You can HTML encode the value in markup with the <%: %> syntax, as shown below.
<span><%: userInput %></span>Or, in Razor syntax, you can HTML encode with @, as shown below.

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A1 - Injection<span>@userInput</span>
The next example shows how to HTML encode a value in code-behind.
var encodedInput = Server.HtmlEncode(userInput);
Managed Code and Non-Managed CodeBoth Java and .Net have the concept of managed and non-managed code. To offer some of these protections
during the invocation of native code, do not declare a native method public. Instead, declare it private andexpose the functionality through a public wrapper method. A wrapper can safely perform any necessary input
validation prior to the invocation of the native method:Java Sample code to call a Native Method with Data Validation in place
Sample 7.13
public final class NativeMethodWrapper {private native void nativeOperation(byte[] data, int offset, int len);public void doOperation(byte[] data, int offset, int len) {// copy mutable input
data = data.clone();// validate input// Note offset+len would be subject to integer overflow.
// For instance if offset = 1 and len = Integer.MAX_VALUE,// then offset+len == Integer.MIN_VALUE which is lower// than data.length.
// Further,// loops of the form// for (int i=offset; i<offset+len; ++i) { ... }// would not throw an exception or cause native code to// crash.
if (offset < 0 || len < 0 || offset > data.length - len) {throw new IllegalArgumentException();}nativeOperation(data, offset, len);}}

Data validations checklist for the Code Reviewer.
• Ensure that a Data Validation mechanism is present.

A1 - Injection• Ensure all input that can (and will) be modified by a malicious user such as HTTP headers, input fields, hidden
fields, drop down lists, and other web components are properly validated.
• Ensure that the proper length checks on all input exist.
• Ensure that all fields, cookies, http headers/bodies, and form fields are validated.
• Ensure that the data is well formed and contains only known good chars if possible.
• Ensure that the data validation occurs on the server side.
• Examine where data validation occurs and if a centralized model or decentralized model is used.
• Ensure there are no backdoors in the data validation model.
• “Golden Rule: All external input, no matter what it is, will be examined and validated.”
Resources:
http://msdn.microsoft.com/en-us/library/vstudio/system.uri

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A2

A2 - Broken Authentication and Session Management

A2

BROKEN AUTHENTICATION AND
SESSION MANAGEMENT

8.1 Overview
Web applications and Web services both use authentication as the primary means of access control from logins via user id and passwords. This control is essential to the prevention of confidential files, data, or web pages
from being access by hackers or users who do not have the necessary access control level.
8.2 Description
Authentication is important, as it is the gateway to the functionality you are wishing to protect. Once a user
is authenticated their requests will be authorized to perform some level of interaction with your applicationthat non-authenticated users will be barred from. You cannot control how users manage their authentication
information or tokens, but you can ensure there is now way to perform application functions without proper
authentication occurring.
There are many forms of authentication with passwords being the most common. Other forms include clientcertificates, biometrics, one time passwords over SMS or special devices, or authentication frameworks such asOpen Authorization (OAUTH) or Single Sign On (SSO).
Typically authentication is done once, when the user logs into a website, and successful authentication results
in a web session being setup for the user (see Session Management). Further (and stronger) authentication
can be subsequently requested if the user attempts to perform a high risk function, for example a bank user
could be asked to confirm an 6 digit number that was sent to their registered phone number before allowing
money to be transferred.
Authentication is just as important within a companies firewall as outside it. Attackers should not be able torun free on a companies internal applications simply because they found a way in through a firewall. Also
separation of privilege (or duties) means someone working in accounts should not be able to modify code ina repository, or application managers should not be able to edit the payroll spreadsheets.
8.3 What to Review
When reviewing code modules which perform authentication functions, some common issues to look out forinclude:• Ensure the login page is only available over TLS. Some sites leave the login page has HTTP, but make the form
submission URL HTTPS so that the users username and password are encrypted when sent to the server. However if the login page is not secured, a risk exists for a man-in-the-middle to modify the form submission URL
to an HTTP URL, and when the user enters their username & password they are sent in the clear.
• Make sure your usernames/user-ids are case insensitive. Many sites use email addresses for usernames andemail addresses are already case insensitive. Regardless, it would be very strange for user ‘smith’ and user
‘Smith’ to be different users. Could result in serious confusion.
• Ensure failure messages for invalid usernames or passwords do not leak information. If the error message
indicates the username was valid, but the password was wrong, then attackers will know that username exists.
If the password was wrong, do not indicate how it was wrong.
• Make sure that every character the user types in is actually included in the password.

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A2 - Broken Authentication and Session Management• Do not log invalid passwords. Many times an e-mail address is used as the username, and those users will
have a few passwords memorized but may forget which one they used on your web site. The first time they
may use a password that in invalid for your site, but valid for many other sites that this user (identified by theusername). If you log that username and password combination, and that log leaks out, this low level compromise on your site could negatively affect many other sites.
• Longer passwords provide a greater combination of characters and consequently make it more difficult for
an attacker to guess. Minimum length of the passwords should be enforced by the application. Passwords
shorter than 10 characters are considered to be weak. Passphrases should be encouraged. For more on password lengths see the OWASP Authentication Cheat Sheet.
• To prevent brute force attacks, implement temporary account lockouts or rate limit login responses. If a user
fails to provide the correct username and password 5 times, then lock the account for X minutes, or implement
logic where login responses take an extra 10 seconds. Be careful though, this could leak the fact that the username is valid to attackers continually trying random usernames, so as an extra measure, consider implementing the same logic for invalid usernames.
• For internal systems, consider forcing the users to change passwords after a set period of time, and store areference (e.g. hash) of the last 5 or more passwords to ensure the user is not simply re-using their old password.
• Password complexity should be enforced by making users choose password strings that include various type ofcharacters (e.g. upper- and lower-case letters, numbers, punctuation, etc.). Ideally, the application would indicate to
the user as they type in their new password how much of the complexity policy their new password meets. For moreon password complexity see the OWASP Authentication Cheat Sheet.
• It is common for an application to have a mechanism that provides a means for a user to gain access to theiraccount in the event they forget their password. This is an example of web site functionality this is invoked by unauthenticated users (as they have not provided their password). Ensure interfaces like this are protected from misuse, if
asking for password reminder results in an e-mail or SMS being sent to the registered user, do not allow the password
reset feature to be used to spam the user by attackers constantly entering the username into this form. Please see
Forgot Password Cheat Sheet for details on this feature.
• It is critical for an application to store a password using the right cryptographic technique. Please see
Password Storage Cheat Sheet for details on this feature.
When reviewing MVC .NET is is important to make sure the application transmits and received over a secure link. It is
recommended to have all web pages not just login pages use SSL.
We need to protect session cookies, which are useful as users credentials.
Sample 8.1
public static void RegisterGlobalFilters(GlobalFilterCollection filters) {“ flters.Add(new RequireHttpsAttribute()); “}In the global.asax file we can review the RegisterGlobalFilters method.

A2 - Broken Authentication and Session Management

The attribute RequireHttpsAttribute() can be used to make sure the application runs over SSL/TLS
It is recommended that this be enabled for SSL/TLS sites.
• For high risk functions, e.g. banking transactions, user profile updates, etc., utilize multi-factor authentication(MFA). This also mitigates against CSRF and session hijacking attacks. MFA is using more than one authentication
factor to logon or process a transaction:• Something you know (account details or passwords)• Something you have (tokens or mobile phones)• Something you are (biometrics)• If the client machine is in a controlled environment utilize SSL Client Authentication, also known as two-way
SSL authentication, which consists of both browser and server sending their respective SSL certificates during the TLS
handshake process. This provides stronger authentication as the server administrator can create and issue client certificates, allowing the server to only trust login requests from machines that have this client SSL certificate installed.
Secure transmission of the client certificate is important.
References• https://www.owasp.org/index.php/Authentication_Cheat_Sheet• http://csrc.nist.gov/publications/nistpubs/800-132/nist-sp800-132.pdf• http://www.codeproject.com/Articles/326574/An-Introduction-to-Mutual-SSL-Authentication• https://cwe.mitre.org/data/definitions/287.html Improper Authentication• OWASP ASVS requirements areas for Authentication (V2)
8.4 Forgot Password
Overview
If your web site needs to have user authentication then most likely it will require user name and password toauthenticate user accesses. However as computer system have increased in complexity, so has authenticating
users has also increased. As a result the code reviewer needs to be aware of the benefits and drawbacks of user
authentication referred to as “Direct Authentication” pattern in this section. This section is going to emphasise
design patterns for when users forget user id and or password and what the code reviewer needs to consider
when reviewing how user id and passwords can be retrieved when forgotten by the user and how to do thisin a secure manner.
General considerations
Notify user by (phone sms, email) such that the user has to click a link in the email that takes them to your site
and ask the user to enter a new password.
Ask user to enter login credentials they already have (Facebook, Twitter, Google, Microsoft Live, OpenID etc) tovalidate user before allowing user to change password.
Send notification to user to confrm registration or forgot password usage.
Send notifications that account information has been changed for registered email. Set appropriate time out
value. I.e. If user does not respond to email within 48 hours then user will be frozen out of system until user
re-affirms password change.
• The identity and shared secret/password must be transferred using encryption to provide data confidentiality.

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A2 - Broken Authentication and Session Management• A shared secret can never be stored in clear text format, even if only for a short time in a message queue.
• A shared secret must always be stored in hashed or encrypted format in a database.
• The organization storing the encrypted shared secret does not need the ability to view or decrypt users
passwords. User password must never be sent back to a user.
• If the client must cache the username and password for presentation for subsequent calls to a Web service
then a secure cache mechanism needs to be in place to protect user name and password.
• When reporting an invalid entry back to a user, the username and or password should not be identified as being invalid. User feed back/error message must consider both user name and password as one item “user credential”.
I.e. “The username or password you entered is incorrect.”
8.5 CAPTCHA
Overview
CAPTCHA (an acronym for “Completely Automated Public Turing test to tell Computers and Humans Apart”.) isan access control technique.
CAPTCHA is used to prevent automated software from gaining access to webmail services like Gmail, Hotmail
and Yahoo to create e-mail spam, automated postings to blogs, forums and wikis for the purpose of promotion(commercial, and or political) or harassment and vandalism and automated account creation.
CAPTCHA’s have proved useful and their use has been upheld in court. Circumventing CAPTCHA has been
upheld in US Courts as a violation Digital Millennium Copyright Act anti-circumvention section 1201(a)(3) andEuropean Directive 2001/29/EC.
General considerations
Code review of CAPTCHA’s the reviewer needs to pay attention to the following rules to make sure the CAPTCHA is built with strong security principals.
• Do not allow the user to enter multiple guesses after an incorrect attempt.
• The software designer and code review need to understand the statics of guessing. I.e. One CAPTCHA
design shows four (3 cats and 1 boat) pictures, User is requested to pick the picture where it is not in the same category of the other pictures. Automated software will have a success rate of 25% by always picking the first picture.
Second depending on the fixed pool of CAPTCHA images over time an attacker can create a database of correct
answers then gain 100% access.
• Consider using a key being passed to the server that uses a HMAC (Hash-based message authenticationcode) the answer.
Text base CAPTCHA’s should adhere to the following security design principals...
1. Randomize the CAPTCHA length: Don’t use a fixed length; it gives too much information to the attacker.
2. Randomize the character size: Make sure the attacker can’t make educated guesses by using several fontsizes / several fonts.
3. Wave the CAPTCHA: Waving the CAPTCHA increases the difficulty for the attacker.
4. Don’t use a complex charset: Using a large charset does not improve significantly the CAPTCHA scheme’sA2 - Broken Authentication and Session Management

security and really hurts human accuracy.
5. Use anti-recognition techniques as a means of strengthening CAPTCHA security: Rotation, scaling androtating some characters and using various font sizes will reduce the recognition efficiency and increase security by making character width less predictable.
6. Keep the line within the CAPTCHAs: Lines must cross only some of the CAPTCHA letters, so that it is
impossible to tell whether it is a line or a character segment.
7. Use large lines: Using lines that are not as wide as the character segments gives an attacker a robust
discriminator and makes the line anti-segmentation technique vulnerable to many attack techniques.
8. CAPTCHA does create issues for web sites that must be ADA (Americans with Disabilities Act of 1990)
compliant. Code reviewer may need to be aware of web accessibilities and security to review the CAPTCHA
implementation where web site is required to be ADA complaint by law.
References• UNITED STATES of AMERICA vs KENNETH LOWSON, KRISTOFER KIRSCH, LOEL STEVENSON Federal Indictment.
February 23, 2010. Retrieved 2012-01-02.
• http://www.google.com/recaptcha/captcha• http://www.ada.gov/anprm2010/web%20anprm_2010.htm• Inaccessibility of CAPTCHA - Alternatives to Visual Turing Tests on the Web http://www.w3.org/TR/turingtest/
8.6 Out-of-Band Communication
Overview
The term ‘out-of-band’ is commonly used when an web application communicates with an end user over achannel separate to the HTTP request/responses conducted through the users’ web browser. Common examples include text/SMS, phone calls, e-mail and regular mail.
Description
The main reason an application would wish to communicate with the end user via these separate channels
is for security. A username and password combination could be sufficient authentication to allow a user tobrowse and use non-sensitive parts of a website, however more sensitive (or risky) functions could requirea stronger form of authentication. A username and password could have been stolen through an infectedcomputer, through social engineering, database leak or other attacks, meaning the web application cannot
put too much in trust a web session providing the valid username and password combination is actually theintended user.
Examples of sensitive operations could include:• Changing password• Changing account details, such as e-mail address, home address, etc• Transferring funds in a banking application• Submitting, modifying or cancelling orders
In these cases many applications will communicate with you via a channel other than a browsing session.
Many large on-line stores will send you confirmation e-mails when you change account details, or purchaseitems. This protects in the case that an attacker has the username and password, if they buy something, thelegitimate user will get an e-mail and have a chance to cancel the order or alert the website that they did notmodify the account.

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When out-of-band techniques are performed for authentication it is termed two-factor authentication. There
are three ways to authenticate:
1. Something you know (e.g. password, passphrase, memorized PIN)
2. Something you have (e.g. mobile phone, cash card, RSA token)
3. Something you are (e.g. iris scan, fingerprint)
If a banking website allows users to initiate transactions online, it could use two-factor authentication by taking 1) the password used to log in and 2) sending an PIN number over SMS to the users registered phone, andthen requiring the user enter the PIN before completing the transaction. This requires something the user
knows (password) and has (phone to receive the PIN).
A ‘chip-and-pin’ banking card will use two-factor authentication, by requiring users to have the card with them(something they have) and also enter a PIN when performing a purchase (something they know). A ‘chip-andpin’ card is not use within the PIN number, likewise knowing the PIN number is useless if you do not have thecard.
What to Review
When reviewing code modules which perform out-of-band functions, some common issues to look out forinclude:1. Recognize the risk of the system being abused. Attackers would like to flood someone with SMS messages
from your site, or e-mails to random people. Ensure:
2. When possible, only authenticated users can access links that cause an out-of-band feature to be invoked(forgot password being an exception).
3. Rate limit the interface, thus users with infected machines, or hacked accounts, can’t use it to flood
out-of-band messages to a user.
4. Do not allow the feature to accept the destination from the user, only use registered phone numbers,
e-mails, addresses.
5. For high risk sites (e.g. banking) the users phone number can be registered in person rather than via theweb site.
6. Do not send any personal or authentication information in the out-of-band communication.
7. Ensure any PINs or passwords send over out-of-band channels have a short life-span and are random.
8. A consideration can be to prevent SMS messages being sent to the device currently conducting thebrowsing session (i.e. user browsing on their iPhone, were the SMS is sent to). However this can be hard to enforce.
If possible give users the choice of band they wish to use. For banking sites Zitmo malware on mobile
devices (see references) can intercept the SMS messages, however iOS devices have not been affected by this malware yet, so users could choose to have SMS PINs sent to their Apple devices, and when on Android they could use
recorded voice messages to receive the PIN.
9. In a typical deployments specialized hardware/software separate from the web application will handle theout-of-band communication, including the creation of temporary PINs and possibly passwords. In this scenario there
is no need to expose the PIN/password to your web application (increasing risk of exposure), instead the web application should query the specialized hardware/software with the PIN/password supplied by the end user, and receiveA2 - Broken Authentication and Session Management

a positive or negative response.
Many sectors including the banking sector have regulations requiring the use of two-factor authentication for certain
types of transactions. In other cases two-factor authentication can reduce costs due to fraud and re-assure customersof the security of a website.
References• https://www.owasp.org/index.php/Forgot_Password_Cheat_Sheet• http://securelist.com/blog/virus-watch/57860/new-zitmo-for-android-and-blackberry/
8.7 Session Management
Overview
A web session is a sequence of network HTTP request and response transactions associated to the same user. Session
management or state is needed by web applications that require the retaining of information or status about eachuser for the duration of multiple requests. Therefore, sessions provide the ability to establish variables – such as accessrights and localization settings – which will apply to each and every interaction a user has with the web application
for the duration of the session.
Description
Code reviewer needs to understand what session techniques the developers used, and how to spot vulnerabilities
that may create potential security risks. Web applications can create sessions to keep track of anonymous users after
the very first user request. An example would be maintaining the user language preference. Additionally, web applications will make use of sessions once the user has authenticated. This ensures the ability to identify the user on
any subsequent requests as well as being able to apply security access controls, authorized access to the user private
data, and to increase the usability of the application. Therefore, current web applications can provide session capabilities both pre and post authentication.
The session ID or token binds the user authentication credentials (in the form of a user session) to the user
HTTP traffic and the appropriate access controls enforced by the web application. The complexity of thesethree components (authentication, session management, and access control) in modern web applications,plus the fact that its implementation and binding resides on the web developer’s hands (as web development
framework do not provide strict relationships between these modules), makes the implementation of a securesession management module very challenging.
The disclosure, capture, prediction, brute force, or fixation of the session ID will lead to session hijacking (orsidejacking) attacks, where an attacker is able to fully impersonate a victim user in the web application. Attackers can perform two types of session hijacking attacks, targeted or generic. In a targeted attack, the attacker’s
goal is to impersonate a specific (or privileged) web application victim user. For generic attacks, the attacker’s
goal is to impersonate (or get access as) any valid or legitimate user in the web application.
With the goal of implementing secure session IDs, the generation of identifiers (IDs or tokens) must meet thefollowing properties:• The name used by the session ID should not be extremely descriptive nor offer unnecessary details about the
purpose and meaning of the ID.
• It is recommended to change the default session ID name of the web development framework to a genericname, such as “id”.
• The session ID length must be at least 128 bits (16 bytes) (The session ID value must provide at least 64 bitsof entropy).

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A2 - Broken Authentication and Session Management• The session ID content (or value) must be meaningless to prevent information disclosure attacks, where an
attacker is able to decode the contents of the ID and extract details of the user, the session, or the inner workings of the web application.
It is recommended to create cryptographically strong session IDs through the usage of cryptographic hashfunctions such as SHA2 (256 bits).
What to Review
Require cookies when your application includes authentication. Code reviewer needs to understand what information is stored in the application cookies. Risk management is needed to address if sensitive informationis stored in the cookie requiring SSL for the cookie.Net ASPX web.config
Sample 8.2<authentication mode=”Forms”><forms loginUrl=”member_login.aspx”
cookieless=”UseCookies”
requireSSL=”true”
path=”/MyApplication” /></authentication>

Java web.xml
Sample 8.3<session-config><cookie-config><secure>true</secure></cookie-config></session-config>

PHP.INI
Sample 8.4
void session_set_cookie_params ( int $lifetime [, string $path [, string $domain [, bool $secure = true [, bool$httponly = true ]]]] )Session Expiration
In reviewing session handling code the reviewer needs to understand what expiration timeouts are neededby the web application or if default session timeout are being used. Insufficient session expiration by the web

A2 - Broken Authentication and Session Management

application increases the exposure of other session-based attacks, as for the attacker to be able to reuse a valid
session ID and hijack the associated session, it must still be active.
Remember for secure coding one of our goals is to reduce the attack surface of our application.
.Net ASPX
Sample 8.5<system.web><sessionState
mode=”InProc”
cookieless=”true”
timeout=”15” /></system.web>

ASPX the developer can change the default time out for a session. This code in the web.config file sets thetimeout session to 15 minutes. The default timeout for an aspx session is 30 minutes.
Java
Sample 8.6<session-config><session-timeout>1</session-timeout></session-config>

PHP
Does not have a session timeout mechanism. PHP developers will need to create their own custom session timeout.
Session Logout/Ending.
Web applications should provide mechanisms that allow security aware users to actively close their session once they
have finished using the web application.
.Net ASPX Session.Abandon() method destroys all the objects stored in a Session object and releases their resources.
If you do not call the Abandon method explicitly, the server destroys these objects when the session times out. You
should use it when the user logs out. Session.Clear() Removes all keys and values from the session. Does not change
session ID. Use this command if you if you don’t want the user to relogin and reset all the session specific data.
Session Attacks
Generally three sorts of session attacks are possible:• Session Hijacking: stealing someone’s session-id, and using it to impersonate that user.
• Session Fixation: setting someone’s session-id to a predefined value, and impersonating them using thatknown value• Session Elevation: when the importance of a session is changed, but its ID is not.

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Session Hijacking• Mostly done via XSS attacks, mostly can be prevented by HTTP-Only session cookies (unless Javascript code
requires access to them).
• It’s generally a good idea for Javascript not to need access to session cookies, as preventing all flavors of XSS
is usually the toughest part of hardening a system.
• Session-ids should be placed inside cookies, and not in URLs. URL information are stored in browser’s history,and HTTP Referrers, and can be accessed by attackers.
• As cookies can be accessed by default from javascript and preventing all flavors of XSS is usually the toughest
part of hardening a system, there is an attribute called “HTTPOnly”, that forbids this access. The session cookie
should has this attribute set. Anyway, as there is no need to access a session cookie from the client, you
should get suspicious about client side code that depends on this access.
• Geographical location checking can help detect simple hijacking scenarios. Advanced hijackers use thesameIP (or range) of the victim.
• An active session should be warned when it is accessed from another location.
• An active users should be warned when s/he has an active session somewhere else (if the policy allows
multiple sessions for a single user).
Session Fixation• If the application sees a new session-id that is not present in the pool, it should be rejected and a new
session-id should be advertised. This is the sole method to prevent fixation.
• All the session-ids should be generated by the application, and then stored in a pool to be checked later for.
Application is the sole authority for session generation.
Session Elevation• Whenever a session is elevated (login, logout, certain authorization), it should be rolled.
• Many applications create sessions for visitors as well (and not just authenticated users). They should
definitely roll the session on elevation, because the user expects the application to treat them securely after
they login.
• When a down-elevation occurs, the session information regarding the higher level should be flushed.
• Sessions should be rolled when they are elevated. Rolling means that the session-id should be changed, andthe session information should be transferred to the new id.
Server-Side Defenses for Session Management.NET ASPX
Generating new session Id’s helps prevent, session rolling, fixation, hijacking.

A2 - Broken Authentication and Session Management

Sample 8.7
public class GuidSessionIDManager : SessionIDManager {public override string CreateSessionID(HttpContext context){
return Guid.NewGuid().ToString();}
public override bool Validate(string id) {try{
Guid testGuid = new Guid(id);if (id == testGuid.ToString())
return true;}catch(Exception e) { throw e }return false;}}
Java
Sample 8.8
request.getSession(false).invalidate();//and then create a new session with
getSession(true) (getSession())

PHP.INI
Sample 8.9
session.use_trans_sid = 0
session.use_only_cookies

References• https://wwww.owasp.org/index.php/SecureFlag

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A3 - Cross-Site Scripting (XSS)

A3

CROSS-SITE SCRIPTING (XSS)

9.1 Overview
What is Cross-Site Scripting (XSS)?
Cross-site scripting (XSS) is a type of coding vulnerability. It is usually found in web applications. XSS enables attackers to inject malicious into web pages viewed by other users. XSS may allow attackers to bypass access controls suchas the same-origin policy may. This is one of the most common vulnerabilities found accordingly with OWASP Top
10. Symantec in its annual threat report found that XSS was the number two vulnerability found on web servers. The
severity/risk of this vulnerability may range from a nuisance to a major security risk, depending on the sensitivity ofthe data handled by the vulnerable site and the nature of any security mitigation implemented by the site’s organization.
Description
There are three types of XSS, Reflected XSS (Non-Persistent), Stored XSS(Persistent), and DOM based XSS. Each ofthese types has different means to deliver a malicious payload to the server. The important takeaway is the consequences are the same.
What to Review
Cross-site scripting vulnerabilities are difficult to identify and remove from a web application
Cross-site scripting flaws can be difficult to identify and remove from a web application. The best practice to search
for flaws is to perform an intense code review and search for all places where user input through a HTTP request
could possibly make its way into the HTML output.
Code reviewer needs to closely review.
1. That untrusted data is not transmitted in the same HTTP responses as HTML or JavaScript.
2. When data is transmitted from the server to the client, untrusted data must be properly encoded and the HTTP
response. Do not assume data from the server is safe. Best practice is to always check data.
3. When introduced into the DOM, untrusted data MUST be introduced using one of the following APIs:a. Node.textContent
b. document.createTextNodec. Element.setAttribute (second parameter only)
Code reviewer should also be aware of the HTML tags (such as <img src…>, <iframe…>, <bgsound src…> etc.) canbe used to transmit malicious JavaScript.
Web application vulnerability automated tools/scanners can help to find Cross-Site scripting flaws. However theycannot find all XSS vulnerabilities, hence manual code reviews are important. Manual code reviews wont catchall either but a defense in depth approach is always the best approach based on your level of risk.
OWASP Zed Attack Proxy(ZAP) is an easy to use integrated penetration-testing tool for finding vulnerabilities in web
applications. ZAP provides automated scanners as well as a set of tools that allow you to find security vulnerabilities
manually. It acts as a web proxy that you point your browser to so it can see the traffic going to a site and allows you
to spider, scan, fuzz, and attack the application. There are other scanners available both open source and commercial.

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Use Microsft’s Anti-XSS library
Another level of help to prevent XSS is to use an Anti-XSS library.
Unfortunately, HtmlEncode or validation feature is not enough to deal with XSS, especially if the user input
needs to be added to JavaScript code, tag attributes, XML or URL. In this case a good option is the Anti-XSS
libray.NET ASPX
1. On ASPX .Net pages code review should check to make sure web config file does not turn off pagevalidation. <pages validateRequest=”false” />2. .Net framework 4.0 does not allow page validation to be turned off. Hence if the programmer wants to turn ofpage validation the developer will need to regress back to 2.0 validation mode. <httpRuntime requestValidationMode=”2.0” />3. Code reviewer needs to make sure page validation is never turned off on anywhere and if it is understand
why and the risks it opens the organization to. <%@ Page Language=”C#” ValidationRequest=”false”.NET MVC
When MVC web apps are exposed to malicious XSS code, they will throw an error like the following one:
Figure 6: Example .Net XSS Framework Error

To avoid this vulnerability, make sure the following code is included:<%server.HtmlEncode(stringValue)%>
The HTMLEncode method applies HTML encoding to a specified string. This is useful as a quick method of
encoding form data and other client request data before using it in your Web application. Encoding data converts potentially unsafe characters to their HTML-encoded equivalent.(MSDN,2013)
JavaScript and JavaScript Frameworks
Both Javascript and Javascript frameworks are now widely use in web applications today. This hinders thecode reviewer in knowing what frameworks do a good job on preventing XSS flaws and which ones don’t.
Code reviewer should check to see what to see if any CVE exists for the framework being used and also checkthat the javascript framework is the latest stable version.

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OWASP References• OWASP XSS Prevention Cheat Sheet• OWASP XSS Filter Evasion Cheat Sheet• OWASP DOM based XSS Prevention Cheat Sheet• Testing Guide: 1st 3 chapters on Data Validation Testing• OWASP Zed Attack Proxy Project
External References• https://www4.symantec.com/mktginfo/whitepaper/ISTR/21347932_GA-internet-security-threat-report-volume-20-2015-social_v2.pdf• https://cwe.mitre.org/data/definitions/79.html• http://webblaze.cs.berkeley.edu/papers/scriptgard.pdf• http://html5sec.org• https://cve.mitre.org
9.2 HTML Attribute Encoding
HTML attributes may contain untrusted data. It is important to determine if any ot the HTML attributes on agiven page contains data from outside the trust boundary.
Some HTML attributes are considered safer than others such as align, alink, alt, bgcolor, border, cellpadding,cellspacing, class, color, cols, colspan, coords, dir, face, height, hspace, ismap, lang, marginheight, marginwidth,
multiple, nohref, noresize, noshade, nowrap, ref, rel, rev, rows, rowspan, scrolling, shape, span, summary, tabindex, title, usemap, valign, value, vlink, vspace, width.
When reviewing code for XSS we need to look for HTML attributes such as the following reviewing code for
XSS we need to look for HTML attributes such as the following:<input type=”text” name=”fname” value=”UNTRUSTED DATA”>
Attacks may take the following format:“><script>/* bad stuff */</script>
What is Attribute encoding?
HTML attribute encoding replaces a subset of characters that are important to prevent a string of charactersfrom breaking the attribute of an HTML element.
This is because the nature of attributes, the data they contain, and how they are parsed and interpreted bya browser or HTML parser is different than how an HTML document and its elements are read; OWASP XSSPrevention Cheat Sheet. Except for alphanumeric characters, escape all characters with ASCII values less than
256 with the &#xHH; format (or a named entity if available) to prevent switching out of the attribute. The reason this rule is so broad is that developers frequently leave attributes unquoted. Properly quoted attributes
can only be escaped with the corresponding quote. Unquoted attributes can be broken out of with many
characters, including [space] % * + , - / ; < = > ^ and |.
Attribute encoding may be performed in a number of ways. Two resources are:
1. HttpUtility.HtmlAttributeEncode

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http://msdn.microsoft.com/en-us/library/wdek0zbf.aspx
2. OWASP Java Encoder Project
https://www.owasp.org/index.php/OWASP_Java_Encoder_ProjectHTML Entity
HTML elements which contain user controlled data or data from untrusted sourced should be reviewed forcontextual output encoding. In the case of HTML entities we need to help ensure HTML Entity encoding is
performed:
Example HTML Entity containing untrusted data:
HTML Body Context <span>UNTRUSTED DATA</span> OR <body>...UNTRUSTED DATA </body> OR<div>UNTRUSTED DATA </div>

HTML Entity Encoding is required& --> &amp; < --> &lt; > --> &gt; “ --> &quot; ‘ --> &#x27;

It is recommended to review where/if untrusted data is placed within entity objects. Searching the sourcecode fro the following encoders may help establish if HTML entity encoding is being done in the applicationand in a consistent manner.
OWASP Java Encoder Project
https://www.owasp.org/index.php/OWASP_Java_Encoder_Project<input type=”text” name=”data” value=”<%=Encode.forHtmlAttribute(dataValue) %>” />OWASP ESAPI
http://code.google.com/p/owasp-esapi-java/source/browse/trunk/src/main/java/org/owasp/esapi/codecs/
HTMLEntityCodec.java
String safe = ESAPI.encoder().encodeForHTML( request.getParameter( “input” ) );JavaScript Parameters
Untrusted data, if being placed inside a JavaScript function/code requires validation. Invalidated data may
break out of the data context and wind up being executed in the code context on a users browser.
Examples of exploitation points (sinks) that are worth reviewing for:

A3 - Cross-Site Scripting (XSS)<script>var currentValue=’UNTRUSTED DATA’;</script>
script> attack: ‘);/* BAD STUFF */<script>someFunction(‘UNTRUSTED DATA’);</

Potential solutions:OWASP HTML Sanitizer ProjecTOWASP JSON Sanitizer Project
ESAPI JavaScript escaping can be call in this manner:
String safe = ESAPI.encoder().encodeForJavaScript( request.getParameter( “input” ) );Please note there are some JavaScript functions that can never safely use untrusted data as input - even if
javascript escaped!
For example (note this is an example of how NOT to use JavaScript):<script> window.setInterval(‘...EVEN IF YOU ESCAPE UNTRUSTED DATA YOU ARE XSSED HERE...’); </script>eval()
Sample 9.1
var txtField = “A1”; var txtUserInput = “’test@google.ie’;alert(1);”; eval( “document.forms[0].” + txtField +“.value =” + A1);

jquerySample 9.2
var txtAlertMsg = “Hello World: “; var txtUserInput = “test<script>alert(1)<\/script>”; $(“#message”).html(
txtAlertMsg +”” + txtUserInput + “”);

Safe usage (use text, not html)$(“#userInput”)

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A3 - Cross-Site Scripting (XSS).text( “test<script>alert(1)<\/script>”);<-- treat user input as text

Nested Contexts
Best to avoid such nested contexts: an element attribute calling a JavaScript function etc. these contexts canreally mess with your mind.

<div onclick=”showError(‘<%=request.getParameter(“errorxyz”)%>’)” >An error occurred </div>
Here we have a HTML attribute(onClick) and within a nested Javascript function call (showError).

When the browser processes this it will first HTML decode the contents of the onclick attribute. It will pass theresults to the JavaScript Interpreter. So we have 2 contextx here...HTML and Javascript (2 browser parsers). We
need to apply “layered” encoding in the RIGHT order:
1. JavaScript encode
2. HTML Attribute Encode so it “unwinds” properly and is not vulnerable.

<div onclick=”showError (‘<%= Encoder.encodeForHtml(Encoder.encodeForJavaScript( request.
getParameter(“error”)%>’)))” >An error occurred ....</div>

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A4

INSECURE DIRECT OBJECT REFERENCE

10.1 Overview
Insecure Direct Object Reference is a commonplace vulnerability with web applications that provide varying
levels of access or exposes an internal object to the user. Examples of what can be exposed are database records, URLs, files, account numbers, or allowing users to bypass web security controls by manipulating URLs.
The user may be authorized to access the web application, but not a specific object, such as a database record,
specific file or even an URL. Potential threats can come from an authorized user of the web application who alters a parameter value that directly points to an object that the user isn’t authorized to access. If the application
doesn’t verify the user for that specific object, it can result in an insecure direct object reference flaw.
10.2 Description
The source of the problem of this risk is based on the manipulation or updating of the data generated previously at server side.
What to Review
SQL Injection
An example of an attack making use of this vulnerability could be a web application where the user has alreadybeen validated. Now the user wants to view open his open invoices via another web page. The application
passes the account number using the URL string. The an application uses unverified data in a SQL call that isaccessing account information:
Sample 10.1
String query = “SELECT * FROM accts WHERE account = ?”; PreparedStatement pstmt = connection.
prepareStatement(query , ... );pstmt.setString( 1, request.getParameter(“acct”));
ResultSet results = pstmt.executeQuery();

The attacker simply modifies the ‘acct’ parameter in their browser to send whatever account number theywant. If the application does not perform user verification, the attacker can access any user’s account, insteadof only the intended customer’s account.
HTTP POST requests
A Cyber Security Analyst (Ibrahim Raafat) found an insecure direct object reference vulnerability with Yahoo!
Suggestions by using Live HTTP Headers check the content in the POST request he could see:prop=addressbook&fid=367443&crumb=Q4.PSLBfBe.&cid=1236547890&cmd= delete_comment

Where parameter ‘fid’ is the topic id and ‘cid’ is the respective comment ID. While testing, he found changing

A4 - Insecure Direct Object Reference

the fid and cid parameter values allow him to delete other comments from the forum, that are actually posted
by another user.
Next, he used the same method to test post deletion mechanism and found a similar vulnerability in that. A
normal HTTP Header POST request of deleting a post is:
POST cmd=delete_item&crumb=SbWqLz.LDP0

He found that, appending the fid (topic id) variable to the URL allows him to delete the respective post of otherusers:POST cmd=delete_item&crumb=SbWqLz.LDP0&fid=xxxxxxxx

After futher analysis he found an attacker could modify the parameters in HTTP POST requests to delete 1.5
million records entered by Yahoo users
Indirect Reference Maps
Moreover, the attackers may find out the internal naming conventions and infer the method names for operation functionality. For instance, if an application has URLs for retrieving detail information of an object like:
Attackers will try to use the following URLS to perform modification on the object:xyz.com/Customers/View/2148102445 or xyz.com/Customers/ViewDetails.aspx?ID=2148102445

Also if web application is returning an object listing part of the directory path or object name an attacker canmodify these.
xyz.com/Customers/Update/2148102445 or xyz.com/Customers/Modify.aspx?ID=2148102445
Or xyz.com/Customers/admin

Data Binding Technique
Another popular feature seen in most of the design frameworks (JSP/Structs, Spring) is data binding, where HTTP
GET request parameters or HTTP POST variables get directly bound to the variables of the corresponding business/
command object. Binding here means that the instance variables of such classes get automatically initialize withthe request parameter values based on their names. Consider a sample design given below; observe that the business logic class binds the business object class binds the business object with the request parameters.
The flaw in such design is that the business objects may have variables that are not dependent on the request parameters. Such variables could be key variables like price, max limit, role, etc. having static values or dependent onsome server side processing logic. A threat in such scenarios is that an attacker may supply additional parametersin request and try to bind values for unexposed variable of business object class.

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Secure Design Recommendation:• An important point to be noted here is that the business/form/command objects must have only thoseinstance variables that are dependent on the user inputs.
• If additional variables are present those must not be vital ones like related to the business rule for the feature.
• In any case the application must accept only desired inputs from the user and the rest must be rejected or left
unbound. And initialization of unexposed of variables, if any must take place after the binding logic.
Review Criteria
Review the application design and check if it incorporates a data binding logic. In case it does, check if businessobjects/beans that get bound to the request parameters have unexposed variables that are meant to have static
values. If such variables are initialized before the binding logic this attack will work successfully.
What the Code Reviewer needs to do:
Code reviewer needs to “map out all locations in the code being reviewed where user input is used to reference
objects directly”. These locations include where user input is used to access a database row, a file, application pages, etc. The reviewer needs to understand if modification of the input used to reference objects can result in the
retrieval of objects the user is not authorized to view.
If untrusted input is used to access objects on the server side, then proper authorization checks must be employed
to ensure data cannot be leaked. Proper input validation will also be required to ensure the untrusted input isproperly understood and used by the server side code. Note that this authorization and input validation must be
performed on the server side; client side code can be bypassed by the attacker.
Binding issues in MVC .NET
A.K.A Over-Posting A.K.A Mass assignments
In MVC framework, mass assignments are a mechanism that allows us to update our models with data coming in arequest in HTTP form fields. As the data that needs to be updated comes in a collection of form fields, a user could
send a request and modify other fields in the model that may not be in the form and the developer didn’t intend
to be updated.
Depending on the models you create, there might be sensitive data that you would not like to be modified. The
vulnerability is exploited when a malicious user modifys a model’s fields, which are not exposed to the user via theview, and the malicious user to change hidden model values adds additional model parameters.
Sample 10.2
public class user{
public int ID { get; set; } <- exposed via view
public string Name { get; set; } <- exposed via view
public bool isAdmin{ get; set; } <-hidden from view}A4 - Insecure Direct Object Reference

Corresponding view (HTML)Sample 10.3
ID: <%= Html.TextBox(“ID”) %> <br>
Name: <%= Html.TextBox(“Name”) %> <br><-- no isAdmin here!

The corresponding HTML for this model contain 2 fields: ID and Name
If an attacker adds the isAdmin parameter to the form and submits they can change the model object above.
So a malicious attacker may change isAdmin=true
Recommendations:
1 - Use a model which does not have values the user should not edit.
2 - Use the bind method and whitelist attributes which can be updated.
3 - Use the controller.UpdateModel method to exclude certain attribute updates.
References• OWASP Top 10-2007 on Insecure Dir Object References• ESAPI Access Reference Map API• ESAPI Access Control API (See isAuthorizedForData(), isAuthorizedForFile(), isAuthorizedForFunction() )• https://www.owasp.org/index.php/Category:OWASP_Application_Security_Verification_Standard_Project• https://cwe.mitre.org/data/definitions/639.html• https://cwe.mitre.org/data/definitions/22.html

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A5

SECURITY MISCONFIGURATION

Many modern applications are developed on frameworks. These frameworks provide the developer less work
to do, as the framework does much of the “housekeeping”. The code developed will extend the functionalityof the framework. It is here that the knowledge of a given framework, and language in which the application
is implemented, is of paramount importance. Much of the transactional functionality may not be visible in thedeveloper’s code and handled in “parent” classes. The reviewer should be aware and knowledgeable of theunderlying framework.
Web applications do not execute in isolation, they typically are deployed within an application server framework, running within an operating system on a physical host, within a network.
Secure operating system configuration (also called hardening) is not typically within the scope of code review.
For more information, see the Center for Internet Security operating system benchmarks.
Networks today consist of much more than routers and switches providing transport services. Filtering switches, VLANs (virtual LANs), firewalls, WAFs (Web Application Firewall), and various middle boxes (e.g. reverse
proxies, intrusion detection and prevention systems) all provide critical security services when configured to
do so. This is a big topic, but outside the scope of this web application code review guide. For a good summary,see the SANS (System Administration, Networking, and Security) Institute Critical Control 10: Secure Configurations for Network Devices such as Firewalls, Routers, and Switches.
Application server frameworks have many security related capabilities. These capabilities are enabled and configured in static configuration files, commonly in XML format, but may also be expressed as annotations within
the code.
11.1 Apache Struts
In struts the struts-config.xml and the web.xml files are the core points to view the transactional functionalityof an application. The struts-config.xml file contains the action mappings for each HTTP request while theweb.xml file contains the deployment descriptor.
The struts framework has a validator engine, which relies on regular expressions to validate the input data.
The beauty of the validator is that no code has to be written for each form bean. (Form bean is the Java object
which received the data from the HTTP request). The validator is not enabled by default in struts. To enable thevalidator, a plug-in must be defined in the <plug-in> section of struts-config.xml. The property defined tells
the struts framework where the custom validation rules are defined (validation.xml) and a definition of theactual rules themselves (validation-rules.xml).
Without a proper understanding of the struts framework, and by simply auditing the Java code, one would not
see any validation being executed, and one does not see the relationship between the defined rules and theJava functions.
The action mappings define the action taken by the application upon receiving a request. Here, in sample
11.1, we can see that when the URL contains “/login” the LoginAction shall be called. From the action mappings we can see the transactions the application performs when external input is received.

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A5 - Security Misconfiguration

Sample 11.1<?xml version=”1.0” encoding=”ISO-8859-1” ?><!DOCTYPE struts-config PUBLIC“-//Apache Software Foundation//DTD Struts Configuration 1.0//EN”“http://jakarta.apache.org/struts/dtds/struts-config_1_0.dtd”><struts-config><form-beans><form-bean name=”login” type=”test.struts.LoginForm” /></form-beans><global-forwards></global-forwards><action-mappings><actionpath=”/login”
type=”test.struts.LoginAction” ><forward name=”valid” path=”/jsp/MainMenu.jsp” /><forward name=”invalid” path=”/jsp/LoginView.jsp” /></action></action-mappings><plug-in className=”org.apache.struts.validator.ValidatorPlugIn”><set-property property=”pathnames”
value=”/test/WEB-INF/validator-rules.xml, /WEB-INF/validation.xml”/></plug-in></struts-config>

11.2 Java Enterprise Edition Declarative Configuration
Some security capabilities are accessible from within a Java program. Programmatic security is done within the
web application using framework specific or standard Java Enterprise Edition (JEE) framework APIs. JEE has the
JEE security model which, uses a role-based security model in which, access to application resources is granted
based on the security role. The security role is a logical grouping of principals (authenticated entities, usually auser), and access is declared by specifying a security constraint on the role.
The constraints and roles are expressed as deployment descriptors expressed as XML elements. Different typesof components use different formats, or schemas, for their deployment descriptors:• Web components may use a web application deployment descriptor in the file web.xml• Enterprise JavaBeans components may use an EJB deployment descriptor named META-INF/ejb-jar.xml
The deployment descriptor can define resources (e.g. servlets accessible via a specific URL), which roles areauthorized to access the resource, and how access is constrained (e.g. via GET but not POST).
The example web component descriptor in sample 11.2, (included in the “web.xml” file) defines a Catalog
servlet, a “manager” role, a SalesInfo resource within the servlet accessible via GET and POST requests, andspecifies that only users with “manager” role, using SSL and successfully using HTTP basic authentication
should be granted access.

A5 - Security Misconfiguration

Sample 11.2<?xml version=”1.0” encoding=”ISO-8859-1”?><web-app xmlns=”http://java.sun.com/xml/ns/j2ee”
xmlns:xsi=”http://www.w3.org/2001/XMLSchema-instance”
xsi:schemaLocation=”http://java.sun.com/xml/ns/j2ee
http://java.sun.com/xml/ns/j2ee/web-app_2_5.xsd” version=?2.5?><display-name>A Secure Application</display-name><servlet><servlet-name>catalog</servlet-name><servlet-class>com.mycorp.CatalogServlet</servlet-class><init-param><param-name>catalog</param-name><param-value>Spring</param-value></init-param><!-- Define Security Roles --><security-role-ref><role-name>MGR</role-name><role-link>manager</role-link></security-role-ref></servlet><security-role><role-name>manager</role-name></security-role><servlet-mapping><servlet-name>catalog</servlet-name><url-pattern>/catalog/*</url-pattern></servlet-mapping><!-- Define A Security Constraint --><security-constraint><!-- Specify the Resources to be Protected --><web-resource-collection><web-resource-name>SalesInfo</web-resource-name><url-pattern>/salesinfo/*</url-pattern><http-method>GET</http-method><http-method>POST</http-method></web-resource-collection><!-- Specify which Users Can Access Protected Resources --><auth-constraint><role-name>manager</role-name></auth-constraint>

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A5 - Security Misconfiguration<!-- Specify Secure Transport using SSL (confidential guarantee) --><user-data-constraint><transport-guarantee>CONFIDENTIAL</transport-guarantee></user-data-constraint></security-constraint><!-- Specify HTTP Basic Authentication Method --><login-config><auth-method>BASIC</auth-method><realm-name>file</realm-name></login-config></web-app>

Security roles can also be declared for enterprise Java beans in the “ejb-jar.xml” file as seen in figure sample 11.3.
Sample 11.3
<ejb-jar><assembly-descriptor><security-role><description>The single application role</description><role-name>TheApplicationRole</role-name></security-role></assembly-descriptor></ejb-jar>

For beans, however, rather than specifying access to resources within servlets, access to bean methods is
specified. The example in sample 11.4 illustrates several types of method access constraints for several beans.
Sample 11.4
<ejb-jar><assembly-descriptor><method-permission><description>The employee and temp-employee roles may access any
method of the EmployeeService bean </description><role-name>employee</role-name><role-name>temp-employee</role-name><method><ejb-name>EmployeeService</ejb-name><method-name>*</method-name></method></method-permission>

A5 - Security Misconfiguration<method-permission><description>The employee role may access the findByPrimaryKey,
getEmployeeInfo, and the updateEmployeeInfo(String) method of
the AardvarkPayroll bean </description><role-name>employee</role-name><method><ejb-name>AardvarkPayroll</ejb-name><method-name>findByPrimaryKey</method-name></method><method><ejb-name>AardvarkPayroll</ejb-name><method-name>getEmployeeInfo</method-name></method><method><ejb-name>AardvarkPayroll</ejb-name><method-name>updateEmployeeInfo</method-name><method-params><method-param>java.lang.String</method-param></method-params></method></method-permission><method-permission><description>The admin role may access any method of the
EmployeeServiceAdmin bean </description><role-name>admin</role-name><method><ejb-name>EmployeeServiceAdmin</ejb-name><method-name>*</method-name></method></method-permission><method-permission><description>Any authenticated user may access any method of the
EmployeeServiceHelp bean</description><unchecked/><method><ejb-name>EmployeeServiceHelp</ejb-name><method-name>*</method-name></method></method-permission><exclude-list><description>No fireTheCTO methods of the EmployeeFiring bean may be
used in this deployment</description><method><ejb-name>EmployeeFiring</ejb-name><method-name>fireTheCTO</method-name></method></exclude-list>

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A5 - Security Misconfiguration</assembly-descriptor></ejb-jar>
If XML deployment descriptors are used to secure the application, code review should include the “web.xml”
and “ejb-jar.xml” files to ensure that access controls are properly applied to the correct roles, and authentication methods are as expected.
11.3 JEE Annotations
JEE annotations for security are defined in the Java Docs [2]. The available annotations are:• @DeclareRoles• @DenyAll - no roles may invoke the method.
• @PermitAll - all roles may invoke the method.
• @RolesAllowed - roles permitted to invoke the method.
• @RunAs - dynamically run the method as a particular role.
For example the code in sample 11.5 allows employees and managers to add movies to the persistent store,
anyone to list movies, but only managers may delete movies.
Sample 11.5
public class Movies {
private EntityManager entityManager;@RolesAllowed({“Employee”, “Manager”})
public void addMovie(Movie movie) throws Exception {entityManager.persist(movie);}@RolesAllowed({“Manager”})
public void deleteMovie(Movie movie) throws Exception {entityManager.remove(movie);}@PermitAllpublic List<Movie> getMovies() throws Exception {Query query = entityManager.createQuery(“SELECT m from Movie as m”);
return query.getResultList();}}Code review should look for such annotations. If present, ensure they reflect the correct roles and permissions,and are consistent with any declared role permissions in the “ejb-jar.xml” file.

A5 - Security Misconfiguration

11.4 Framework Specific Configuration
Apache Tomcat
The server.xml[3] file should be reviewed to ensure security related parameters are configured as expected. The tomcat server.xml file defines many security related parameters.
Table 12: Apache Tomcat Security Parameters
Parameter

Description

ServerThis is the shutdown port.

Connectors

maxPostSize, maxParameterCount, server, SSLEnabled, secure, ciphers.

Host

autoDeploy, deployOnStartup, deployXML

Context

crossContext, privileged, allowLinking

FilterTomcat provides a number of filters which may be configured to incoming requests

Filters are especially powerful, and a code review should validate they are used unless there is a compelling reason
not to.
Jetty
Jetty adds several security enhancements:• Limiting form content• Obfuscating passwords
The maximum form content size and number of form keys can be configured at server and web application levelin the “jetty-web.xml” file.
Sample 11.6
<Configure class=”org.eclipse.jetty.webapp.WebAppContext”>…
<Set name=”maxFormContentSize”>200000</Set><Set name=”maxFormKeys”>200</Set></Configure><configure class=”org.eclipse.jetty.server.Server”>...
<Call name=”setAttribute”><Arg>org.eclipse.jetty.server.Request.maxFormContentSize</Arg><Arg>100000</Arg></Call><Call name=”setAttribute”><Arg>org.eclipse.jetty.server.Request.maxFormKeys</Arg><Arg>2000</Arg></Call></configure>

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Jetty also supports the use of obfuscated passwords in jetty XML files where a plain text password is usuallyneeded. sample 11.7 shows example code setting the password for a JDBC Datasource with obfuscation (theobfuscated password is generated by Jetty org.eclipse.jetty.util.security.Password utility).
Sample 11.7
<New id=”DSTest” class=”org.eclipse.jetty.plus.jndi.Resource”><Arg></Arg><Arg>jdbc/DSTest</Arg><Arg><New class=”com.jolbox.bonecp.BoneCPDataSource”><Set name=”driverClass”>com.mysql.jdbc.Driver</Set><Set name=”jdbcUrl”>jdbc:mysql://localhost:3306/foo</Set><Set name=”username”>dbuser</Set><Set name=”password”><Call class=”org.eclipse.jetty.util.security.Password” name=”deobfuscate”><Arg>OBF:1ri71v1r1v2n1ri71shq1ri71shs1ri71v1r1v2n1ri7</Arg></Call></Set><Set name=”minConnectionsPerPartition”>5</Set><Set name=”maxConnectionsPerPartition”>50</Set><Set name=”acquireIncrement”>5</Set><Set name=”idleConnectionTestPeriod”>30</Set></New></Arg></New>JBoss AS
JBoss Application Server, like Jetty, allows password obfuscation (called password masking in JBoss) in its XML
configuration files. After using JBoss password utility to create password mask, replace any occurrence of amasked password in XML configuration files with the following annotation.
Sample 11.8
<annotation>@org.jboss.security.integration.password.Password(securityDomain=MASK_NAME,methodName=setPROPERTY_NAME)</annotation>

See Masking Passwords in XML Configuration in the JBoss Security Guide.

A5 - Security Misconfiguration

Oracle WebLogic
WebLogic server supports additional deployment descriptors in the “weblogic.xml” file as shown in table 13.
Table 13: Weblogic Security Parameters
Parameter

Description

externally-definedRole to principal mappings are externally defined in WebLogic Admin Console

run-as-principal-name

Assign a principal to a role when running as that role

run-as-role-assignment

Contains the run-as-principal-name descriptor

security-permission

Contains security-permission-spec descriptor

FilterTomcat provides a number of filters which may be configured to incoming requests

security-permission-spec

Specify application permissions [6]security-role-assignment

Explicitly assign principals to a role

More information on WebLogic additional deployment descriptors may be found at weblogic.xml Deployment
Descriptors.
For general guidelines on securing web applications running within WebLogic, see the Programming WebLogic Security guide and the NSA’s BEA WebLogic Platform Security Guide.
11.5 Programmatic Configuration: J2EE
The J2EE API for programmatic security consists of methods of the EJBContext interface and the HttpServletRequest interface. These methods allow components to make business-logic decisions based on the security
role of the caller or remote user (there are also methods to authenticate users, but that is outside the scope of
secure deployment configuration).
The J2EE APIs that interact with J2EE security configuration include:• getRemoteUser, which determines the user name with which the client authenticated• isUserInRole, which determines whether a remote user is in a specific security role.
• getUserPrincipal, which determines the principal name of the current user and returns a java.security.
Use of these programmatic APIs should be reviewed to ensure consistency with the configuration. Specifically,
the security-role-ref element should be declared in the deployment descriptor with a role-name subelement
containing the role name to be passed to the isUserInRole method.
The code in sample 11.9 demonstrates the use of programmatic security for the purposes of programmatic
login and establishing identities and roles. This servlet does the following:• displays information about the current user.
• prompts the user to log in.
• prints out the information again to demonstrate the effect of the login method.

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A5 - Security Misconfiguration• Iogs the user out.
• prints out the information again to demonstrate the effect of the logout method.
Sample 11.9
package enterprise.programmatic_login;
import java.io.*;import java.net.*;import javax.annotation.security.DeclareRoles;
import javax.servlet.*;import javax.servlet.http.*;@DeclareRoles(“javaee6user”)
public class LoginServlet extends HttpServlet {/*** Processes requests for both HTTP GET and POST methods.
* @param request servlet request* @param response servlet response*/
protected void processRequest(HttpServletRequest request,
HttpServletResponse response)throws ServletException, IOException {response.setContentType(“text/html;charset=UTF-8”);
PrintWriter out = response.getWriter();try {String userName = request.getParameter(“txtUserName”);
String password = request.getParameter(“txtPassword”);
out.println(“Before Login” + “<br><br>”);out.println(“IsUserInRole?..”+ request.isUserInRole(“javaee6user”)+”<br>”);out.println(“getRemoteUser?..” + request.getRemoteUser()+”<br>”);out.println(“getUserPrincipal?..”+ request.getUserPrincipal()+”<br>”);out.println(“getAuthType?..” + request.getAuthType()+”<br><br>”);try {request.login(userName, password);} catch(ServletException ex) {out.println(“Login Failed with a ServletException..”+ ex.getMessage());
return;}out.println(“After Login...”+”<br><br>”);out.println(“IsUserInRole?..”A5 - Security Misconfiguration+ request.isUserInRole(“javaee6user”)+”<br>”);out.println(“getRemoteUser?..” + request.getRemoteUser()+”<br>”);out.println(“getUserPrincipal?..”+ request.getUserPrincipal()+”<br>”);out.println(“getAuthType?..” + request.getAuthType()+”<br><br>”);request.logout();
out.println(“After Logout...”+”<br><br>”);out.println(“IsUserInRole?..”+ request.isUserInRole(“javaee6user”)+”<br>”);out.println(“getRemoteUser?..” + request.getRemoteUser()+”<br>”);out.println(“getUserPrincipal?..”+ request.getUserPrincipal()+”<br>”);out.println(“getAuthType?..” + request.getAuthType()+”<br>”);} finally {out.close();
}
}
...
}

More detailed information can be found in the Java EE Tutorial: Using Programmatic Security with Web Applications.
11.6 Microsoft IIS
ASP.NET / IIS applications use an optional XML-based configuration file, named web.config, to maintain application configuration settings. This covers issues such as authentication, authorization, error pages, HTTP
settings, debug settings, web service settings, etc. Without knowledge of these files, a transactional analysis
would be very difficult and not accurate.
In IIS 7 there is a configuration system which affects the hierarchy level and how one file can inherit from
Figure 7: IIS Configuration Filesmachine.config(root) web.config

ApplicationHost.config

web.config

web.config

windows\system32\intsrv

c:\inetpub\wwwroot

d:\MyApp

http://localhost

http://localhost/MyApp

another. The following figure shows how this will work and the location of each file (Aguilar, 2006)

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It is possible to provide a file web.config at the root of the virtual directory for a web application. If the file isabsent, the default configuration settings in machine.config will be used. If the file is present, any settings in
Sample 11.10<authentication mode=”Forms”><forms name=”name”
loginUrl=”url”
protection=”Encryption”
timeout=”30” path=”/”requireSSL=”true|”slidingExpiration=”false”><credentials passwordFormat=”Clear”><user name=”username” password=”password”/></credentials></forms><passport redirectUrl=”internal”/></authentication>

web.config will override the default settings.
Many of the important security settings are not set in the code, but in the framework configuration files.
Knowledge of the framework is of paramount importance when reviewing framework-based applications.
Table 14: Parameters In The Web.config File
Parameter

Description

authentication mode

The default authentication mode is ASP.NET forms-based authentication.

loginUrl

Specifies the URL where the request is redirected for login if no valid authentication cookie is found.

protection

Specifies that the cookie is encrypted using 3DES or DES but DV is not performed on the cookie. Beware of plaintext
attacks.

timeout

Cookie expiry time in minutes.

Some examples of framework specific parameters in the web.config file are shown in table 14.
11.7 Framework Specific Configuration: Microsoft IIS
Security features can be configured in IIS using the Web.config (application level) or ApplicationHost.config (server
level) file, in the <system.webServer><security> section. The types of features that may be configured include:• Permitted authentication methods• Authorization rules• Request filters and limits• Use of SSL• Source IP address filtering• Error handling
The Web.config and ApplicationHost.config files should be included in code review. The <system.web-

A5 - Security Misconfiguration

Server><security> sections should be reviewed to ensure all security configuration is as expected.
For guidelines on securing the overall configuration of Microsoft IIS, see the IIS supports basic, client certificate,digest, IIS client certificate, and Windows authentication methods. They are configured in the <system.webServer><security><authentication> section.
The example in sample 11.11 disables anonymous authentication for a site named MySite, then enables both
Sample 11.11<location path=”MySite”><system.webServer><security><authentication><anonymousAuthentication enabled=”false” /><basicAuthentication enabled=”true” defaultLogonDomain=”MySite” /><windowsAuthentication enabled=”true” /></authentication></security></system.webServer></location>

basic authentication and windows authentication for the site.
IIS authorization configuration allows specification of users access to the site or server and is configured in the<system.webServer><security><authorization> section.
The configuration in sample 11.12 removes the default IIS authorization settings, which allows all users accessto Web site or application content, and then configures an authorization rule that allows only users with adSample 11.12<configuration><system.webServer><security><authorization><remove users=”*” roles=”” verbs=”” /><add accessType=”Allow” users=”” roles=”Administrators” /></authorization></security></system.webServer></configuration

ministrator privileges to access the content.
IIS supports filtering, including enforcing limits, on incoming HTTP requests. Table 15 shows many of the IIS
security parameters that can be set.

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Table 15: IIS Security Parameters
Parameter

Function

denyUrlSequences

A list of prohibited URL patterns

fileExtensions

Allowed or prohibited file extensions

hiddenSegments

URLs that cannot be browsed

requestLimits

URL, content, query string, and HTTP header length limits

verbs

Allowed or prohibited verbs

alwaysAllowedUrls

URLs always permitted

alwaysAllowedQueryStrings

Query strings always allowed

denyQueryStringSequences

Prohibited query strings

filteringRules

Custom filtering rules

These parameters are configured in the <system.webServer><security><requestFiltering> section. The example in sample 11.13:• Denies access to two URL sequences. The first sequence prevents directory transversal and the second sequence prevents access to alternate data streams.
• Sets the maximum length for a URL to 2KB and the maximum length for a query string to 1KB.

Sample 11.13<configuration><system.webServer><security><requestFiltering><denyUrlSequences><add sequence=”..” /><add sequence=”:” /></denyUrlSequences><fileExtensions allowUnlisted=”false” /><requestLimits maxUrl=”2048” maxQueryString=”1024” /><verbs allowUnlisted=”false” /></requestFiltering></security></system.webServer></configuration>

A5 - Security Misconfiguration• Denies access to unlisted file name extensions and unlisted HTTP verbs.
IIS allows specifying whether SSL is supported, is required, whether client authentication is supported or required, and cipher strength. It is configured in the <system.webServer><security><access> section. The exSample 11.14<location path=”MySite”><system.webServer><security><access sslFlags=”ssl”></security></system.webServer></location>
ample in figure A5.13 specifies SSL as required for all connections to the site MySite.
IIS allows restrictions on source IP addresses or DNS names. It is configured in the <system.webServer><security><ipSecurity> section as shown in sample 11.15 where the example configuration denies access to the
Sample 11.15<location path=”Default Web Site”><system.webServer><security><ipSecurity><add ipAddress=”192.168.100.1” /><add ipAddress=”169.254.0.0” subnetMask=”255.255.0.0” /></ipSecurity></security></system.webServer></location>

IP address 192.168.100.1 and to the entire 169.254.0.0 network:
Detailed information on IIS security configuration can be found at IIS Security Configuration. Specific security
feature configuration information can be found at Authentication, Authorization, SSL, Source IP, Request Filtering, and Custom Request Filtering[12].
11.8 Programmatic Configuration: Microsoft IIS
Microsoft IIS security configuration can also be programmatically set from various languages:• appcmd.exe set config• C#• Visual Basic• JavaScript
For example, disabling anonymous authentication for a site named MySite, then enabling both basic authentication and windows authentication for the site (as done via configuration in the section above) can be accom-

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Sample 11.16
appcmd.exe set config “MySite” -section:system.webServer/security/authentication/anonymousAuthentication /enabled:”False” /commit:apphost
appcmd.exe set config “MySite” -section:system.webServer/security/authentication/basicAuthentication /enabled:”True” /commit:apphost
appcmd.exe set config “MySite” -section:system.webServer/security/authentication/windowsAuthentication /enabled:”True” /commit:apphostplished from the command line using the commands in figure sample 11.16.
Sample 11.17
using System;
using System.Text;
using Microsoft.Web.Administration;internal static class Sample {private static void Main() {using(ServerManager serverManager = new ServerManager()) {Configuration config = serverManager.GetApplicationHostConfiguration();ConfigurationSection anonymousAuthenticationSection =config.GetSection(“system.webServer/security/authentication/anonymousAuthentication”, “MySite”);
anonymousAuthenticationSection[“enabled”] = false;
ConfigurationSection basicAuthenticationSection =config.GetSection(“system.webServer/security/authentication/basicAuthentication”, “MySite”);
basicAuthenticationSection[“enabled”] = true;
ConfigurationSection windowsAuthenticationSection =config.GetSection(“system.webServer/security/authentication/windowsAuthentication”, “MySite”);
windowsAuthenticationSection[“enabled”] = true;

}

}

}

serverManager.CommitChanges();

Alternatively the same authentication setup can be coded programmatically as in sample 11.17.
When reviewing source code, special attention should be paid to configuration updates in security sections.
11.9 Further IIS Configurations
Filtering Requests and URL Rewriting

A5 - Security Misconfiguration

Request Filtering was introduced in IIS7 and it has replaced the functionality UrlScan add-on for IIS 6.0. Thisbuilt-in security feature allows to filter undesired URL request but it is also possible to configure different kindsof filtering. To begin with, it is important to understand how the IIS pipeline works when a request is done. The
Figure 8: IIS Request FilteringFilesHTTP
REQUEST

REQUEST FILTERING(HIGH PRIORITY)
BEGIN REQUESTURL REWRITE MODULE(MEDIUM PRIORITY)AUTHENTICATE REQUESTAUTHORIZE REQUESTRESOLVE CACHEEND REQUESTHTTPRESPONSE

following diagram shows the order in these modules(Yakushev, 2008)

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Sample 11.18<configuration><system.webServer><security><requestFiltering><denyUrlSequences><add sequence=”..” /><add sequence=”:” /></denyUrlSequences><fileExtensions allowUnlisted=”false” /><requestLimits maxUrl=”2048” maxQueryString=”1024” /><verbs allowUnlisted=”false” /></requestFiltering></security></system.webServer></configuration>
Request filtering can be setup through the IIS interface or on the web.config file. Example:(Yakushev, 2008)

Sample 11.19
using System;
using System.Text;
using Microsoft.Web.Administration;internal static class Sample{
private static void Main(){using (ServerManager serverManager = new ServerManager()){
Configuration config = serverManager.GetWebConfiguration(“Default Web Site”);
ConfigurationSection requestFilteringSection = config.GetSection(“system.webServer/security/requestFiltering”);
ConfigurationElementCollection denyUrlSequencesCollection =requestFilteringSection.GetCollection(“denyUrlSequences”);
ConfigurationElement addElement = denyUrlSequencesCollection.CreateElement(“add”);
addElement[“sequence”] = @”..”;
denyUrlSequencesCollection.Add(addElement);
ConfigurationElement addElement1 = denyUrlSequencesCollection.CreateElement(“add”);
addElement1[“sequence”] = @”:”;
denyUrlSequencesCollection.Add(addElement1);
ConfigurationElement addElement2 = denyUrlSequencesCollection.CreateElement(“add”);
addElement2[“sequence”] = @”\”;
denyUrlSequencesCollection.Add(addElement2);

A5 - Security Misconfiguration

}

}

}

serverManager.CommitChanges();This can also be done through the application code, for example:(Yakushev, 2008)
Filtering Double - Encoded Requests
This attack technique consists of encoding user request parameters twice in hexadecimal format in order tobypass security controls or cause unexpected behavior from the application. It’s possible because the webserver accepts and processes client requests in many encoded forms.
By using double encoding it’s possible to bypass security filters that only decode user input once. The second
decoding process is executed by the backend platform or modules that properly handle encoded data, but
don’t have the corresponding security checks in place.
Attackers can inject double encoding in pathnames or query strings to bypass the authentication schema andsecurity filters in use by the web application.
There are some common character that are used in Web applications attacks. For example, Path Traversal attacks use “../” (dot-dot-slash) , while XSS attacks use “<” and “>” characters. These characters give a hexadecimal representation that differs from normal data.
For example, “../” (dot-dot-slash) characters represent %2E%2E%2f in hexadecimal representation. When the% symbol is encoded again, its representation in hexadecimal code is %25. The result from the double encoding process ”../”(dot-dot-slash) would be %252E%252E%252F:• The hexadecimal encoding of “../” represents “%2E%2E%2f”• Then encoding the “%” represents “%25”• Double encoding of “../” represents “%252E%252E%252F”
If you do not want IIS to allow doubled-encoded requests to be served, use the following (IIS Team,2007):
Sample 11.20<configuration><system.webServer><security><requestFiltering
allowDoubleEscaping=”false”></requestFiltering></security></system.webServer></configuration>

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Filter High Bit Characters
This allows or rejects all requests to IIS that contain non-ASCII characters . When this occurs error code 404.12.
is displayed to the user . The UrlScan (IIS6 add-on) equivalent is AllowHighBitCharacters.
Sample 11.21<configuration><system.webServer><security><requestFiltering
allowHighBitCharacters=”true”></requestFiltering></security></system.webServer></configuration>

Filter Based on File Extensions
Using this filter you can allow IIS to a request based on file extensions, the error code logged is 404.7. The AllowExtensions and DenyExtensions options are the UrlScan equivalents.
Sample 11.22<configuration><system.webServer><security><requestFiltering><fileExtensions allowUnlisted=”true” ><add fileExtension=”.asp” allowed=”false”/></fileExtensions></requestFiltering></security></system.webServer></configuration>

Filter Based on Request Limits
When IIS rejects a request based on request limits, the error code logged is:• 404.13 if the content is too long.
• 404.14 if the URL is too large.
• 404.15 if the query string is too long.
This can be used to limit a long query string or too much content sent to an application which you cannotchange the source code to fix the issue.

A5 - Security Misconfiguration

Sample 11.23<configuration><system.webServer><security><requestFiltering><requestLimits
maxAllowedContentLength=”30000000”
maxUrl=”260”
maxQueryString=”25”/></requestFiltering></security></system.webServer></configuration>

Filter by Verbs
When IIS reject a request based on this feature, the error code logged is 404.6. This corresponds to the UseAllowVerbs, AllowVerbs, and DenyVerbs options in UrlScan.
In case you want the application to use only certain type of verb, it is necessary to firt set the allowUnlisted to‘false’ and then set the verbs that you would like to allow (see example)
Sample 11.24<configuration><system.webServer><security><requestFiltering><verbs allowUnlisted=”false”><add verb=”GET” allowed=”true” /></verbs></requestFiltering></security></system.webServer></configuration>

Filter Based on URL Sequences
This feature defines a list of sequences that IIS can reject when it is part of a request. When IIS reject a request
for this feature, the error code logged is 404.5.This corresponds to the DenyUrlSequences feature in UrlScan.
This is a very powerful feature. This avoids a given character sequence from ever being attended by IIS:
Sample 11.25<configuration><system.webServer>

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A5 - Security Misconfiguration<security><requestFiltering><denyUrlSequences><add sequence=”..”/></denyUrlSequences></requestFiltering></security></system.webServer></configuration>

Filter Out Hidden Segments
In case you want IIS to serve content in binary directory but not the binary, you can apply this configuration.
Sample 11.26<configuration><system.webServer><security><requestFiltering><hiddenSegments><add segment=”BIN”/></hiddenSegments></requestFiltering></security></system.webServer></configuration>

Password protection and sensitive information
The web.config files might include sensitive information in the connection strings such as database passwords,
mail server user names among others.
Sections that are required to be encrypted are:
• <appSettings>. This section contains custom application settings.
• <connectionStrings>. This section contains connection strings.
• <identity>. This section can contain impersonation credentials.
• <sessionState>. This section contains the connection string for the out-of-process session state provider.
Passwords and user names contained in a <connectionstring> section should be encrypted. ASP.NET allows
you to encrypt this information by using the functionality aspnet_regiis .This utility is found in the installed.NET framework under the folder%windows%\Microsoft.NET\Framework\v2.0.50727
You can specify the section you need to encrypt by using the command:
aspnet_regiis -pef sectiontobeencryoted
Encrypting sections in Web.Config file
Even though encrypting sections is possible, not all sections can be encrypted, specifically sections that are

A5 - Security Misconfiguration

read before user code is run. The following sections cannot be encrypted:• <processModel>• <runtime>• <mscorlib>• <startup>• <system.runtime.remoting>• <configProtectedData>• <satelliteassemblies>• <cryptographySettings>• <cryptoNameMapping>• <cryptoClasses>
Machine-Level RSA key container or User-Level Key Containers
Encrypting a single file using machine-level RSA key has its disadvantages when this file is moved to otherservers. In this case, user-level RSA key container is strongly advised. The RSAProtectedConfigurationProvider
supports machine-level and user-level key containers for key storage.
RSA machine key containers are stored in the following folder:\Documents and Settings\All Users\Application Data\Microsoft\Crypto\RSA\MachineKeys
User Key Container
When the application that needs to be protected is in a shared hosting environment and protection of sensitive data cannot be accessible to other applications, the user key container is strongly recommended.
In this case each application should have a separate identity.
RSA user-level key containers are stored in the following folder:\Documents and Settings\{UserName}\Application Data\Microsoft\Crypto\RSA
IIS configurations
Depending on the version of IIS that must be configured, it is important to revise some of its settings which
can comprise security in the server.
Trust level
The trust level is a set of Code Access Security permissions granted to an application within a hosting environment. These are defined using policy files. Depending on the trust level that must be configured, it is possible
to grant FULL, HIGH, MEDIUM, LOW or MINIMAL level. The ASP.NET host does not apply any additional policy
to applications that are running at the full-trust level.
Example:Sample 11.27<system.web><securityPolicy><trustLevel name=”Full” policyFile=”internal”/></securityPolicy></system.web>

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Lock Trust Levels
In the .NET framework web.config file is possible to lock applications from changing their trust level
This file is found at:
C:\Windows\Microsoft.NET\Framework\{version}\CONFIG
The following example shows how to lock 2 different application configuration trust levels (MSDN, 2013)
Sample 11.28<configuration><location path=”application1” allowOverride=”false”><system.web><trust level=”High” /></system.web></location><location path=”application2” allowOverride=”false”><system.web><trust level=”Medium” /></system.web></location></configuration>

References• Yakushev Ruslan , 2008 “IIS 7.0 Request Filtering and URL Rewriting “ available at http://www.iis.net/learn/
extensions/url-rewrite-module/iis-request-filtering-and-url-rewriting (Last accessed on 14 July, 2013)• OWASP, 2009 “Double Encoding” available at https://www.owasp.org/index.php/Double_Encoding (Last accessed on 14 July, 2013)• IIS Team, 2007 “Use Request Filtering “ available at http://www.iis.net/learn/manage/configuring-security/
use-request-filtering (Last accessed on 14 July, 2013)• Aguilar Carlos ,2006 “The new Configuration System in IIS 7” available at http://blogs.msdn.com/b/carlosag/
archive/2006/04/25/iis7configurationsystem.aspx (Last accessed on 14 July, 2013)• MSDN, 2013 . How to: Lock ASP.NET Configuration Settings available at http://msdn.microsoft.com/en-us/
library/ms178693.aspx (Last accessed on 14 July, 2013)11.10 Strongly Named Assemblies
During the build process either QA or Developers are going to publish the code into executable formats. Usually this consists of an exe or and one or several DLL’s. During the build/publish process a decision needs to bemade to sign or not sign the code.
Signing your code is called creating “strong names” by Microsoft. If you create a project using Visual Studio anduse Microsofts “Run code analysis” most likely your will encounter a Microsoft design error if the code is notstrong named; “Warning 1 CA2210 : Microsoft.Design : Sign ‘xxx.exe’ with a strong name key.”

A5 - Security Misconfiguration

Code review needs to be aware if strong naming is being used, benefits and what threat vectors strong naming
helps prevent or understand the reasons for not using strong naming.
A strong name is a method to sign an assembly’s identity using its text name, version number, culture information, a public key and a digital signature. (Solis, 2012)• Strong naming guarantees a unique name for that assembly.
• Strong names protect the version lineage of an assembly. A strong name can ensure that no one can produce a subsequent version of your assembly. Users can be sure that a version of the assembly they are loading
comes from the same publisher that created the version the application was built with.
The above two points are very important if you are going to use Global Assembly Cache (GAC).
• Strong names provide a strong integrity check and prevent spoofing. Passing the .NET Framework security
checks guarantees that the contents of the assembly have not been changed since it was built.
Note, however, that strong names in and of themselves do not imply a level of trust like that provided, forexample, by a digital signature and supporting certificate. If you use the GAC assemblies remember the assemblies are not verified each time they load since the GAC by design is a locked-down, admin-only store.
What strong names can’t prevent is a malicious user from stripping the strong name signature entirely, modifying the assembly, or re-signing it with the malicious user’s key.
The code reviewer needs to understand how the strong name private key will be kept secure and managed.
This is crucible if you decide strong name signatures are a good fit for your organization.
If principle of least privilege is used so code is not or less susceptible to be access by the hacker and the GAC isnot being used strong names provides less benefits or no benefits at all.
How to use Strong Naming
Signing tools
In order to create a strong name assembly there are a set of tools and steps that you need to follow
Using Visual Studio
In order to use Visual Studio to create a Strongly Named Assembly, it is necessary to have a copy of the public/
private key pair file. It is also possible to create this pair key in Visual Studio
In Visual Studio 2005, the C#, Visual Basic, and Visual J# integrated development environments (IDEs) allow you
to generate key pairs and sign assemblies without the need to create a key pair using Sn.exe (Strong Name
Tool).
These IDEs have a Signing tab in the Project Designer. . The use of the AssemblyKeyFileAttribute to identify key
file pairs has been made obsolete in Visual Studio 2005.
The following figure illustrates the process done by the compiler

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Figure 9: C# Strong Naming• ASSEMBLY BINARY• SIMPLE NAME
• VERSION NUMBERASSEMBLY• CULTURE INFOC#VBCOMPILER

Manifest
Version Number
Culture Info
Public Key

PUBLIC KEY

Simple Name
Metadata
CIL
Digital Signature

PRIVATE KEY

Using Strong Name tool
The Sign Tool is a command-line tool that digitally signs files, verifies signatures in files, or time stamps files.
The Sign Tool is not supported on Microsoft Windows NT, Windows Me, Windows 98, or Windows 95.
In case you aren’t using the “Visual Studio Command Prompt” (Start >> Microsoft Visual Studio 2010 >> Visual
Studio Tools >> Visual Studio Command Prompt (2010)) you can locate sn.exe at %ProgramFiles%\Microsoft
SDKs\Windows\v7.0A\bin\sn.exe
The following command creates a new, random key pair and stores it in keyPair.snk.
sn -k keyPair.snk
The following command stores the key in keyPair.snk in the container MyContainer in the strong name CSP.
sn -i keyPair.snk MyContainer
The following command extracts the public key from keyPair.snk and stores it in publicKey.snk.
sn -p keyPair.snk publicKey.snk
The following command displays the public key and the token for the public key contained in publicKey.snk.
sn -tp publicKey.snk
The following command verifies the assembly MyAsm.dll.
sn -v MyAsm.dll
The following command deletes MyContainer from the default CSP.
sn -d MyContainer
Using the Assembly Linker(AI.exe)

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This tool is automatically installed with Visual Studio and with the Windows SDK. To run the tool, we recommend that you use the Visual Studio Command Prompt or the Windows SDK Command Prompt (CMD Shell).
These utilities enable you to run the tool easily, without navigating to the installation folder. For more information, see Visual Studio and Windows SDK Command Prompts.
If you have Visual Studio installed on your computer:On the taskbar, click Start, click All Programs, click Visual Studio, click Visual Studio Tools, and then click Visual
Studio Command Prompt.
-orIf you have the Windows SDK installed on your computer:On the taskbar, click Start, click All Programs, click the folder for the Windows SDK, and then click Command
Prompt (or CMD Shell).
At the command prompt, type the following:
al sources options
Remarks
All Visual Studio compilers produce assemblies. However if you have one or more modules (metadata withouta manifest) you can use Al.exe to create an assembly with the manifest in a separate file.
To install assemblies in the cache, remove assemblies from the cache, or list the contents of the cache, use the
Global Assembly Cache Tool (Gacutil.exe).
The following command creates an executable file t2a.exe with an assembly from the t2.netmodule module.
The entry point is the Main method in MyClass.
al t2.netmodule /target:exe /out:t2a.exe /main:MyClass.Main
Use Assembly attributes
You can insert the strong name information in the code directly. For this, depending on where the key file islocated you can use AssemblyKeyFileAttribute or AssemblyKeyNameAttribute
Use Compiler options :use /keyfile or /delaysign
Safeguarding the key pair from developers is necessary to maintain and guarantee the integrity of the assemblies. The public key should be accessible, but access to the private key is restricted to only a few individuals.
When developing assemblies with strong names, each assembly that references the strong-named target assembly contains the token of the public key used to give the target assembly a strong name. This requires thatthe public key be available during the development process.
You can use delayed or partial signing at build time to reserve space in the portable executable (PE) file for thestrong name signature, but defer the actual signing until some later stage (typically just before shipping theassembly).
You can use /keyfile or /delaysign in C# and VB.NET (MSDN)
References• http://msdn.microsoft.com/en-us/library/wd40t7ad(v=vs.80).aspx• http://msdn.microsoft.com/en-us/library/c405shex(v=vs.110).aspx• http://msdn.microsoft.com/en-us/library/k5b5tt23(v=vs.80).aspx

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11.10 Round Tripping
Round Tripping is a reverse engineering technique that allows and attacker to decompile an assembly from acertain application. Ildasm.exe can be used for this purpose, and ILAsm is used to recompiled the assembly.
The MSIL Disassembler( Ildasm.exe) is a companion tool to the MSIL Assembler (Ilasm.exe). Ildasm.exe takes aportable executable (PE) file that contains Microsoft intermediate language (MSIL) code and creates a text file
suitable as input to Ilasm.exe. This tool is automatically installed with Visual Studio and with the Windows SDK.
The importance of Obfuscation
As mentioned before, Round Tripping is a technique used to reverse engineer assemblies. Therefore, if you
want to avoid your assemblies being reversed engineered or even worse, that the code is victim of malicious
manipulation using the Ildasm and Ilasm tools, then it is advisable to apply it. There are different kinds of products that can be used for this purpose such as DeepSea, Crypto or Dotfuscator.
Using Obfuscation
The most effective technique used to avoid reverse engineering and tampering of assemblies is the use of
Obfuscation. Visual Studio contains a version of Dotfuscator. This program is accessible by choosing on the VS
menu, Tools Dotfuscator(Community Edition menu command). Note: This tools is not available in Express
versions
To obfuscate your assemblies:• Build the project in VS Studio• Tools--> Dotfuscator Community Edition• A screen prompts asking for which project type, choose ‘Creat New Project’ and click OK• On the Input tab of the Dotfuscator interface, click ‘Browse and Add assembly to list’Browse for the compiled application
ASPNetConfigs
Introduction
Securing resources in ASP.NET applications is a combination of configuration settings in the Web.config file
but also, it’s important to remember that the IIS configurations play also a big part on this. It’s an integrated
approach which provides a total framework of security.
The following highlights the most important aspects of ASP.NET configuration settings within the web.config
file. For a total overview see chapter ASP.NET security (https://www.owasp.org/index.php/CRV2_FrameworkSpecIssuesASPNet)
Secure Configuration Values
Sensitive Information saved in config files should be encrypted. Encryption keys stored in the machineKey
element for example or connectionstrings with username and passwords to login to database.
Lock ASP.NET Configuration settings
You can lock configuration settings in ASP.NET configuration files (Web.config files) by adding an allowOverride attribute to a location element

A5 - Security Misconfiguration

Configure directories using Location Settings
Through the <location> element you can establish settings for specific folders and files. The Path attribute isused to specify the file or subdirectory. This is done in the Web.config file example:
Sample 11.29<location path=”.” ><section1 .../><section2 ... /></location><location path=”Default Web Site” ><section1 … /><section2 … /></location<location path=”Default Web Site/MyApplication/Admin/xyz.html” ><section1 ... /><section2 ... /></location>

Configure exceptions for Error Code handlingShowing and handling the correct error code when a user sends a bad request or invalid parameters is an important configuration subject. Logging these errors are also an excellent help when analyzing potential attacks
to the application.
It is possible to configure these errors in the code or in the Web.Config file
The HttpException method describes an exception that occurred during the processing of HTTP requests.For
example:Sample 11.30
if (string.IsNullOrEmpty(Request[“id”]))
throw new HttpException(400, “Bad request”);

or in the Web.config file:
Sample 11.31<configuration><system.web><customErrors mode=”On” defaultRedirect=”ErrorPage.html”
redirectMode=”ResponseRewrite”>

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A5 - Security Misconfiguration<error statusCode=”400” redirect=”BadRequest.html” /><error statusCode=”404” redirect=”FileNotFound.html” /></customErrors></system.web></configuration>

Input validation
Anything coming from external sources can be consider as input in a web application. Not only the user inserting data through a web form, but also data retrieved from a web service or database, also headers sentfrom the browsers fall under this concept. A way of defining when input is safe can be done through outlining
a trust boundary.
Defining what is known as trust boundary can help us to visualize all possible untrusted inputs. One of thoseare user input.ASP.NET has different types of validations depending on the level of control to be applied. By
default, web pages code is validated against malicious users. The following is a list types of validations used
(MSDN, 2013):
Figure 10: IIS Input Validation
Type of validation

Control to use

Description

Required entry

RequiredFieldValidator

Ensures that the user does not skip an entry.

Comparison to a value

CompareValidator

Compares a user’s entry against a constant value, against the value of anothercontrol (using a comparison operator such as less than, equal, or greater than),or for a specific data type.

Range checking

RangeValidator

Checks that a user’s entry is between specified lower and upper boundaries.
You can check ranges within pairs of numbers, alphabetic characters, and dates.

Pattern matching

RegularExpressionValidator

Checks that the entry matches a pattern defined by a regular expression. This
type of validation enables you to check for predictable sequences of characters,such as those in e-mail addresses, telephone numbers, postal codes, and so on.

User-defined

CustomValidator

Checks the user’s entry using validation logic that you write yourself. This typeof validation enables you to check for values derived at run time.

References
MSDN, 2013 “Securing ASP.NET Configurations” available at
http://msdn.microsoft.com/en-us/library/ms178699%28v=vs.100%29.aspx (Last Viewed, 25th July 2013)11.11 .NET Authentication Controls
In the .NET, there are Authentication tags in the configuration file. The <authentication> element configures
the authentication mode that your applications use. The appropriate authentication mode depends on how
your application or Web service has been designed. The default Machine.config setting applies a secure Windows authentication default as shown below
authentication Attributes:mode=”[Windows|Forms|Passport|None]”<authentication mode=”Windows” />A5 - Security Misconfiguration

Forms Authentication Guidelines
To use Forms authentication, set mode=“Forms” on the <authentication> element. Next,
configure Forms authentication using the child <forms> element. The following fragment shows a secure<forms> authentication element configuration:
Sample 11.32<authentication mode=”Forms”><forms loginUrl=”Restricted\login.aspx” Login page in an SSL protected folder
protection=”All” Privacy and integrity
requireSSL=”true” Prevents cookie being sent over httptimeout=”10” Limited session lifetime
name=”AppNameCookie” Unique per-application name
path=”/FormsAuth” and pathslidingExpiration=”true” > Sliding session lifetime</forms></authentication>

Use the following recommendations to improve Forms authentication security:• Partition your Web site.
• Set protection=“All”.
• Use small cookie time-out values.
• Consider using a fixed expiration period.
• Use SSL with Forms authentication.
• If you do not use SSL, set slidingExpiration = “false”.
• Do not use the <credentials> element on production servers.
• Configure the <machineKey> element.
• Use unique cookie names and paths.
Classic ASP
For classic ASP pages, authentication is usually performed manually by including the user information in session variablesafter validation against a DB, so you can look for something like:
Session (“UserId”) = UserName
Session (“Roles”) = UserRoles
Code Review .Net Manage Code.NET Managed code is less vulnerable to common vulnerabilities found in unmanaged code such as Buffer
Overflows and memory corruption however there could be issues in the code that can affect performance andsecurity. The following is a summary of the recommended practices to look for during the code review. Also, it
is worth mentioning some tools that can make the work easier on this part and they can help you understandand pin point flaws in your code
Code Access Security
This supports the execution of semi-trusted code, preventing several forms of security threats. The following isa summary of possible vulnerabilities due to improper use of Code Access security:

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Table 16: Code Access Security Vulnerabilities
Vulnerability

Implications

Improper use of link demands or asserts

The code is susceptible to luring attacks

Code allows untrusted callers

Malicious code can use the code to perform sensitive operations and access resources

Declarative security
Use declarative security instead of imperative whenever possible.
Example of declarative syntax(MSDN[2], 2013):
Sample 11.33[MyPermission(SecurityAction.Demand, Unrestricted = true)]
public class MyClass{
public MyClass(){//The constructor is protected by the security call.
}public void MyMethod(){//This method is protected by the security call.
}public void YourMethod(){//This method is protected by the security call.
}}

Unmanaged code
Even though C# is a strong type language, it is possible to use unmanaged code calls by using the ‘unsafe’
code. “Check that any class that uses an unmanaged resource, such as a database connection across method
calls, implements the IDisposable interface. If the semantics of the object are such that a Close method is morelogical than a Dispose method, provide a Close method in addition to Dispose”.
Exception handlingManage code should use exception handling for security purposes among other reasons. Make sure that you
follow these recommendations:*Avoid exception handling in loops, use try/catch block if it is necessary.
*Identify code that swallows exceptions*Use exceptions handling for unexpected conditions and not just to control the flow in the application
Tools
FxCop
FxCop is an analysis tool that analyses binary assemblies, not source code. The tool has a predefined set ofrules and it is possible to configure and extend them.
Some of the available rules regarding security are (CodePlex, 2010):

A5 - Security Misconfiguration

Table 17: FxCop Flags
Rule

Description

EnableEventValidationShouldBeTrue

Verifies if the EnableEventValidation directive is disabled on a certain page.

ValidateRequestShouldBeEnabled

Verifies if the ValidateRequest directive is disabled on a certain page.

ViewStateEncryptionModeShouldBeAlways

Verifies if the ViewStateEncryptionMode directive is not set to Never on a certain page.

EnableViewStateMacShouldBeTrue

Verifies if the EnableViewStateMac directive is not set to false on a certain page.

EnableViewStateShouldBeTrue

Verifies if the EnableViewState directive is not set to false on a certain page.

ViewStateUserKeyShouldBeUsed

Verifies if the Page.ViewStateUserKey is being used in the application to prevent CSRF.

DebugCompilationMustBeDisabled

Verifies that debug compilation is turned off. This eliminates potential performance and security issues related to debug code enabled and additional extensive error messages being
returned.

CustomErrorPageShouldBeSpecified

Verifies that the CustomErrors section is configured to have a default URL for redirecting uses
in case of error.

FormAuthenticationShouldNotContainFormAuthentica
tionCredentials

Verifies that no credentials are specified under the form authentication configuration.

EnableCrossAppRedirectsShouldBeTrue

Verifies that system.web.authentication.forms enableCrossAppRedirects is set to true. The
settings indicate if the user should be redirected to another application url after the authentication process. If the setting is false, the authentication process will not allow redirection toanother application or host. This helps prevent an attacker to force the user to be redirected
to another site during the authentication process. This attack is commonly called Open redirect and is used mostly during phishing attacks.

FormAuthenticationProtectionShouldBeAll

Verifies that the protection attribute on the system.web.authentication.forms protection is
set to All which specifies that the application use both data validation and encryption to help
protect the authentication cookie.

FormAuthenticationRequireSSLShouldBeTrue

Verifies that the requireSSL attribute on the system.web.authentication.forms configuration
element is set to True which forces the authentication cookie to specify the secure attribute.
This directs the browser to only provide the cookie over SSL.

FormAuthenticationSlidingExpirationShouldBeFalse

Verifies that system.web.authentication.forms slidingExpiration is set to false when the site
is being served over HTTP. This will force the authentication cookie to have a fixed timeout
value instead of being refreshed by each request. Since the cookie will traverse over clear text
network and could potentially be intercepted, having a fixed timeout value on the cookie
will limit the amount of time the cookie can be replayed. If the cookie is being sent only over
HTTPS, it is less likely to be intercepted and having the slidingExpiration setting to True willcause the timeout to be refreshed after each request which gives a better user experience.

HttpCookiesHttpOnlyCookiesShouldBeTrue

Verifies that the system.web.httpCookies httpOnlyCookies configuration setting is set to True
which forces all cookies to be sent with the HttpOnly attribute.

HttpCookiesRequireSSLShouldBeTrue

Verifies that the system.web.httpCookies requireSSL configuration is set to True which forces
all cookies to be sent with the secure attribute. This indicates the browser to only provide thecookie over SSL.

TraceShouldBeDisabled

Verifies that the system.web.trace enabled setting is set to false which disables tracing. It is
recommended to disable tracing on production servers to make sure that an attacker cannot
gain information from the trace about your application. Trace information can help an attacker probe and compromise your application.

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Rule

Description

AnonymousAccessIsEnabled

Looks in the web.config file to see if the authorization section allows anonymous access.

RoleManagerCookieProtectionShouldBeAll

Verifies that the system.web.rolemanager cookieProtection is set to All which enforces thecookie to be both encrypted and validated by the server.

RoleManagerCookieRequireSSLShouldBeTrue

Verifies that the system.web.rolemanager cookieRequireSSL attribute is set to True which
forces the role manager cookie to specify the secure attribute. This directs the browser to only
provide the cookie over SSL.

RoleManagerCookieSlidingExpirationShouldBeTrue

Verifies that the system.web.rolemanager cookieSlidingExpiration is set to false when the site
is being served over HTTP. This will force the authentication cookie to have a fixed timeout
value instead of being refreshed by each request. Since the cookie will traverse over clear text
network and could potentially be intercepted, having a fixed timeout value on the cookie
will limit the amount of time the cookie can be replayed. If the cookie is being sent only over
HTTPS, it is less likely to be intercepted and having the slidingExpiration setting to True willcause the timeout to be refreshed after each request which gives a better user experience.

PagesEnableViewStateMacShouldBeTrue

Verifies that the viewstate mac is enabled.

PagesEnableEventValidationMustBeTrue

Verifies that event validation is enabled.

HttpRuntimeEnableHeaderCheckingShouldBeTrue

Verifies that the system.web.httpRuntime enableHeaderChecking attribute is set to true. The
setting indicates whether ASP.NET should check the request header for potential injection
attacks. If an attack is detected, ASP.NET responds with an error. This forces ASP.NET to apply
the ValidateRequest protection to headers sent by the client. If an attack is detected the application throws HttpRequestValidationException.

PagesValidateRequestShouldBeEnabled

Verify that validateRequest is enabled.

PagesViewStateEncryptionModeShouldBeAlways

Verifies that the viewstate encryption mode is not configured to never encrypt.

CustomErrorsModeShouldBeOn

Verifies that the system.web.customErrors mode is set to On or RemoteOnly. This disable detailed error message returned by ASP.NET to remote users.

MarkVerbHandlersWithValidateAntiforgeryToken

ValidateAntiforgeryTokenAttribute is used to protect against potential CSRF attacks against
ASP.NET MVC applications.

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A6

SENSITIVE DATA EXPOSURE

Many web applications do not properly protect sensitive data, such as credit cards, tax IDs, and authentication
credentials. Attackers may steal or modify such weakly protected data to conduct credit card fraud, identity
theft, or other crimes. Sensitive data deserves extra protection such as encryption at rest or in transit, as well
as special precautions when exchanged with the browser.
12.1 Cryptographic Controls
Software developers, architects and designers are at the forefront of deciding which category a particularapplication resides in. Cryptography provides for security of data at rest (via encryption), enforcement of data
integrity (via hashing/digesting), and non-repudiation of data (via signing). To ensure this cryptographic codeadequately protections the data, all source code must use a standard (secure) algorithms with strong key sizes.
Common flaws when implementing cryptographic code includes the use of non-standard cryptographic algorithms, custom implementation of cryptography (standard & non-standard) algorithms, use of standard algorithms which are cryptographically insecure (e.g. DES), and the implementation of insecure keys can weaken
the overall security posture of any application. Implementation of the aforementioned flaws enable attackers
to use cryptanalytic tools and techniques to decrypt sensitive data.
12.2 Description
Many companies handle sensitive information for their customers, for instance medical details or credit card numbers,
and industry regulations dictate this sensitive information must be encrypted to protect the customers’ information.
In the medical industry the HIPAA regulations advise businesses what protections must be applied to medical data,in the financial industry many regulations cover PII (personally identifiable information) controls.
Regardless of the financial impact of regulatory penalties, there are many business reasons to protect (though encryption or hashing) the information processed by an application, including privacy and fraud detection/protection.
All sensitive data that the application handles should be identified and encryption should be enforced. Similarly a decision should be made as to whether sensitive data must be encrypted in transit (i.e. being sent fromone computer to another) and/or at rest (i.e. stored in a DB, file, keychain, etc.):
1) Protection in transit; this typically means using the SSL/TLS layer to encrypt data travelling on the HTTP
protocol, although it can also include FTPS, or even SSL on TCP. Frameworks such as IIS and Apache Struts
come with SSL/TLS functionality included, and thus the developer will not be coding the actual TLS encryption, but instead will be configuring a framework to provide TLS security.
However the decisions made here, even at an architectural level, need to be well informed, and a discussion on
TLS design decisions is covered in section 1.3.
2) Protection at rest; this can include encryption of credit cards in the database, hashing of passwords,
producing message authentication codes (MACs) to ensure a message has not been modified between computers. Where TLS code will come with a framework, code to encrypt or hash data to be stored will typicallyneed to use APIs provided by cryptographic libraries.
The developer will not be writing code to implement the AES algorithm (OpenSSL or CryptoAPI will do that), thedeveloper will be writing modules to use an AES implantation in the correct way. Again the correct decisions need tobe made regarding up-to-date algorithms, key storage, and other design decisions, which are covered in section 1.4.

A6 - Sensitive Data Exposure

Cryptography Definitions
Before diving into discussions on encrypting traffic and data, some terminology used in the realm of cryptography is defined in table 18.
Table 18: Cryptographic Definitions
Term

Description

Encoding

Transforming data from one form into another, typically with the aim of making the data easier to work with. For example encoding binary data (which could not be printed to a screen into printable ASCII format which can be copy/
pasted. Note that encoding does not aim to hide the data, the method to return the encoded data back to its originalform will be publically known.

Entropy

Essentially this is randomness. Cryptographic functions will need to work with some form of randomness to allow thesource data to be encrypted in such a way that an attacker cannot reverse the encryption without the necessary key.
Having a good source of entropy is essential to any cryptographic algorithm.

Hashing

Non-reversible transformation of data into what is called a ‘fingerprint’ or ‘hashvalue’. Input of any size can be taken
and always results in the same size of output (for the algorithm). The aim is not to convert the fingerprint back into thesource data at a later time, but to run the hash algorithm over two sets of data to determine if they produce the same
fingerprint. This would show that data has not been tampered with.

Salt

A non-secret value that can be added to a hashing algorithm to modify the fingerprint result. One attack againsthashing algorithms is a ‘rainbow table attack’ where all source values are pre-computed and a table produced. The
attacker can then take a fingerprint, look it up in the table, and correspond it to the original data. Using a unique salt
value for each data to be hashed protects against rainbow tables, as a rainbow table for each salt value would need tobe created, which would greatly extend the time taken by an attacker. The salt is not a secret and can be stored or sentwith the fingerprint.

Encryption

Transformation of source data into an encrypted form that can be reversed back to the original source. Typically thealgorithms used to encrypt are publically known, but rely on a secret ‘key’ to guide the transformation. An attacker
without the key should not be able to transform the data back into the original source.

Symmetric Encryption

A form of encryption where the same key is known to both the sender and the receiver. This is a fast form of encryption,
however requires a secure, out-of-band method to pass the symmetric key between the sender and receiver.

Public-Key Encryption(PKI)

A form of encryption using two keys, one to encrypt the data, and one to decrypt the data back to its original form. Thisis a slower method of encryption however one of the keys can be publically known (referred to as a ‘public key’). The
other key is called a ‘private key’ and is kept secret. Any data encrypted with the public key can be decrypted back into
its original form using the private key. Similarly any data encrypted with the private key can be decrypted back to its
original form using the public key.

Certificate

An association between an entity (e.g. person, company) and a public key. Typically this forms part of a public-key
infrastructure where certain trusted entities (e.g. Certificate Authorities in internet TLS) perform validation of an entitles
credentials and assert (using their own certificate) that a declared public key belongs to the entity.

12.3 What to Review: Protection in Transit
The terms Secure Socket Layer (SSL) and Transport Layer Security (TLS) are often used interchangeably. In
fact, SSL v3.1 is equivalent to TLS v1.0. However, different versions of SSL and TLS are supported by modern
web browsers and by most modern web frameworks and platforms. Note that since developments in attacks
against the SSL protocol have shown it to be weaker against attacks, this guide will use the term TLS to refer totransport layer security over the HTTP or TCP protocols.
The primary benefit of transport layer security is the protection of web application data from unauthorized
disclosure and modification when it is transmitted between clients (web browsers) and the web application
server, and between the web application server and back end and other non-browser based enterprise components.
In theory, the decision to use TLS to protect computer to computer communication should be based on thenature of the traffic or functionality available over the interface. If sensitive information is passing over the

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A6 - Sensitive Data Exposureinterface, TLS will prevent eavesdroppers from being able to view or modify the data. Likewise if the interface
allows money to be transferred, or sensitive functions to be initiated, then TLS will protect the associated login
or session information authorizing the user to perform those functions. However with the price of certificates
dropping, and TLS configuration within frameworks becoming easier, TLS protection of an interface is not alarge endeavor and many web sites are using TLS protections for their entire site (i.e. there are only HTTPS
pages, no HTTP pages are available).
The server validation component of TLS provides authentication of the server to the client. If configured to require client side certificates, TLS can also play a role in client authentication to the server. However, in practiceclient side certificates are not often used in lieu of username and password based authentication models forclients.
Using Validated Implementations
The US government provides a list of software that has been validated to provide a strong and secure implementation of various cryptographic functions, including those used in TLS. This list is referred to as the FIPS
140-2 validated cryptomodules.
A cryptomodule, whether it is a software library or a hardware device, implements cryptographic algorithms(symmetric and asymmetric algorithms, hash algorithms, random number generator algorithms, and message
authentication code algorithms). The security of a cryptomodule and its services (and the web applications
that call the cryptomodule) depend on the correct implementation and integration of each of these threeparts. In addition, the cryptomodule must be used and accessed securely. In order to leverage the benefits of
TLS it is important to use a TLS service (e.g. library, web framework, web application server) which has been
FIPS 140-2 validated. In addition, the cryptomodule must be installed, configured and operated in either anapproved or an allowed mode to provide a high degree of certainty that the FIPS 140-2 validated cryptomodule is providing the expected security services in the expected manner.
When reviewing designs or code that is handling TLS encryption, items to look out for include:• Use TLS for the login pages and any authenticated pages. Failure to utilize TLS for the login landing page
allows an attacker to modify the login form action, causing the user’s credentials to be posted to an arbitrary
location. Failure to utilize TLS for authenticated pages after the login enables an attacker to view the unencrypted session ID and compromise the user’s authenticated session.
• Use TLS internally when transmitting sensitive data or exposing authenticated functionality. All networks,
both external and internal, which transmit sensitive data must utilize TLS or an equivalent transport layer security mechanism. It is not sufficient to claim that access to the internal network is “restricted to employees”.
Numerous recent data compromises have shown that the internal network can be breached by attackers. In
these attacks, sniffers have been installed to access unencrypted sensitive data sent on the internal network.
• Prefer all interfaces (or pages) being accessible only over HTTPS. All pages which are available over TLS mustnot be available over a non-TLS connection. A user may inadvertently bookmark or manually type a URL to a
HTTP page (e.g. http://example.com/myaccount) within the authenticated portion of the application.
• Use the “secure” and “http-only” cookie flags for authentication cookies. Failure to use the “secure” flag
enables an attacker to access the session cookie by tricking the user’s browser into submitting a request to an
unencrypted page on the site. The “http-only” flag denies JavaScript functions access to the cookies contents.
• Do not put sensitive data in the URL. TLS will protect the contents of the traffic on the wire, including the URL
when transported, however remember that URL’s are visible in browser history settings, and typically written

A6 - Sensitive Data Exposure

to server logs.
• Prevent the caching of sensitive data. The TLS protocol provides confidentiality only for data in transit but it
does not help with potential data leakage issues at the client or intermediary proxies.
• Use HTTP Strict Transport Security (HSTS) for high risk interfaces. HSTS will prevent any web clients fromattempting to connect to your web site over a non-TLS protocol. From a server-side point of view this may
seem irrelevant if no non-TLS pages are provided, however a web site setting up HSTS does protect clientsfrom other attacks (e.g. DNS cache poisioning).
• Use 2048 key lengths (and above) and SHA-256 (and above). The private key used to generate the cipher key
must be sufficiently strong for the anticipated lifetime of the private key and corresponding certificate. The
current best practice is to select a key size of at least 2048 bits. Note that attacks against SHA-1 have shown
weakness and the current best practice is to use at least SHA-256 or equivalent.
• Only use specified, fully qualified domain names in your certificates. Do not use wildcard certificates, or RFC
1918 addresses (e.g. 10.* or 192.168.*). If you need to support multiple domain names use Subject Alternate
Names (SANs) which provide a specific listing of multiple names where the certificate is valid. For example thecertificate could list the subject’s CN as example.com, and list two SANs: abc.example.com and xyz.example.
com. These certificates are sometimes referred to as “multiple domain certificates”.
• Always provide all certificates in the chain. When a user receives a server or host’s certificate, the certificate
must be validated back to a trusted root certification authority. This is known as path validation. There canbe one or more intermediate certificates in between the end-entity (server or host) certificate and root certificate. In addition to validating both endpoints, the client software will also have to validate all intermediate
certificates, which can cause failures if the client does not have the certificates. This occurs in many mobile
platforms.
12.4 What to Review: Protection at Rest
As a general recommendation, companies should not create their own custom cryptographic libraries and algorithms. There is a huge distinction between groups, organizations, and individuals developing cryptographic algorithms and those that implement cryptography either in software or in hardware. Using an establishedcryptographic library that has been developed by experts and tested by the industry is the safest way to implement cryptographic functions in a company’s code. Some common examples of libraries used in various
languages and environments are covered in the below table.
Table 19: Popular Cryptographic implementations according to environment
Language

Libraries

Discussion

C# .NETClass libraries
within ‘System.Security.
Cryptography’

For applications coded in C#.NET there are class libraries and implementations within the ‘System.Security.Cryptography’ that should be used. This namespace within .NET aims to provide a number of warppers that do not require
proficient knowledge of cryptography in order to use it.

C/C++(Win32)CryptoAPI
and DPAPIFor C/C++ code running on Win32 platforms, the CreyptoAPI and DPAPI are recommended.

C/C++(Linux)OpenSSL,
NSS, boringssl

For C/C++ on Linux/Unix operating systems, us OpenSSL, NSS, or one of the many forks of these libraries.

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ASP

CryptoAPI
and DPAPIClassis ASP pages do not have direct access to cryptographic functions, so the only way is to create COM wrappers in
Visual C++ or Visual Basic, implementing calls to CryptoAPI or DPAPI. Then call them from ASP pages using the Server.
CreateObject method.

Java

Java Cryptography
Extension,
BouncyCastle, Spring
Security

JCE is a standard API that any cryptographic library can implement to provide cryptographic functions to the developer.
Oracle provide a list of companies that act as Cryptographic Service Providers and/or offer clean room implementationsof the JCE. BouncyCastle is on of the more popular implementations. Spring Secuirty is also popular in application
where Spring is already being utilized.

A secure way to implement robust encryption mechanisms within source code is by using FIPS [7] compliant algorithms with the use of the Microsoft Data Protection API (DPAPI) [4] or the Java Cryptography Extension (JCE) [5].
A company should identify minimum standards for the following when establishing your cryptographic code
strategy:• Which standard algorithms are to be used by applications• Minimum key sizes to be supported• What types of data must be encrypted
When reviewing code handling cryptography look out for:• Is strong enough encryption algorithms being used, and is the implementation of those algorithms FIPS-140 complaint.
• Is the right type of cryptographic algorithm being used, is data being hashed that should be encrypted with asymmetric key? If there is no way to safely transfer the symmetric key to the other party, is public key cryptographic
algorithms being employed?
• In any cryptographic system the protection of the key is the most important aspect. Exposure of the symmetric or
private key means the encrypted data is no longer private. Tightly control who has access to enter or view the keys,
and how the keys are used within applications.
• Any code implementing cryptographic processes and algorithms should be reviewed and audited against a set ofcompany or regulatory specifications. High level decisions need to be made (and continually revisited) as to what anorganization considers ‘strong encryption’ to be, and all implementation instances should adhere to this standard.
• Cryptographic modules must be tested under high load with multithreaded implementations, and each piece ofencrypted data should be checked to ensure it was encrypted and decrypted correctly.
• In .Net check for examples of cryptography in the MSDN Library Security Practices: .NET Framework 2.0 Security
Practices at a Glanceo Check that the Data Protection API (DPAPI) is being used.
o Verify no proprietary algorithms are being used.
o Check that RNGCryptoServiceProvider is used for PRNG.
o Verify key length is at least 128 bits.
• In ASP perform all of these checks on the COM wrapper as ASP does not have direct access to cryptographic functions
o Check that the Data Protection API (DPAPI) or CryptoAPI is being used into COM object• P122 - bslightly incorrect
points indents quickly on a call.

bottom of page isin terms of bullet
again let’s coverA6 - Sensitive Data Exposure

o Verify no proprietary algorithms are being used
o Check that RNGCryptoServiceProvider is used for PRNG
o Verify key length is at least 128 bits
• For Java check that the Java Cryptography Extension (JCE) is being usedo Verify no proprietary algorithms are being used
o Check that SecureRandom (or similar) is used for PRNG
o Verify key length is at least 128 bits
Bad Practice: Use of Insecure Cryptographic Algorithms
The DES and SHA-0 algorithms are cryptographically insecure. The example in sample 12.1 outlines a cryptographic
module using DES (available per using the Java Cryptographic Extensions) which should not be used. Additionally,
SHA-1 and MD5 should be avoided in new applications moving forward.
Sample 12.1
package org.badexample.crypto;<snip>
try {/** Step 1. Generate a DES key using KeyGenerator */KeyGenerator keyGen = KeyGenerator.getInstance(“DES”);
SecretKey secretKey = keyGen.generateKey();/** Step2. Create a Cipher by specifying the following parameters*
a. Algorithm name - here it is DES*
b. Mode - here it is CBC*
c. Padding - PKCS5Padding */Cipher desCipher = Cipher.getInstance(“DES/CBC/PKCS5Padding”);<snip>

Good Practice: Use Strong Entropy
The source code in sample 12.2 outlines secure key generation per use of strong entropy:
Sample 12.2
package org.owasp.java.crypto;
import java.security.SecureRandom;
import java.security.NoSuchAlgorithmException;
import sun.misc.BASE64Encoder;/*** This program provides the functionality for Generating a Secure Random Number.
* There are 2 ways to generate a Random number through SecureRandom.
* 1. By calling nextBytes method to generate Random Bytes

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A6 - Sensitive Data Exposure* 2. Using setSeed(byte[]) to reseed a Random object*/
public class SecureRandomGen {public static void main(String[] args) {try {// Initialize a secure random number generator
SecureRandom secureRandom = SecureRandom.getInstance(“SHA512”);// Method 1 - Calling nextBytes method to generate Random Bytes
byte[] bytes = new byte[512];
secureRandom.nextBytes(bytes);// Printing the SecureRandom number by calling secureRandom.nextDouble()
System.out.println(“ Secure Random # generated by calling nextBytes() is “ + secureRandom.
nextDouble());// Method 2 - Using setSeed(byte[]) to reseed a Random object
int seedByteCount = 10;byte[] seed = secureRandom.generateSeed(seedByteCount);
secureRandom.setSeed(seed);
System.out.println(“ Secure Random # generated using setSeed(byte[]) is “ + secureRandom.
nextDouble());} catch (NoSuchAlgorithmException noSuchAlgo){
System.out.println(“ No Such Algorithm exists “ + noSuchAlgo);}}}
Good Practice: Use Strong Algorithms
Below illustrates the implementation of AES (available per Using the Java Cryptographic Extensions):
Sample 12.3
package org.owasp.java.crypto;
import javax.crypto.KeyGenerator;
import javax.crypto.SecretKey;
import javax.crypto.Cipher;
import java.security.NoSuchAlgorithmException;
import java.security.InvalidKeyException;
import java.security.InvalidAlgorithmParameterException;
import javax.crypto.NoSuchPaddingException;

A6 - Sensitive Data Exposure

import javax.crypto.BadPaddingException;
import javax.crypto.IllegalBlockSizeException;
import sun.misc.BASE64Encoder;/*** This program provides the following cryptographic functionalities* 1. Encryption using AES* 2. Decryption using AES** High Level Algorithm :* 1. Generate a DES key (specify the Key size during this phase)* 2. Create the Cipher* 3. To Encrypt : Initialize the Cipher for Encryption* 4. To Decrypt : Initialize the Cipher for Decryption*/
public class AES {public static void main(String[] args) {String strDataToEncrypt = new String();
String strCipherText = new String();
String strDecryptedText = new String();try{/*** Step 1. Generate an AES key using KeyGenerator*
Initialize the keysize to 128*/
KeyGenerator keyGen = KeyGenerator.getInstance(“AES”);
keyGen.init(128);
SecretKey secretKey = keyGen.generateKey();/*** Step2. Create a Cipher by specifying the following parameters*
a. Algorithm name - here it is AES*/
Cipher aesCipher = Cipher.getInstance(“AES”);/*** Step 3. Initialize the Cipher for Encryption*/
aesCipher.init(Cipher.ENCRYPT_MODE,secretKey);/*** Step 4. Encrypt the Data*
1. Declare / Initialize the Data. Here the data is of type String*
2. Convert the Input Text to Bytes

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3. Encrypt the bytes using doFinal method*/
strDataToEncrypt = “Hello World of Encryption using AES “;byte[] byteDataToEncrypt = strDataToEncrypt.getBytes();byte[] byteCipherText = aesCipher.doFinal(byteDataToEncrypt);
strCipherText = new BASE64Encoder().encode(byteCipherText);
System.out.println(“Cipher Text generated using AES is “ +strCipherText);/***/ Step 5. Decrypt the Data*
1. Initialize the Cipher for Decryption*
2. Decrypt the cipher bytes using doFinal method*/
aesCipher.init(Cipher.DECRYPT_MODE,secretKey,aesCipher.getParameters());
byte[] byteDecryptedText = aesCipher.doFinal(byteCipherText);
strDecryptedText = new String(byteDecryptedText);
System.out.println(“ Decrypted Text message is “ +strDecryptedText);}
catch (NoSuchAlgorithmException noSuchAlgo){
System.out.println(“ No Such Algorithm exists “ + noSuchAlgo);}
catch (NoSuchPaddingException noSuchPad){
System.out.println(“ No Such Padding exists “ + noSuchPad);}
catch (InvalidKeyException invalidKey){
System.out.println(“ Invalid Key “ + invalidKey);}
catch (BadPaddingException badPadding){
System.out.println(“ Bad Padding “ + badPadding);}
catch (IllegalBlockSizeException illegalBlockSize){
System.out.println(“ Illegal Block Size “ + illegalBlockSize);}
catch (InvalidAlgorithmParameterException invalidParam){
System.out.println(“ Invalid Parameter “ + invalidParam);}}}A6 - Sensitive Data Exposure

1.1.4 References[1] Bruce Schneier, Applied Cryptography, John Wiley & Sons, 2nd edition, 1996.
[2] Michael Howard, Steve Lipner, The Security Development Lifecycle, 2006, pp. 251 - 258[3] .NET Framework Developer’s Guide, Cryptographic Services, http://msdn2.microsoft.com/en-us/library/93bskf9z.aspx[4] Microsoft Developer Network, Windows Data Protection, http://msdn2.microsoft.com/en-us/library/ms995355.aspx[5] Sun Developer Network, Java Cryptography Extension, http://java.sun.com/products/jce/[6] Sun Developer Network, Cryptographic Service Providers and Clean Room Implementations, http://java.sun.
com/products/jce/jce122_providers.html[7] Federal Information Processing Standards, http://csrc.nist.gov/publications/fips/
12.5 Encryption, Hashing & Salting
A cryptographic hash algorithm, also called a hash function, is a computer algorithm designed to provide a random
mapping from an arbitrary block of data (string of binary data) and return a fixed-size bit string known as a “message
digest” and achieve certain security.
Cryptographic hashing functions are used to create digital signatures, message authentication codes (MACs), otherforms of authentication and many other security applications in the information infrastructure. They are also used
to store user passwords in databases instead of storing the password in clear text and help prevent data leakage insession management for web applications. The actual algorithm used to create a cryptology function varies per implementation (SHA-256, SHA-512, etc.)
Never accept in a code review an algorithm created by the programmer for hashing. Always use cryptographic functions that are provided by the language, framework, or common (trusted) cryptographic libraries. These functions arewell vetted and well tested by experience cryptographers.
In the United States in 2000, the department of Commerce Bureau of Export revised encryption export regulations.
The results of the new export regulations it that the regulations have been greatly relaxed. However if the code is to
be exported outside of the source country current export laws for the export and import counties should be reviewed
for compliance.
Case in point is if the entire message is hashed instead of a digital signature of the message the National Security
Agency (NSA) considers this a quasi-encryption and State controls would apply.
It is always a valid choice to seek legal advice within the organization if the code review is being done to ensure legal
compliance.
With security nothing is secure forever. This is especially true with cryptographic hashing functions. Some hashingalgorithms such as Windows LanMan hashes are considered completely broken. Others like MD5, which in the past
were considered safe for password hash usage, have known issues like collision attacks (note that collision attacks do
not affect password hashes). The code reviewer needs to understand the weaknesses of obsolete hashing functions
as well as the current best practices for the choice of cryptographic algorithms.

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Working with Salts
The most common programmatic issue with hashing is:• Not using a salt value• Using a salt the salt value is too short• Same salt value is used in multiple hashes.
The purpose of a salt is to make it harder for an attacker to perform pre-computed hashing attack (e.g., using rainbow
tables). Take for example that the SHA512 has of ‘password’ is as shown in row 1 of table 20, and any attacker with arainbow table will spot the hash value corresponding to ‘password’. Taking into consideration it takes days or weeks
to compute a rainbow table to values up to around 8 or 10 characters, the effort to produce this table is worth it when
an application is not using any salts.
Now take a scenario where an application adds a salt of ‘WindowCleaner’ to all passwords entered. Now the hash of‘password’ becomes the hash of ‘passwordWindowCleaner’, which is shown in row 2 of table 20. This is unlikely to bein the attackers rainbow table, however the attacker can now spend the next 10 days (for example) computing a new
rainbow table with ‘WindowCleaner’ on the end of every 8 to 10 character string and they again can now decode ourhashed database of passwords.
At a last step, an application can create a random salt for each entry, and store that salt in the DB with the hashedpassword. Now for user1, the random salt is ‘a0w8hsdfas8ls587uas87’, meaning the password to be hashed is ‘passworda0w8hsdfas8ls587uas87’, shown in row 3 of table 20, and for user2, the random salt is ‘8ash87123klnf9d8dq3w’,
meaning the password to be hashed is ‘password8ash87123klnf9d8dq3w’, shown in row 4 of table X, and repeat for
all users.
Now an attacker would need a rainbow table for each users’ password they mean to decrypt – whereas before it took
10 days to decrypt all of the DB passwords using the same salt, now it takes 10 days to create a rainbow table foruser1’s password, and another 10 days for user2’s password, etc. If there were 100,000 users, that’s now 100,000 x 10
days = 1,000,000 days or 2738 years, to create rainbow tables for all the users.
As can be seen, the salt does not need to be secret, as it is the fact that unique salts are used for each user thatslows down an attacker.
Table 20: Salt usages and associated fingerprints
Method to hash ‘password’Fingerprint

No salt

B109F3BBBC244EB82441917ED06D618B9008DD09B3
BEFD1B5E07394C706A8BB980B1D7785E5976EC049B
46DF5F1326AF5A2EA6D103FD07C95385FFAB0CACBC86

Salt = ‘WindowCleaner’

E6F9DCB1D07E5412135120C0257BAA1A27659D41DC7
7FE2DE4C345E23CB973415F8DFDFFF6AA7F0AE0BDD
61560FB028EFEDF2B5422B40E5EE040A0223D16F06F

Salt = ‘a0w8hsdfas8ls587uas87’

5AA762E7C83CFF223B5A00ADA939FBD186C4A2CD01
1B0A7FE7AF86B8CA5420C7A47B52AFD2FA6B9BB172
22ACF32B3E13F8C436447C36364A5E2BE998416A103A

Salt = ‘8ash87123klnf9d8dq3w’

8058D43195B1CF2794D012A86AC809BFE73254A82C8C
E6C10256D1C46B9F45700D040A6AC6290746058A63E5
0AAF8C87ABCD5C3AA00CDBDB31C10BA6D12A1A7

A6 - Sensitive Data Exposure

One way to generate a salt value is using a pseudo-random number generator, as shown in sample 12.4 below.
Sample 12.4
private int minSaltSize = 8;
private int maxSaltSize = 24;
private int saltSize;
private byte[] GetSalt(string input) {byte[] data;byte[] saltBytes;
RNGCryptoServiceProvider rng = new RNGCryptoServiceProvider();saltBytes = new byte[saltSize];
rng.GetNonZeroBytes(saltBytes);
data = Encoding.UTF8.GetBytes(input);byte[] dataWithSaltBytes =new byte[data.Length + saltBytes.Length];for (int i = 0; i < data.Length; i++)
dataWithSaltBytes[i] = data[i];for (int i = 0; i < saltBytes.Length; i++)
dataWithSaltBytes[data.Length + i] = saltBytes[i];return dataWithSaltBytes;}

Note that a salt value does not need to possess the quality of a cryptographically secure randomness.
Best practices is to use a cryptographically function to create the salt, create a salt value for each hash value,
and a minimum value of 128 bits (16 characters). The bits are not costly so don’t save a few bits thinking you
gain something back in performance instead use a value of 256-bit salt value. It is highly recommended.
Best Practices
Industry leading Cryptographer’s are advising that MD5 and SHA-1 should not be used for any applications.
The United State FEDERAL INFORMATION PROCESSING STANDARDS PUBLICATION (FIPS) specifies seven cryptographic hash algorithms — SHA-1, SHA-224, SHA-256, SHA-384, SHA-512, SHA-512/224, and SHA-512/256
are approved for federal use.
The code reviewer should consider this standard because the FIPS is also widely adopted by the information
technology industry.
The code reviewer should raise a red flag if MD5 and SHA-1 are used and a risk assessment be done to understand why these functions would be used instead of other better-suited hash functions. FIPS does allow that
MD5 can be used only when used as part of an approved key transport scheme where no security is provided
by the algorithm.
sample 12.5 below shows an example function which could implement a generic hash feature for an application.

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Sample 12.5
App Code File:<add key=”HashMethod” value=”SHA512”/>
C# Code:1: preferredHash = HashAlgorithm.Create((string)ConfigurationManager.AppSettings[“HashMethod”]);
2:
3: hash = computeHash(preferredHash, testString);
4:
5: private string computeHash(HashAlgorithm myHash, string input) {6: byte[] data;
7: data = myHash.ComputeHash(Encoding.UTF8.GetBytes(input));
8: sb = new StringBuilder();
9: for (int i = 0; i < data.Length; i++) {10:
sb.Append(data[i].ToString(“x2”));
11: }12: return sb.ToString();
13: }Line 1 lets us get our hashing algorithm we are going to use from the config file. If we use the machine config
file our implementation would be server wide instead of application specific.
Line 3 allows us to use the config value and set it according as our choice of hashing function. ComputHash
could be SHA-256 or SHA-512.
References
http://valerieaurora.org/hash.html (Lifetimes of cryptographic hash functions)
http://docs.oracle.com/javase/6/docs/api/java/security/SecureRandom.html
http://msdn.microsoft.com/en-us/library/system.security.cryptography.rngcryptoserviceprovider.aspx
http://csrc.nist.gov/publications/fips/fips180-4/fips-180-4.pdf
Ferguson and Schneier (2003) Practical Cryptography (see Chapter 6; section 6.2 Real Hash Functions)12.6 Reducing the attack surface
The Attack Surface of an application is a description of the entry/exit points, the roles/entitlements of the users, andthe sensitivity of the data held within the application. For example, entry points such as login screens, HTML forms,
file upload screens, all introduce a level of risk to the application. Note that the code structure also forms part of
the Attack Surface, in that the code checking authentication, or crypto, etc., is exercised by critical functions on theapplication.
Description
The attack surface of a software environment is a description of the entry points where an attacker can try to manipulate an application, it typically takes the form of a systems diagram where all entry points (interfaces) are pointed out.

A6 - Sensitive Data Exposure

Michael Howard (at Microsoft) and other researchers have developed a method for measuring the Attack Surface ofan application, and to track changes to the Attack Surface over time, called the Relative Attack Surface Quotient (RSQ).
It is assumed that the application Attack Surface is already known, probably through some previous threat modelingexercise, or Architectural Risk Analysis. Therefor the entry and exit points are known, the sensitivity of the data within
the application is understood, and the various users of the system, and their entitlements, have been mapped inrelation to the functions and data.
From a code review point of view, the aim would be to ensure the change being reviewed is not unnecessarily increasing the Attack Surface. For example, is the code change suddenly using HTTP where only HTTPS was used
before? Is the coder deciding to write their own hash function instead of using the pre-existing (and well exercised/tested) central repository of crypto functions? In some development environments the Attack Surface changes canbe checked during the design phase if such detail is captured, however at code review the actual implementation isreflected in the code and such Attack Surface exposures can be identified.
You can also build up a picture of the Attack Surface by scanning the application. For web apps you can use a tool likethe OWASP Zed Attack Proxy Project (ZAP), Arachni, Skipfish, w3af or one of the many commercial dynamic testingand vulnerability scanning tools or services to crawl your app and map the parts of the application that are accessible
over the web. Once you have a map of the Attack Surface, identify the high risk areas, then understand what compensating controls you have in place.
Note that backups of code and data (online, and on offline media) are an important but often ignored part of a system’s Attack Surface. Protecting your data and IP by writing secure software and hardening the infrastructure will all
be wasted if you hand everything over to bad guys by not protecting your backups.
What to Review
When reviewing code modules from an Attack Surface point of view, some common issues to look out for include:• Does the code change modify the attack surface? By applying the change to the current Attack Surface of the application does it open new ports or accept new inputs? If it does could the change be done in a way that does notincrease the attack surface? If a better implementation exists then that should be recommended, however if there is
no way to implement the code without increasing the Attack Surface, make sure the business knows of the increasedrisk.
• Is the feature unnecessarily using HTTP instead of HTTPS?
• Is the function going to be available to non-authenticated users? If no authentication is necessary for the function
to be invoked, then the risk of attackers using the interface is increased. Does the function invoke a backend task that
could be used to deny services to other legitimate users?
o E.g. if the function writes to a file, or sends an SMS, or causes a CPU intensive calculation, could an attacker write ascript to call the function many times per second and prevent legitimate users access to that task?
• Are searches controlled? Search is a risky operation as it typically queries the database for some criteria and returns
the results, if attacker can inject SQL into query then they could access more data than intended.
• Is important data stored separately from trivial data (in DB, file storage, etc). Is the change going to allow unauthenticated users to search for publicly available store locations in a database table in the same partition as the username/password table? Should this store location data be put into a different database, or different partition, to reduce therisk to the database information?

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A6 - Sensitive Data Exposure• If file uploads are allowed, are they be authenticated? Is there rate limiting? Is there a maximum file size for eachupload or aggregate for each user? Does the application restrict the file uploads to certain types of file (by checking
MIME data or file suffix). Is the application is going to run virus checking?
• If you have administration users with high privilege, are their actions logged/tracked in such a way that they a) can’t
erase/modify the log and b) can’t deny their actions?
o Are there any alarms or monitoring to spot if they are accessing sensitive data that they shouldn’t be? This could
apply to all types of users, not only administrators.
• Will changes be compatible with existing countermeasures, or security code, or will new code/countermeasuresneed to be developed?
• Is the change attempting to introduce some non-centralized security code module, instead of re-using or extending
an existing security module?
• Is the change adding unnecessary user levels or entitlements that will complicate the attack surface.
• If the change is storing PII or confidential data, is all of the new information absolutely necessary? There is little value
in increasing the risk to an application by storing the social security numbers of millions of people, if the data is neverused.
• Does application configuration cause the attack surface to vary greatly depending on configuration settings, and isthat configuration simple to use and alert the administrator when the attack surface is being expanded?
• Could the change be done in a different way that would reduce the attack surface, i.e instead of making help items
searchable and storing help item text in a database table beside the main username/password store, providing static
help text on HTML pages reduces the risk through the ‘help’ interface.
• Is information stored on the client that should be stored on the server?
1.2.3 References
https://www.owasp.org/index.php/Attack_Surface_Analysis_Cheat_Sheet
https://www.owasp.org/index.php/OWASP_Zed_Attack_Proxy_Project
http://www.cs.cmu.edu/~wing/publications/Howard-Wing03.pdf

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A7 - Missing Function Level Access Control

A7

MISSING FUNCTION LEVEL ACCESS CONTROL

Most web applications verify function level access rights before making that functionality visible in the UI. However,applications need to perform the same access control checks on the server when each function is accessed. If requests
are not verified, attackers will be able to forge requests in order to access functionality without proper authorization.
13.1 Authorization
Authorization is as important as authentication. Access to application functionality and access to all data should be
authorized. For data access authorization, application logic should check if the data belongs to the authenticateduser, or if the user should be able to access that data.
Placement of security checks is a vital area of review in an application design. Incorrect placement can render theapplied security controls useless, thus it is important to review the application design and determine the correctnessof such checks. Many web application designs are based on the concept of Model-View-Controller (MVC) that have acentral controller which listens to all incoming request and delegates control to appropriate form/business processing logic. Ultimately the user is rendered with a view. In such a layered design, when there are many entities involvedin processing a request, developers can go wrong in placing the security controls in the incorrect place, for example
some application developers feel “view” is the right place to have the authorization checks.
Authorization issues cover a wide array of layers in a web application; from the functional authorization of a user togain access to a particular function of the application at the application layer, to the Database access authorizationand least privilege issues at the persistence layer.
In most of the applications the request parameters or the URL’s serve as sole factors to determine the processing logic.
In such a scenario the elements in the request, which are used for such identifications, may be subject to manipulation attacks to obtain access to restricted resources or pages in the application.
There are two main design methods to implements authorization: Role Base Access Control (RBAC) and Access Control Lists (ACLs). RBAC is used when assigning users to roles, and then roles to permissions. This is a more logicalmodeling of actual system authorization. It also allows administrators to fine-grain and re-check role-permission assignments, while making sure that every role has the permissions it is supposed to have (and nothing more or less).
Thus assigning users to roles should reduce the chance of human-error. Many web frameworks allow roles to be
assigned to logged in users, and custom code can check the session information to authorize functionality based on
the current users role.
13.2 Description
It seems logical to restrict the users at the page/view level they won’t be able to perform any operation in the application. But what if instead of requesting for a page/view an unauthorized user tries to request for an internal action
such as to add/modify any data in the application? It will be processed but the resultant view will be denied to theuser; because the flaw lies in just having a view based access control in the applications. Much of the business logic
processing (for a request) is done before the “view” is executed. So the request to process any action would get processed successfully without authorization.
In an MVC based system given this issue is shown in figure 11 below, where the authentication check is present in the view action.

A7 - Missing Function Level Access Control

Figure 11: MVC Access Control

1. REQUEST

2READ CONFIG FILE

3

RETURN ACTION MAPPINGURL = ACTIONCONTROLLERSERVIET

Config.xml

4

INTANTIATE ACTION CLASS ANDCALLS EXECUTE METHOD

5RETURN DATA TO BE VIEWED/EDITEDACTIONCLASSEXECUTE{}

6
RENDER VIEW AND DATA

AUTH
CHECK

7. RESPONSE

Authentication
check built
inside the view

VIEW

In this example neither the controller servlet (central processing entity) nor the action classes have any access
control checks. If the user requests an internal action such as add user details, without authentication, it will
get processed, but the only difference is that the user will be shown an error page as resultant view will bedisallowed to the user. A similar flaw is observed in ASP.NET applications where the developers tend to mix thecode for handling POSTBACK’s and authentication checks. Usually it is observed that the authentication checkin the ASP.NET pages are not applied for POSTBACKs, as indicated below. Here, if an attacker tries to access thepage without authentication an error page will be rendered. Instead, if the attacker tries to send an internal
POSTBACK request directly without authentication it would succeed.
Unused or undeclared actions or functionality can present in the configuration files. Such configurations thatare not exposed (or tested) as valid features in the application expand the attack surface and raise the risk to
the application. An unused configuration present in a configuration file is shown in sample 13.1, where the‘TestAction’ at the end of the file has been left over from the testing cycle and will be exposed to external users.
It is likely this action would not be checked on every release and could expose a vulnerability.
Sample 13.1
<mapping><url>/InsecureDesign/action/AddUserDetails</url><action>Action.UserAction</action>

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A7 - Missing Function Level Access Control<success>JSP_WithDesign/Success.jsp</success></mapping><mapping><url>/InsecureDesign/action/ChangePassword</url><action>Action.ChangePasswordAction</action><success>JSP_WithDesign/Success.jsp</success></mapping><mapping><url>/InsecureDesign/action/test</url><action>Action.TestAction</action><success>JSP_WithDesign/Success.jsp</success></mapping>
Another popular feature seen in most of the design frameworks today is data binding, where the request
parameters get directly bound to the variables of the corresponding business/command object. Binding heremeans that the instance variables of such classes get automatically initialized with the request parameter values based on their names. The issue with this design is that the business objects may have variables that are
not dependent on the request parameters. Such variables could be key variables like price, max limit, role etc.
having static values or dependent on some server side processing logic. A threat in such scenarios is that an attacker may supply additional parameters in request and try to bind values for unexposed variable of businessobject class. In this case the attacker can send an additional “price” parameter in the request which binds withthe unexposed variable “price” in business object, thereby manipulating business logic.
What to Review
It is imperative to place all validation checks before processing any business logic and in case of ASP.NET applications
independent of the POSTBACKs. The security controls like authentication check must be place before processing any
request.
The use of filters is recommended when authorization is being implemented in MVC 3 and above as .NET MVC 3
introduced a method in global.asax called RegisterGlobalFilters which can be used to default deny access to URL’s in
the application.
Sample 13.2
public static void RegisterGlobalFilters(GlobalFilterCollection filters){
filters.Add(new HandleErrorAttribute());
filters.Add(new System.Web.Mvc.AuthorizeAttribute());}

It is recommended when reviewing MVC3/4 .NET to take a look at how authorization is being implemented. The line
above, “filters.Add(new System.Web.Mvc.AuthorizeAttribute());” default denies access to any request without a valid

A7 - Missing Function Level Access Control

session. If this is implemented we may need to provide unauthorized access to certain pages such as a registration
page, public welcome page or a login page.
The directive “AllowAnonymous” is used to provide access to public pages with no valid session required. The code
may look like this:Sample 13.3
[AllowAnonymous]
public ActionResult LogMeIn(string returnUrl)

When reviewing code for authorization, the following considerations can be checked for:• Every entry point should be authorized. Every function should be authorized.
• Authorization checks should be efficient, and implemented in a central code base such that it can be appliedconsistently.
• In cases where authorization fails, a HTTP 403 not authorized page should be returned.
• When using RBAC, there must be some way for the application to report on the currently provisioned users of thesystem and their associated roles. This allows the business to periodically audit the user access to the system andensure it is accurate. For example, if a user is provisioned as an admin on the system, then that user changes job toanother department, it could be the case that the admin role is no longer appropriate.
• There should be an easy method to change or remove a user’s role (in RBAC systems). Adding, modifying or removing a user from a role should result in audit logs.
• For roles that are higher risk, addition, modification and deletion of those roles should involve multiple levels of
authorization (e.g. maker/checker), this may be tracked within the application itself, or through some centralizedrole application. Both the functionality and code of the system controlling roles should be part of the review scope.
• At a design level attempt to keep the range of roles simple. Applications with multiple permission levels/roles often
increases the possibility of conflicting permission sets resulting in unanticipated privileges.
• In application architectures with thick clients (i.e. mobile apps or binaries running on a PC) do not attempt toperform any authorization in the client code, as this could be bypassed by an attacker. In browser based applications
do not perform any authorization decisions in JavaScript.
• Never base authorization decisions on untrusted data. For example do not use a header, or hidden field, from theclient request to determine the level of authorization a user will have, again this can be manipulated by an attacker.
• Follow the principle of ‘complete mediation’, where authorization is checked at every stage of a function. For example, if an application has four pages to browse through to purchase an item (browse.html, basket.html, inputPayment.html, makePayment.html) then check user authorization at every page, and stage within pages, instead of only
performing a check in the first page.

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A7 - Missing Function Level Access Control• By default deny access to any page, and then use authorization logic to explicitly allow access based on roles/
ACL rules.
• Where to hold authorization data, DB or session?
• Remove all redundant/test/unexposed business logic configurations from the file
• The business/form/command objects must have only those instance variables that are dependent on theuser inputs.

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A8 - Cross-Site Request Forgery (CSRF)

A8CROSS-SITE REQUEST FORGERY (CSRF)

A CSRF attack forces a logged-on victim’s browser to send a forged HTTP request, including the victim’s session
cookie and any other automatically included authentication information, to a vulnerable web application. This
allows the attacker to force the victim’s browser to generate requests the vulnerable application thinks arelegitimate requests from the victim.
14.1 Description
CSRF is an attack which forces an end user to execute unwanted actions on a web application in which they are
currently authenticated. With a little help of social engineering (like sending a link via email/chat), an attacker
may force the users of a web application to execute actions of the attacker’s choosing. A successful CSRF exploit can compromise end user data, and protected functionality, in the case of a normal privileged user. If thetargeted end user is the administrator account, this can compromise the entire web application.
The impact of a successful cross-site request forgery attack is limited to the capabilities exposed by the vulnerable application. For example, this attack could result in a transfer of funds, changing a password, or purchasing an item in the user’s context. In effect, CSRF attacks are used by an attacker to make a target system
perform a function (funds Transfer, form submission etc.) via the target’s browser without knowledge of thetarget user, at least until the unauthorized function has been committed.
CSRF is not the same as XSS (Cross Site Scripting), which forces malicious content to be served by a trusted
website to an unsuspecting victim. Cross-Site Request Forgery (CSRF, a.k.a C-SURF or Confused-Deputy) attacks are considered useful if the attacker knows the target is authenticated to a web based system. They only
work if the target is logged into the system, and therefore have a small attack footprint. Other logical weaknesses also need to be present such as no transaction authorization required by the user. In effect CSRF attacks
are used by an attacker to make a target system perform a function (Funds Transfer, Form submission etc..) via
the target’s browser without the knowledge of the target user, at least until the unauthorized function has
been committed. A primary target is the exploitation of “ease of use” features on web applications (One-click
purchase).
Impacts of successful CSRF exploits vary greatly based on the role of the victim. When targeting a normal user,a successful CSRF attack can compromise end-user data and their associated functions. If the targeted end
user is an administrator account, a CSRF attack can compromise the entire Web application. The sites that are
more likely to be attacked are community Websites (social networking, email) or sites that have high dollar
value accounts associated with them (banks, stock brokerages, bill pay services). This attack can happen even
if the user is logged into a Web site using strong encryption (HTTPS). Utilizing social engineering, an attacker
will embed malicious HTML or JavaScript code into an email or Website to request a specific ‘task url’. The task
then executes with or without the user’s knowledge, either directly or by utilizing a Cross-site Scripting flaw
(ex: Samy MySpace Worm).
How They WorkCSRF attacks work by sending a rogue HTTP request from an authenticated user’s browser to the application,
which then commits a transaction without authorization given by the target user. As long as the user is authenticated and a meaningful HTTP request is sent by the user’s browser to a target application, the application does not know if the origin of the request is a valid transaction or a link clicked by the user (that was, say,in an email) while the user is authenticated to the application. The request will be authenticated as the requestfrom the users browser will automatically include the ‘Cookie’ header, which is the basis for authentication. So

A8 - Cross-Site Request Forgery (CSRF)

an attacker makes the victim perform actions that they didn’t intend to, such as purchase an item. Sample
14.1 shows an example an HTTP POST to a ticket vendor to purchase a number of tickets.
Sample 14.1
POST http://TicketMeister.com/Buy_ticket.htm HTTP/1.1
Host: ticketmeister
User-Agent: Mozilla/5.0 (Macintosh; U; PPC Mac OS X Mach-O;) Firefox/1.4.1
Cookie: JSPSESSIONID=34JHURHD894LOP04957HR49I3JE383940123K
ticketId=ATHX1138&to=PO BOX 1198 DUBLIN 2&amount=10&date=11042008
The response of the vendor is to acknowledge the purchase of the tickets:
HTTP/1.0 200 OK
Date: Fri, 02 May 2008 10:01:20 GMT
Server: IBM_HTTP_ServerContent-Type: text/xml;charset=ISO-8859-1
Content-Language: en-US
X-Cache: MISS from app-proxy-2.proxy.ie
Connection: close<?xml version=”1.0” encoding=”ISO-8859-1”?><pge_data> Ticket Purchased, Thank you for your custom.
</pge_data>

What to Review
This issue is simple to detect, but there may be compensating controls around the functionality of the application
which may alert the user to a CSRF attempt. As long as the application accepts a well formed HTTP request and therequest adheres to some business logic of the application CSRF shall work.
By checking the page rendering we need to see if any unique identifiers are appended to the links rendered by theapplication in the user’s browser. If there is no unique identifier relating to each HTTP request to tie a HTTP request to
the user, we are vulnerable. Session ID is not enough, as the session ID shall be sent automatically if a user clicks on arogue link, as the user is already authenticated.
Prevention Measures That Do NOT Work
Examples of attempted CSRF prevent techniques which attackers can bypass are listed in table 21, these measures
should not be used in sensitive applications and should fail code review.
Table 21: Unsuccessful Countermeasures For Csrf Attacks
MeasureDescription

Using a Secret Cookie

Remember that all cookies, even the secret ones, will be submitted with every request. All authentication tokens
will be submitted regardless of whether or not the end-user was tricked into submitting the request. Furthermore, session identifiers are simply used by the application container to associate the request with a specificsession object. The session identifier does not verify that the end-user intended to submit the request.

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Only Accepting POST Requests

Applications can be developed to only accept POST requests for the execution of business logic. The misconception is that since the attacker cannot construct a malicious link, a CSRF attack cannot be executed. Unfortunately, this logic is incorrect. There are numerous methods in which an attacker can trick a victim into submitting aforged POST request, such as a simple form hosted in an attacker’s website with hidden values. This form can betriggered automatically by JavaScript or can be triggered by the victim who thinks the form will do something
else.

Salt = ‘a0w8hsdfas8ls587uas87’

Multi-Step transactions are not an adequate prevention of CSRF. As long as an attacker can predict or deduceeach step of the completed transaction, then CSRF is possible.

URL Rewriting

This might be seen as a useful CSRF prevention technique as the attacker cannot guess the victim’s session ID.
However, the user’s credential is exposed over the URL.

Password, transferring funds, purchasing, the application may not simply execute the request, but should respond to the request with another request for the user’s password, or an out-of-band (i.e. two-factor) authentication item.
Preventing CSRF
Checking if the request has a valid session cookie is not enough, the application needs to have a unique identifier assocaited with every HTTP request sent to the application. CSRF requests (from an attackers e-mail) will
not have this valid unique identifier. The reason CSRF requests won’t have this unique request identifier is theunique ID is rendered as a hidden field, or within the URL, and is appended to the HTTP request once a link/button press is selected. The attacker will have no knowledge of this unique ID, as it is random and rendered
dynamically per link, per page.
Application logic to prevent CSRF then include:
1. A list of unique IDs is compiled prior to delivering the page to the user. The list contains all valid unique IDs
generated for all links on a given page. The unique ID could be derived from a secure random generator suchas SecureRandom for J2EE and could be stored in the session or another centralized cache.
2. A unique ID is appended to each link/form on the requested page prior to being displayed to the user.
3. The application checks if the unique ID passed with the HTTP request is valid for a given request. If theunique ID passed with the HTTP request is valid for a given request.
4. If the unique ID is not present, terminate the user session and display an error to the user.
General Recommendation: Synchronizer Token Pattern
In order to facilitate a “transparent but visible” CSRF solution, developers are encouraged to adopt the Synchronizer Token Pattern http://www.corej2eepatterns.com/Design/PresoDesign.htm. The synchronizer
token pattern requires the generating of random “challenge” tokens that are associated with the user’s current
session. These challenge tokens are then inserted within the HTML forms and links associated with sensitive
server-side operations. When the user wishes to invoke these sensitive operations, the HTTP request should
include this challenge token. It is then the responsibility of the server application to verify the existence andcorrectness of this token. By including a challenge token with each request, the developer has a strong control
to verify that the user actually intended to submit the desired requests. Inclusion of a required security token in
HTTP requests associated with sensitive business functions helps mitigate CSRF attacks as successful exploitation assumes the attacker knows the randomly generated token for the target victim’s session. This is analogous to the attacker being able to guess the target victim’s session identifier. The following synopsis describes a
general approach to incorporate challenge tokens within the request.
When a Web application formulates a request (by generating a link or form that causes a request when submitted or

A8 - Cross-Site Request Forgery (CSRF)

clicked by the user), the application should include a hidden input parameter with a common name such as “CSRFToken”. The value of this token must be randomly generated such that it cannot be guessed by an attacker. Consider
leveraging the java.security.SecureRandom class for Java applications to generate a sufficiently long random token.
Alternative generation algorithms include the use of 256-bit BASE64 encoded hashes. Developers that choose thisgeneration algorithm must make sure that there is randomness and uniqueness utilized in the data that is hashed togenerate the random token.
Sample 14.2
<form action=”/transfer.do” method=”post”><input type=”hidden” name=”CSRFToken”
value=”OWY4NmQwODE4ODRjN2Q2NTlhMmZlYWE...
wYzU1YWQwMTVhM2JmNGYxYjJiMGI4MjJjZDE1ZDZ...
MGYwMGEwOA==”>…
</form>Depending on the risk level of the product, it may only generate this token once for the current session. After initial
generation of this token, the value is stored in the session and is utilized for each subsequent request until the session
expires. When a request is issued by the end-user, the server-side component must verify the existence and validity ofthe token in the request as compared to the token found in the session. If the token was not found within the requestor the value provided does not match the value within the session, then the request should be aborted, token should
be reset and the event logged as a potential CSRF attack in progress.
To further enhance the security of this proposed design, consider randomizing the CSRF token parameter name andor value for each request. Implementing this approach results in the generation of per-request tokens as opposed toper-session tokens. Note, however, that this may result in usability concerns. For example, the “Back” button browser
capability is often hindered as the previous page may contain a token that is no longer valid. Interaction with thisprevious page will result in a CSRF false positive security event at the server. Regardless of the approach taken, developers are encouraged to protect the CSRF token the same way they protect authenticated session identifiers, suchas the use of SSLv3/TLS.
Disclosure of Token in URL
Many implementations of this control include the challenge token in GET (URL) requests as well as POST requests.
This is often implemented as a result of sensitive server-side operations being invoked as a result of embedded links
in the page or other general design patterns. These patterns are often implemented without knowledge of CSRF andan understanding of CSRF prevention design strategies. While this control does help mitigate the risk of CSRF attacks,
the unique per-session token is being exposed for GET requests. CSRF tokens in GET requests are potentially leaked atseveral locations: browser history, HTTP log files, network appliances that make a point to log the first line of an HTTP
request, and Referer headers if the protected site links to an external site.
In the latter case (leaked CSRF token due to the Referer header being parsed by a linked site), it is trivially easy for thelinked site to launch a CSRF attack on the protected site, and they will be able to target this attack very effectively,
since the Referer header tells them the site as well as the CSRF token. The attack could be run entirely from javascript,
so that a simple addition of a script tag to the HTML of a site can launch an attack (whether on an originally malicious
site or on a hacked site). This attack scenario is easy to prevent, the referer will be omitted if the origin of the request

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is HTTPS. Therefore this attack does not affect web applications that are HTTPS only.
The ideal solution is to only include the CSRF token in POST requests and modify server-side actions that have statechanging affect to only respond to POST requests. This is in fact what the RFC 2616 requires for GET requests. If sensitive server-side actions are guaranteed to only ever respond to POST requests, then there is no need to include thetoken in GET requests.
Viewstate (ASP.NET)
ASP.NET has an option to maintain your ViewState. The ViewState indicates the status of a page when submitted tothe server. The status is defined through a hidden field placed on each page with a <form runat=”server”> control.
Viewstate can be used as a CSRF defense, as it is difficult for an attacker to forge a valid Viewstate. It is not impossible
to forge a valid Viewstate since it is feasible that parameter values could be obtained or guessed by the attacker.
However, if the current session ID is added to the ViewState, it then makes each Viewstate unique, and thus immune
to CSRF.
To use the ViewStateUserKey property within the Viewstate to protect against spoofed post backs add the following
in the OnInit virtual method of the page-derived class (This property must be set in the Page.Init event)
Sample 14.3
protected override OnInit(EventArgs e) {base.OnInit(e);if (User.Identity.IsAuthenticated)
ViewStateUserKey = Session.SessionID; }To key the Viewstate to an individual using a unique value of your choice use “(Page.ViewStateUserKey)”. This must
be applied in Page_Init because the key has to be provided to ASP.NET before Viewstate is loaded. This option has
been available since ASP.NET 1.1. However, there are limitations on this mechanism. Such as, ViewState MACs are only
checked on POSTback, so any other application requests not using postbacks will happily allow CSRF.
Double Submit Cookies
Double submitting cookies is defined as sending a random value in both a cookie and as a request parameter, withthe server verifying if the cookie value and request value are equal.
When a user authenticates to a site, the site should generate a (cryptographically strong) pseudorandom value andset it as a cookie on the user’s machine separate from the session id. The site does not have to save this value in any
way. The site should then require every sensitive submission to include this random value as a hidden form value (orother request parameter) and also as a cookie value. An attacker cannot read any data sent from the server or modify
cookie values, per the same-origin policy. This means that while an attacker can send any value he wants with a malicious CSRF request, the attacker will be unable to modify or read the value stored in the cookie. Since the cookie value
and the request parameter or form value must be the same, the attacker will be unable to successfully submit a form
unless he is able to guess the random CSRF value.
Direct Web Remoting (DWR) Java library version 2.0 has CSRF protection built in as it implements the double cookie
submission transparently.
The above CSRF prevents rely on the use of a unique token and the Same-Origin Policy to prevent CSRF by maintaining a secret token to authenticate requests. The following methods can prevent CSRF by relying upon similar rules

A8 - Cross-Site Request Forgery (CSRF)

that CSRF exploits can never break.
Checking The Referer Header
Although it is trivial to spoof the referer header on your own browser, it is impossible to do so in a CSRF attack. Checking the referer is a commonly used method of preventing CSRF on embedded network devices because it does
not require a per-user state. This makes a referer a useful method of CSRF prevention when memory is scarce. Thismethod of CSRF mitigation is also commonly used with unauthenticated requests, such as requests made prior toestablishing a session state which is required to keep track of a synchronization token.
However, checking the referer is considered to be a weaker from of CSRF protection. For example, open redirect
vulnerabilities can be used to exploit GET-based requests that are protected with a referer check and some organizations or browser tools remove referrer headers as a form of data protection. There are also common implementation
mistakes with referer checks. For example if the CSRF attack originates from an HTTPS domain then the referer will beomitted. In this case the lack of a referer should be considered to be an attack when the request is performing a statechange. Also note that the attacker has limited influence over the referer. For example, if the victim’s domain is “site.
com” then an attacker have the CSRF exploit originate from “site.com.attacker.com” which may fool a broken referer
check implementation. XSS can be used to bypass a referer check.
In short, referer checking is a reasonable form of CSRF intrusion detection and prevention even though it is not acomplete protection. Referer checking can detect some attacks but not stop all attacks. For example, if the HTTP
referrer is from a different domain and you are expecting requests from your domain only, you can safely block thatrequest.
Checking The Origin Header
The Origin HTTP Header standard was introduced as a method of defending against CSRF and other Cross-Domain
attacks. Unlike the referer, the origin will be present in HTTP request that originates from an HTTPS URL. If the origin
header is present, then it should be checked for consistency.
Challenge-Response
Challenge-Response is another defense option for CSRF. As mentioned before it is typically used when the functionality being invoked is high risk. While challenge-response is a very strong defense to CSRF (assuming proper
implementation), it does impact user experience. For applications in need of high security, tokens (transparent) andchallenge-response should be used on high risk functions.
The following are some examples of challenge-response options:• CAPTCHA• Re-Authentication (password)• One-time Token
No Cross-Site Scripting (XSS) Vulnerabilities
Cross-Site Scripting is not necessary for CSRF to work. However, any cross-site scripting vulnerability can be used todefeat token, Double-Submit cookie, referer and origin based CSRF defenses. This is because an XSS payload cansimply read any page on the site using a XMLHttpRequest and obtain the generated token from the response, and include that token with a forged request. This technique is exactly how the MySpace (Samy) worm defeated MySpace’s
anti CSRF defenses in 2005, which enabled the worm to propagate. XSS cannot defeat challenge-response defenses
such as Captcha, re-authentication or one-time passwords. It is imperative that no XSS vulnerabilities are present toensure that CSRF defenses can’t be circumvented.

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A9 - Using Components with Known Vulnerabilities

A9

USING COMPONENTS WITH KNOWN
VULNERABILITIESComponents, such as libraries, frameworks, and other software modules, almost always run with full privileges. If a vulnerable component is exploited, such an attack can facilitate serious data loss or server takeover.
Applications using components with known vulnerabilities may undermine application defenses and enable
a range of possible attacks and impacts.
15.1 Description
Today it would be rare for an application or software component to be developed without the re-use of someopen source or paid-for library or framework. This makes a lot of sense as these frameworks and libraries are
already developed and working, and have had a good degree of testing applied. However these third party
components can also be a source of security vulnerabilities when an attacker finds a flaw in the components
code, in fact this flaw has added attraction since the attacker knows the exploit will work on everyone who isusing the component.
This issue has matured to such a state that flaws/exploits for popular frameworks, libraries and operating systems are sold on underground markets for large sums of money.
What to Review
There is really no code to review for this topic, unless your organization has taken it upon itself to review the code of
the component (assuming its open source and not a closed source third party library), in which case the code review
would be similar to any other audit review. However code review can be used within the larger company-wide tracking or audit mechanisms that lets the organization know what third party code it is using.
Regardless of the size of company, the use of third party components, and their versions, should be tracked to ensure the organization can be alerted when any security vulnerabilities are flagged. For smaller companies with 1 or
2 products this tracking could be as easy as a spreadsheet or wiki page, however for larger companies with 100s of
applications or products, the task of tracking developer use of third party frameworks and libraries is equally as large
as the risk posed by those libraries.
If a company has 20 products and each of those products use 5 or 6 third party components (e.g. Apache web servers,
OpenSSL crypto libraries, Java libraries for regex and DB interactions, etc.) that leaves the company with over 100
external sources where security vulnerabilities can come from. If the company suddenly hears of a heartbleed typevulnerability, it has to be able to react and upgrade those affected applications, or take other countermeasures, toprotect itself and its customers.
Controlling the Ingredients
One method used by larger companies to limit their exposure to third party vulnerabilities is to control which libraries can be used by their developers. For example they could specify that developers should use a certain version of
OpenSSL as the crypto library instead of other options.
This allows management and risk controllers to know their risk profile to vulnerabilities on the market, if a bug appears in bouncycastle, they know they are not exposed (i.e. some developer didn’t use bouncycastle on one of the
products, because it’s not on the list of crypto libraries to use). On the other hand, if there is a bug in OpenSSL, all theireggs are in that basket and they need to upgrade immediately.

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There will obviously be technical challenges to limiting the choices of third party components, and such a policy
could be unpopular with developers who’ll want to use the latest and greatest framework, but the first step to securing a product is knowing what ingredients you’ve made it with.
How can such a policy be tracked or enforced? At some point the library or framework, in the form of .dll/.so or as
source code, will be integrated into the codeline.
Such integrations should be subject to code review, and as a task of this code review the reviewer can check:
1. The library is one that can be used in the product suite (or maybe is already used and the developer is simply
unaware, in which case the review should be rejected and the original integration used)
2. Any tracking or auditing software (even a basic spread sheet) is updated to reflect that the product is using thethird party library. This allows for rapid remediation if a vulnerability appears, meaning the product will be patched.
SledgeHammer to Crack a Nut
One last responsibility of the reviewer will be to ensure the correct third party library is being used for the functionality needed. Many libraries come with vast amounts of functionality which may not be used. For example, is thedeveloper including a library to perform their regex’s, but which also will include other functionality not used/neededby the application? This increases the attack surface of the application and can cause unexpected behavior whenthat extra code opens a port and communicates to the internet.
If the reviewer thinks too much functionality/code is being introduced they can advise to turn off non-used functionality, or better still find a way to not include that functionality in the product (e.g. by stripping out code, or hardcoding
branches so unused functions are never used).
The OWASP project “OWASP Dependency Check” can provide a measure of automation for library checking(https://www.owasp.org/index.php/OWASP_Dependency_Check)

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A10 - Unvalidated Redirects And Forwards

A10 UNVALIDATED REDIRECTS AND FORWARDS
Web applications frequently redirect and forward users to other pages and websites, and use untrusted data to determine the destination pages. Without proper validation, attackers can redirect victims to phishing or malware sites,or use forwards to access unauthorized pages.
16.1 Description
Unvalidated redirects and forwards are possible when a web application accepts untrusted input that could cause
the web application to redirect the request to a URL contained within untrusted input. By modifying untrusted URL
input to a site, an attacker may successfully launch a phishing scam and steal user credentials.
As the server name in the modified link is identical to the original site, phishing attempts may have a more trustworthy appearance. Invalidated redirect and forward attacks can also be used to maliciously craft a URL that would
pass the application’s access control check and then forward the attacker to privileged functions that they would
normally not be able to access.
Redirects
Redirect functionality on a web site allows a user’s browser to be told to go to a different page on the site. This can bedone to improve user interface or track how users are navigating the site.
To provide the redirect functionality a site may have a specific URL to perform the redirect:• http://www.example.com/utility/redirect.cgi
This page will take a parameter (from URL or POST body) of ‘URL’ and will send back a message to the user’s browser
to go to that page, for example:• http://www.example.com/utility/redirect.cgi?URL=http://www.example.com/viewtxn.html
However this can be abused as an attacker can attempt to make a valid user click on a link that appears to be forwww.example.com but which will invoke the redirect functionality on example.com to cause the users browser to go
to a malicious site (one that could look like example.com and trick the user into entering sensitive or authentication
information:• http://www.example.com/utiltiy/redirect cgi?URL=http://attacker.com/fakelogin.html
Forwards
Forwards are similar to redirects however the new page is not retrieved by the users browser (as occurred with theredirect) but instead the server framework will obtain the forwarded page and return it to the users browser. This isachieved by ‘forward’ commands within Java frameworks (e.g. Struts) or ‘Server.Transfer’ in .Net. As the forward is performed by the server framework itself, it limits the range of URLs the attacker can exploit to the current web site (i.e.
attacker cannot ‘forward’ to attacker.com), however this attack can be used to bypass access controls. For example,
where a site sends the forwarded page in the response:• If purchasing, forward to ‘purchase.do’• If cancelling, forward to ‘cancelled.do’
This will then be passed as a parameter to the web site:• http://www.example.com/txn/acceptpayment.html?FWD=purchaseA10 - Unvalidated Redirects And Forwards

If instead an attacker used the forward to attempt to access to a different page within the web site, e.g. admin.do, thenthey may access pages that they are not authorized to view, because authorization is being applied on the ‘acceptpayment’ page, instead of the forwarded page.
What to Review
If any part of the URL being forwarded, or redirected, to is based on user input, then the site could be at risk. Ensure:• All redirects/forwards are constructed based on a whitelist, or• All redirtects/forwards use reletive paths to ensure they stay on the trusted site
Redirects
The following examples demonstrate unsafe redirect and forward code. The following Java code receives the URL
from the ‘url’ parameter and redirects to that URL.
Sample 16.1
response.sendRedirect(request.getParameter(“url”));

The following PHP code obtains a URL from the query string and then redirects the user to that URL.
Sample 16.2
$redirect_url = $_GET[‘url’];
header(“Location: “ . $redirect_url);

A similar example of C# .NET Vulnerable Code:Sample 16.3
string url = request.QueryString[“url”];
Response.Redirect(url);

The above code is vulnerable to an attack if no validation or extra method controls are applied to verify thecertainty of the URL. This vulnerability could be used as part of a phishing scam by redirecting users to a malicious site. If user input has to be used as part of the URL to be used, then apply strict validation to the input,
ensuring it cannot be used for purposes other than intended.
Note that vulnerable code does not need to explicitly call a ‘redirect’ function, but instead could directly modify the response to cause the client browser to go to the redirected page. Code to look for is shown in table 22.

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Table 22: Redirect Risks
Method of Redirection

Description

Redirect Response(note 301 and 307 responses will
also cause a redirect)

HTTP/1.1 302 Found
Location: http://www.attacker.com/page.html

Meta Tag<html><head><meta http-equiv=”Refresh”
content=”0;url=http://attacker.com/page.html”>

JavaScript<script type=”text/javascript”><!—
window.location=”http://attacker.com/page.html”//-->

Refresh Header

HTTP/1.1 200 OK
Refresh=0; url=http://attacker.com/page.html

Where an attacker has posted a redirecting URL on a forum, or sends in an e-mail, the web site can check thereferer header to ensure the user is coming from a page within the site, although this countermeasure will not
apply if the malicious URL is contained within the site itself.
Consider creating a whitelist of URLs or options that a redirect is allowed to go to, or deny the ability for theuser input to determine the scheme or hostname of the redirect. A site could also encode (or encrypt) the
URL value to be redirected to such that an attacker cannot easily create a malicious URL parameter that, whenunencoded (or unencypted), will be considered valid.
Forwards
The countermeasure for forwards is to either whitelist the range of pages that can be forwarded to (similar to
redirects) and to enforce authentication on the forwarded page as well as the forwarding page. This meansthat even if an attacker manages to force a forward to a page they should not have access to, the authentication check on the forwarded page will deny them access.
Note on J2EE
There is a noted flaw related to the “sendRedirect” method in J2EE applications. For example:• response.sendRedirect(“home.html”);
This method is used to send a redirection response to the user who then gets redirected to the desired web
component whose URL is passed an argument to the method. One such misconception is that execution flowin the Servlet/JSP page that is redirecting the user stops after a call to this method. Note that if there is code
present after the ‘If’ condition it will be executed.
The fact that execution of a servlet or JSP continues even after sendRedirect() method, also applies to Forward
method of the RequestDispatcher Class. However, <jsp:forward> tag is an exception, it is observed that theexecution flow stops after the use of <jsp:forward> tag.
After issue a redirect or forward, terminate code flow using a “return” statement.
References• OWASP Article on Open Redirects https://www.owasp.org/index.php/Open_redirectA10 - Unvalidated Redirects And Forwards• CWE Entry 601 on Open Redirects http://cwe.mitre.org/data/definitions/601.html• WASC Article on URL Redirector Abuse http://projects.webappsec.org/w/page/13246981/URL%20Redirector%20Abuse• Google blog article on the dangers of open redirects http://googlewebmastercentral.blogspot.
com/2009/01/open-redirect-urls-is-your-site-being.html• Preventing Open Redirection Attacks (C#) http://www.asp.net/mvc/tutorials/security/preventing-open-redirection-attacks

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HTML 5

HTML5
HTML5 was created to replace HTLML4, XHTML and the HTML DOM Level 2. The main purpose of this new standard is to provide dynamic content without the use of extra proprietary client side plugins. This allows designers
and developers to create exceptional sites providing a great user experience without having to install any additional plug-ins into the browser.
17.1 Description
Ideally users should have the latest web browser installed but this does not happens as regularly as security experts advice, therefore the website should implement 2 layer controls, one layer independent from browser type,second, as an additional control.
What to Review: Web Messaging
Web Messaging (also known as Cross Domain Messaging) provides a means of messaging between documentsfrom different origins in a way that is generally safer than the multiple hacks used in the past to accomplish thistask. The communication API is as follows:
However, there are still some recommendations to keep in mind:• When posting a message, explicitly state the expected origin as the second argument to ‘postMessage’ rather
than ‘*’ in order to prevent sending the message to an unknown origin after a redirect or some other means of thetarget window’s origin changing.
• The receiving page should always:
o Check the ‘origin’ attribute of the sender to verify the data is originating from the expected location.
o Perform input validation on the ‘data’ attribute of the event to ensure that it’s in the desired format.
• Don’t assume you have control over the ‘data’ attribute. A single Cross Site Scripting flaw in the sending page
allows an attacker to send messages of any given format.
• Both pages should only interpret the exchanged messages as ‘data’. Never evaluate passed messages as code (e.g.
via ‘eval()’) or insert it to a page DOM (e.g. via ‘innerHTML’), as that would create a DOM-based XSS vulnerability.
• To assign the data value to an element, instead of using an insecure method like ‘element.innerHTML = data’, use
the safer option: ‘element.textContent = data;’• Check the origin properly exactly to match the FQDN(s) you expect. Note that the following code: ‘ if(message.
orgin.indexOf(“.owasp.org”)!=-1) { /* ... */ }’ is very insecure and will not have the desired behavior as ‘www.owasp.
org.attacker.com’ will match.
• If you need to embed external content/untrusted gadgets and allow user-controlled scripts (which is highly
discouraged), consider using a JavaScript rewriting framework such as Google’s Caja or check the information
on sandboxed frames.

HTML 5What to Review: Cross Origin Resource Sharing
Cross Origin Resource Sharing or CORS is a mechanism that enables a web browser to perform “cross-domain”
requests using the XMLHttpRequest L2 API in a controlled manner. In the past, the XMLHttpRequest L1 API
only allowed requests to be sent within the same origin as it was restricted by the same origin policy.
Cross-Origin requests have an Origin header that identifies the domain initiating the request and is automatically included by the browser in the request sent to the server. CORS defines the protocol between a web
browser and a server that will determine whether a cross-origin request is allowed. In order to accomplish
this goal, there are HTTP headers that provide information on the messaging context including: Origin, Access-Control-Request-Method, Access-Control-Request-Headers, Access-Control-Allow-Origin, Access-Control-Allow-Credentials, Access-Control-Allow-Methods, Access-Control-Allow-Headers.
The CORS specification mandates that for non simple requests, such as requests other than GET or POST or requests
that uses credentials, a pre-flight OPTIONS request must be sent in advance to check if the type of request will havea negative impact on the data. The pre-flight request checks the methods, headers allowed by the server, and if
credentials are permitted, based on the result of the OPTIONS request, the browser decides whether the request isallowed or not.
Items to note when reviewing code related to CORS includes:• Ensure that URLs responding with ‘Access-Control-Allow-Origin: *’ do not include any sensitive content or information that might aid attacker in further attacks. Use the ‘Access-Control-Allow-Origin’ header only on chosen URLs thatneed to be accessed cross-domain. Don’t use the header for the whole domain.
• Allow only selected, trusted domains in the ‘Access-Control-Allow-Origin’ header. Prefer whitelisting domains overblacklisting or allowing any domain (do not use ‘*’ wildcard nor blindly return the ‘Origin’ header content without any
checks).
• Keep in mind that CORS does not prevent the requested data from going to an unauthenticated location. It’s still
important for the server to perform usual Cross-Site Request Forgery prevention.
• While the RFC recommends a pre-flight request with the ‘OPTIONS’ verb, current implementations might not perform this request, so it’s important that “ordinary” (‘GET’ and ‘POST’) requests perform any access control necessary.
• Discard requests received over plain HTTP with HTTPS origins to prevent mixed content bugs.
• Don’t rely only on the Origin header for Access Control checks. Browser always sends this header in CORS requests,
but may be spoofed outside the browser. Application-level protocols should be used to protect sensitive data.
What to Review: WebSockets
Traditionally the HTTP protocol only allows one request/response per TCP connection. Asynchronous JavaScript and XML (AJAX) allows clients to send and receive data asynchronously (in the background without apage refresh) to the server, however, AJAX requires the client to initiate the requests and wait for the server
responses (half-duplex). HTML5 WebSockets allow the client/server to create a ‘full-duplex’ (two-way) communication channels, allowing the client and server to truly communicate asynchronously. WebSockets conduct
their initial ‘upgrade’ handshake over HTTP and from then on all communication is carried out over TCP channels.
The following is sample code of an application using Web Sockets:

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Sample 17.1
[Constructor(in DOMString url, optional in DOMString protocol)]interface WebSocket{ readonly attribute DOMString URL;// ready state const unsigned short CONNECTING = 0;
const unsigned short OPEN = 1;
const unsigned short CLOSED = 2;
readonly attribute unsigned short readyState;
readonly attribute unsigned long bufferedAmount;// networking
attribute Function onopen;
attribute Function onmessage;attribute Function onclose;boolean send(in DOMString data);
void close();};WebSocket implements EventTarget;
var myWebSocket = new WebSocket(“ws://www.websockets.org”);
myWebSocket.onopen = function(evt) { alert(“Connection open ...”); };myWebSocket.onmessage = function(evt) { alert( “Received Message: “ + evt.data); };myWebSocket.onclose = function(evt) { alert(“Connection closed.”); };When reviewing code implementing websockets, the following items should be taken into consideration:• Drop backward compatibility in implemented client/servers and use only protocol versions above hybi-00. Popular
Hixie-76 version (hiby-00) and older are outdated and insecure.
• The recommended version supported in latest versions of all current browsers is rfc6455 RFC 6455 (supported by
Firefox 11+, Chrome 16+, Safari 6, Opera 12.50, and IE10).
• While it’s relatively easy to tunnel TCP services through WebSockets (e.g. VNC, FTP), doing so enables access to thesetunneled services for the in-browser attacker in case of a Cross Site Scripting attack. These services might also becalled directly from a malicious page or program.
• The protocol doesn’t handle authorization and/or authentication. Application-level protocols should handle that
separately in case sensitive data is being transferred.
• Process the messages received by the websocket as data. Don’t try to assign it directly to the DOM nor evaluate ascode. If the response is JSON, never use the insecure eval() function; use the safe option JSON.parse() instead.
• Endpoints exposed through the ‘ws://’ protocol are easily reversible to plain text. Only ‘wss://’ (WebSockets over SSL/
TLS) should be used for protection against Man-In-The-Middle attacks.
• Spoofing the client is possible outside a browser, so the WebSockets server should be able to handle incorrect/
malicious input. Always validate input coming from the remote site, as it might have been altered.
• When implementing servers, check the ‘Origin:’ header in the Websockets handshake. Though it might be spoofedHTML 5outside a browser, browsers always add the Origin of the page that initiated the Websockets connection.
• As a WebSockets client in a browser is accessible through JavaScript calls, all Websockets communication can be
spoofed or hijacked through Cross Site Scripting. Always validate data coming through a WebSockets connection.
5.1.5 What to Review: Server-Sent Events
Server sent events seem similar to WebSockets, however they do not use a special protocol (they re-used HTTP) andthey allow the client browser to solely listen for updates (messages) from the server, thereby removing the need forthe client to send any polling or other messages up to the server.
When reviewing code that is handling server sent events, items to keep in mind are:• Validate URLs passed to the ‘EventSource’ constructor, even though only same-origin URLs are allowed.
• As mentioned before, process the messages (‘event.data’) as data and never evaluate the content as HTML or scriptcode.
• Always check the origin attribute of the message (‘event.origin’) to ensure the message is coming from atrusted domain. Use a whitelist.

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Same Origin Policy

SAME ORIGIN POLICYSame Origin Policy (SOP), also called Single Origin Policy is a part of web application security model. Same Origin
Policy has vulnerabilities that the code reviewer needs to take into consideration. SOP covers three main areas of
web development, Trust, Authority, and Policy. Same Origin Policy is made of the combination of three components(Scheme, Hostname and Port).
18.1 Description
Internet Explorer has two major exceptions when it comes to same origin policy:
1. Trust Zones: if both domains are in highly trusted zone e.g, corporate domains, then the same origin limitations are not applied.
2. Port: IE doesn’t include port into Same Origin components, therefore http://yourcompany.com:81/index.
html and http://yourcompany.com/index.html are considered from same origin and no restrictions are applied.
These exceptions are non-standard and not supported in any of other browser but would be helpful if developing an
app for Windows RT (or) IE based web application.
The following figure displays the various parts of the URL:
Figure 12

foo://username:password@example.com:8042/over/there/index.dtb?type=animal&name=narwhal#nose

userinfo

hostname

port

query
fragment

authority

path

interpretable as keys

schemenameinterpretable as values

hierarchical partpath

interpretable as filename

interpretable as filename

Same Origin Policy• foo://username:password@example.com:8042/over/there/index.dtb?type=animal&name =narwhal#nose
18.2 What to Review• If application allows user-supplied data in the URL then the code reviewer needs to make sure the path,query or Fragment Id Code data is validated.
• Make sure user-supplied scheme name or authority section has good input validation. This is a major code
injection and phishing risk. Only permit prefixes needed by the application. Do not use blacklisting. Code reviewer should make sure only whitelisting is used for validation.
• Make sure authority section should only contain alphanumerics, “-“, and “.” And be followed by “/”, “?”,”#”. The
risk here an IDN homograph attack.
• Code reviewer needs to make sure the programmer is not assuming default behavior because the programmers browser properly escapes a particular character or browser standard says the character will be escaped
properly before allowing any URL-derived values are put inside a database query or the URL is echoed backto the user.
• Resources with a MIME type of image/png are treated as images and resources with MIME type of text/html
are treated as HTML documents. Web applications can limit that content’s authority by restricting its MIME
type. For example, serving user-generated content as image/png is less risky than serving user-generated
content as text/html.
• Privileges on document and resources should grant or withhold privileges from origins as a whole (rather than discriminating between individual documents within an origin). Withholding privileges is ineffective
because the document without the privilege can usually obtain the privilege anyway because SOP does not
isolate documents within an origin.

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Reviewing Logging CodeREVIEWING LOGGING CODE
Applications log messages of varying intensity and to varying sinks. Many logging APIs allow you to set the
granularity of log message from a state of logging nearly all messages at level ‘trace’ or ‘debug’ to only logging
the most important messages at level ‘critical’. Where the log message is written to is also a consideration,
sometimes it can be written to a local file, other times to a database log table, or it could be written over a
network link to a central logging server.
The volume of logging has to be controlled since the act of writing messages to the log uses CPU cycles, thus
writing every small detail to a log will use up more resources (CPU, network bandwidth, disk space). Couple
that with the fact that the logs have to be parsed or interpreted by a tool or human in order for them to be useful, the consumer of the log could have to parse through thousands of lines to find a message of consequence.
19.1 Description
Logs can vary by type; for example a log may simply contain application state or process data, allowing support or development track what the system is doing when a bug has occurred. Other logs may be specific tosecurity, only logging important information that a central security system will have interest in. Further logs
could be used for business purposes, such as billing.
Application logs can be powerful as the application business logic has the most information about the user(e.g. identity, roles, permissions) and the context of the event (target, action, outcomes), and often this data
is not available to either infrastructure devices, or even closely-related applications. Application logging is an
important feature of a production system, especially to support personnel and auditors, however it is often
forgotten and is rarely described in sufficient detail in design/requirement documentation. The level and content of security monitoring, alerting and reporting needs to be set during the requirements and design stageof projects, and should be proportionate to the information security risks. Application logging should also be
consistent within the application, consistent across an organization’s application portfolio and use industry
standards where relevant, so the logged event data can be consumed, correlated, analyzed and managed bya wide variety of systems.
All types of applications may send event data to remote systems, either directly over a network connection, or asynchronously though a daily/weekly/monthly secure copy of the log to some centralized log collection and management system (e.g. SIEM or SEM) or another application elsewhere.
If the information in the log is important, and could possibly be used for legal matters, consider how the source (log)
can be verified, and how integrity and non-repudiation can be enforced. Log data, temporary debug logs, and backups/copies/extractions, must not be destroyed before the duration of the required data retention period, and mustnot be kept beyond this time. Legal, regulatory and contractual obligations may impact on these periods.
Server applications commonly write event log data to the file system or a database (SQL or NoSQL), however logging
could be required on client devices such as applications installed on desktops and on mobile devices may use local
storage and local databases. Consider how this client logging data is transferred to the server.
What to Review
When reviewing code modules from a logging point of view, some common issues to look out for include:

Reviewing Logging Code• When using the file system, it is preferable to use a separate partition than those used by the operating system,other application files and user generated content• For file-based logs, apply strict permissions concerning which users can access the directories, and the permissionsof files within the directories• In web applications, the logs should not be exposed in web-accessible locations, and if done so, should haverestricted access and be configured with a plain text MIME type (not HTML)• When using a database, it is preferable to utilize a separate database account that is only used for writing log
data and which has very restrictive database, table, function and command permissions• Consider what types of messages should be logged:o Input validation failures e.g. protocol violations, unacceptable encodings, invalid parameter names and valueso Output validation failures e.g. database record set mismatch, invalid data encoding
o Authentication successes and failureso Authorization (access control) failureso Session management failures e.g. cookie session identification value modificationo Connection timings• Consider what each log message should contain:o Date and time, in some common format (also makes sense to ensure all nodes of an application are synced
through something like NTP
o User performing the action
o Action being performed/attemptedo Information on the client, e.g. IP address, source port, user-agento External classifications e.g. NIST Security Content Automation Protocol (SCAP), Mitre Common Attack Pattern Enumeration and Classification (CAPEC)o Perform sanitization on all event data to prevent log injection attacks e.g. carriage return (CR), line feed (LF)
and delimiter characters (and optionally to remove sensitive data)• If writing to databases, read, understand and apply the SQL injection cheat sheet• Ensure logging is implemented and enabled during application security, fuzz, penetration and performance testing

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Reviewing Logging Code• Ensure logging cannot be used to deplete system resources, for example by filling up disk space or exceeding
database transaction log space, leading to denial of service• The logging mechanisms and collected event data must be protected from mis-use such as tampering intransit, and unauthorized access, modification and deletion once stored• Store or copy log data to read-only media as soon as possible• Consider what should not be logged:o Session identification values (consider replacing with a hashed value if needed to track session specific events)o Sensitive personal data and some forms of personally identifiable information (PII)o Authentication passwords (successful or unsuccessful)o Database connection stringso Keyso Data of a higher security classification than the logging system is allowed to store
References• See NIST SP 800-92 Guide to Computer Security Log Management for more guidance.
• Mitre Common Event Expression (CEE)• PCISSC PCI DSS v2.0 Requirement 10 and PA-DSS v2.0 Requirement 4• Other Common Log File System (CLFS), Microsoft

Error Handling

ERROR HANDLING
Proper error handling is important in two ways:
1. It may affect the state of the application. The initial failure to prevent the error may cause the application to traverse
into an insecure state. This covers the key premise of ‘failing securely’, errors induced should not leave the applicationin an insecure state. Resources should be locked down and released, sessions terminated (if required), and calculations
or business logic should be halted (depending on the type of error, of course).
2. It may leak system information to a user. An important aspect of secure application development is to prevent
information leakage. Error messages give an attacker great insight into the inner workings of an application. Weakerror handling also aids the attacker, as the errors returned may assist them in constructing correct attack vectors.
A generic error page for most errors is recommended, this approach makes it more difficult for attackers to identify
signatures of potentially successful attacks such as blind SQL injection using booleanization ‘’<should include
reference>’’ or analysis of response time characteristics.
20.1 Description
The purpose of reviewing the Error Handling code is to assure that the application fails safely under all possible
error conditions, expected and unexpected. No sensitive information is presented to the user when an error occurs.
A company coding guidelines should include sections on Error Handling and how it should be controlled by anapplication suite, this will allow developers to code against this guidelines as well as review against them.A company
coding guidelines should include sections on Error Handling and how it should be controlled by an application suite,
this will allow developers to code against this guidelines as well as review against them.
For example, SQL injection is much tougher to successfully execute without some healthy error messages. It lessens
the attack footprint, and an attacker would have to resort to using “blind SQL injection” which is more difficult andtime consuming.
A well-planned error/exception handling guideline is important in a company for three reasons:
1. Good error handling does not give an attacker any information which is a means to an end, attacking the application
2. A proper centralised error strategy is easier to maintain and reduces the chance of any uncaught errors “bubblingup” to the front end of an application.
3. Information leakage could lead to social engineering exploits, for example if the hosting companies name isreturned, or some employees name can be seen.
Regardless of whether the development language provide checked exceptions or not, reviewers should
remember:• Not all errors are exceptions. Don’t rely on exception handling to be your only way of handling errors, handle allcase statement ‘default’ sections, ensure all ‘if’ statements have their ‘else’ clauses covered, ensure that all exits froma function (e.g. return statements, exceptions, etc.) are covered. RAII concepts (e.g. auto pointers and the like) arean advantage here. In languages like Java and C#, remember that errors are different from exceptions (different
hierarchy) and should be handled.

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Error Handling• Catching an exception is not automatically handling it. You’ve caught your exception, so how do you handle
it? For many cases this should be obvious enough, based on your business logic, but for some (e.g. out ofmemory, array index out of bounds, etc.) the handling many not be so simple.
• Don’t catch more that you can handle. Catch all clauses (e.g. ‘catch(Exception e)’ in Java & C# or ‘catch(...)
in C++) should be avoided as you will not know what type of exception you are handling, and if you don’tknow the exception type, how do you accurately handle it? It could be that the downstream server is not
responding, or a user may have exceeded their quota, or you may be out of memory, these issues should be
handled in different ways and thus should be caught in exception clauses that are specific.
When an exception or error is thrown, we also need to log this occurrence. Sometimes this is due to bad
development, but it can be the result of an attack or some other service your application relies on failing. Thishas to be imagined in the production scenario, if your application handles ‘failing securely’ by returning anerror response to the client, and since we don’t want to leak information that error will be generic, we need tohave some way of identifying why the failure occurred. If your customer reports 1000’s of errors occurred lastnight, you know that customer is going to want to know why. If you don’t have proper logging and traceability
coded into your application then you will not be able to establish if those errors were due to some attempted
hack, or an error in your business logic when handling a particular type of error.
All code paths that can cause an exception to be thrown should check for success in order for the exception
not to be thrown. This could be hard to impossible for a manual code review to cover, especially for large
bodies of code. However if there is a debug version of the code, then modules/functions could throw relevant
exceptions/errors and an automated tool can ensure the state and error responses from the module is as
expected. This then means the code reviewer has the job of ensuring all relevant exceptions/errors are tested
in the debug code.
What to Review
When reviewing code it is recommended that you assess the commonality within the application from anerror/exception handling perspective. Frameworks have error handling resources which can be exploited toassist in secure programming, and such resources within the framework should be reviewed to assess if theerror handling is “wired-up” correctly. A generic error page should be used for all exceptions if possible as thisprevents the attacker from identifying internal responses to error states. This also makes it more difficult for
automated tools to identify successful attacks.
For JSP struts this could be controlled in the struts-config.xml file, a key file when reviewing the wired-up struts
environment:
Sample 20.1
<exception key=”bank.error.nowonga”
path=”/NoWonga.jsp”
type=”mybank.account.NoCashException”/>

Specification can be done for JSP in web.xml in order to handle unhandled exceptions. When unhandled
exceptions occur, but are not caught in code, the user is forwarded to a generic error page:Error Handling

Sample 20.2
<error-page><exception-type>UnhandledException</exception-type><location>GenericError.jsp</location></error-page>
Also in the case of HTTP 404 or HTTP 500 errors during the review you may find:Sample 20.3
<error-page><error-code>500</error-code><location>GenericError.jsp</location></error-page>For IIS development the ‘Application_Error()’ handler will allow the application to catch all uncaught exceptions andhandle them in a consistent way. Note this is important or else there is a chance your exception informaiton could besent back to the client in the response.
For Apache development, returning failures from handlers or modules can prevent an further processing by the
Apache engine and result in an error response from the server. Response headers, body, etc can be set by by thehandler/module or can be configured using the“ErrorDocument”configuration. We should use a localized description
string in every exception, a friendly error reason such as “System Error – Please try again later”. When the user sees anerror message, it will be derived from this description string of the exception that was thrown, and never from theexception class which may contain a stack trace, line number where the error occurred, class name, or method name.
Do not expose sensitive information like exception messages. Information such as paths on the local file system isconsidered privileged information; any internal system information should be hidden from the user. As mentioned
before, an attacker could use this information to gather private user information from the application or componentsthat make up the app.
Don’t put people’s names or any internal contact information in error messages. Don’t put any “human” information,
which would lead to a level of familiarity and a social engineering exploit.
What to Review: Failing Securely
There can be many different reasons why an application may fail, for example:• The result of business logic conditions not being met.
• The result of the environment wherein the business logic resides fails.
• The result of upstream or downstream systems upon which the application depends fail.
• Technical hardware / physical failure.

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Failures are like the Spanish Inquisition; popularly nobody expected the Spanish Inquisition (see Monty Python) but inreal life the Spanish knew when an inquisition was going to occur and were prepared for it, similarly in an application,
though you don’t expect errors to occur your code should be prepared for them to happen. In the event of a failure,
it is important not to leave the “doors” of the application open and the keys to other “rooms” within the application
sitting on the table. In the course of a logical workflow, which is designed based upon requirements, errors may occur
which can be programmatically handled, such as a connection pool not being available, or a downstream server
returning a failure.
Such areas of failure should be examined during the course of the code review. It should be examined if resources
should be released such as memory, connection pools, file handles etc.
The review of code should also include pinpointing areas where the user session should be terminated or invalidated.
Sometimes errors may occur which do not make any logical sense from a business logic perspective or a technicalstandpoint, for example a logged in user looking to access an account which is not registered to that user. Such
conditions reflect possible malicious activity. Here we should review if the code is in any way defensive and kills
the user’s session object and forwards the user to the login page. (Keep in mind that the session object should be
examined upon every HTTP request).
What to Review: Potentially Vulnerable CodeJava
In Java we have the concept of an error object; the Exception object. This lives in the Java package java.lang and isderived from the Throwable object. Exceptions are thrown when an abnormal occurrence has occurred. Anotherobject derived from Throwable is the Error object, which is thrown when something more serious occurs. The Error
object can be caught in a catch clause, but cannot be handled, the best you can do is log some information about
the Error and then re-throw it.
Information leakage can occur when developers use some exception methods, which ‘bubble’ to the user UI due to apoor error handling strategy. The methods are as follows:• printStackTrace()• getStackTrace()
Also important to know is that the output of these methods is printed in System console, the same as System.out.
println(e) where there is an Exception. Be sure to not redirect the outputStream to PrintWriter object of JSP, byconvention called “out”, for example:• printStackTrace(out);
Note it is possible to change where system.err and system.out write to (like modifying fd 1 & 2 in bash or C/C++),
using the java.lang.system package:• setErr() for the System.err field, and• setOut() for the System.out field.
This could be used on a process wide basis to ensure no output gets written to standard error or standard out (which
can be reflected back to the client) but instead write to a configured log file.
C#.NET
In .NET a System.Exception object exists and has commonly used child objects such as ApplicationException and
SystemException are used. It is not recommended that you throw or catch a SystemException this is thrown by
runtime.

Error Handling

When an error occurs, either the system or the currently executing application reports it by throwing an exception
containing information about the error, similar to Java. Once thrown, an exception is handled by the application orby the default exception handler. This Exception object contains similar methods to the Java implementation such as:• StackTrace• Source• Message• HelpLink
In .NET we need to look at the error handling strategy from the point of view of global error handling and the handlingof unexpected errors. This can be done in many ways and this article is not an exhaustive list. Firstly, an Error Event isthrown when an unhandled exception is thrown.
This is part of the TemplateControl class, see reference:• http://msdn.microsoft.com/library/default.asp?url=/library/enus/cpref/html/
frlrfSystemWebUITemplateControlClassErrorTopic.asp
Error handling can be done in three ways in .NET, executed in the following order:• On the aspx or associated codebehind page in the Page_Error.
• In the global.asax file’s Application_Error (as mentioned before).
• In the web.config file’s customErrors section.
It is recommended to look in these areas to understand the error strategy of the application.
Classic ASP
Unlike Java and .NET, classic ASP pages do not have structured error handling in try-catch blocks. Instead they have a
specific object called “err”. This makes error handling in a classic ASP pages hard to do and prone to design errors onerror handlers, causing race conditions and information leakage. Also, as ASP uses VBScript (a subtract of Visual Basic),
sentences like “On Error GoTo label” are not available. In classic ASP there are two ways to do error handling, the firstis using the err object with a “On Error Resume Next” and “On Error GoTo 0”.
Sample 20.4
Public Function IsInteger (ByVal Number)
Dim Res, tNumber
Number = Trim(Number)tNumber=Number
On Error Resume Next
‘If an error occurs continue execution
Number = CInt(Number)‘if Number is a alphanumeric string a Type Mismatch error will occur
Res = (err.number = 0)
‘If there are no errors then return true
On Error GoTo 0‘If an error occurs stop execution and display error
re.Pattern = “^[\+\-]? *\d+$”‘only one +/- and digits are allowed
IsInteger = re.Test(tNumber) And Res
End Function

The second is using an error handler on an error page (http://support.microsoft.com/kb/299981).

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Sample 20.5
Dim ErrObj
set ErrObj = Server.GetLastError()‘Now use ErrObj as the regular err object

C++
In the C++ language, any object or built in type can be thrown. However there is a STL type std::exception
which is supposed to be used as the parent of any user defined exception, indeed this type is used in the
STL and many libraries as the parent type of all exceptions. Having std::exception encourages developers tocreate a hierarchy of exceptions which match the exception usage and means all exceptions can be caught bycatching a std::exception object (instead of a ‘catch (...)’ block).
Unlike Java, even errors that you can’t recover from (e.g. std::bad_alloc which means your out of memory)derive from std::exception, so a ‘catch( std::exception& e)’ is similar to ‘catch (...)’ except that it allows you accessto the exception so you can know what occurred and possibly print some error information using e.what().
There are many logging libraries for C++, so if your codebase uses a particular logging class look for usages ofthat logger for anywhere sensitive information can be written to the logs.
What to Review: Error Handling in IIS
Page_Error is page level handling which is run on the server side in .NET. Below is an example but the error
information is a little too informative and hence bad practice.
Sample 20.6
<script language=”C#” runat=”server”>
Sub Page_Error(Source As Object, E As EventArgs)
Dim message As String = Request.Url.ToString()& Server.GetLastError().ToString()
Response.Write(message) // display message
End Sub</script>
The text in the example above has a number of issues. Firstly, it displays the HTTP request to the user in theform of Request.Url.ToString().
Assuming there has been no data validation prior to this point, we are vulnerable to cross site scripting attacks.
Secondly, the error message and stack trace is displayed to the user using Server.GetLastError().ToString()
which divulges internal information regarding the application.
After the Page_Error is called, the Application_Error sub is called.
When an error occurs, the Application_Error function is called. In this method we can log the error and redirect
to another page. In fact catching errors in Application_Error instead of Page_Error would be an example ofcentralizing errors as described earlier.

Error Handling

Sample 20.7<%@ Import Namespace=”System.Diagnostics” %><script language=”C#” runat=”server”>
void Application_Error(Object sender, EventArgs e) {String Message = “\n\nURL: http://localhost/” + Request.Path+ “\n\nMESSAGE:\n “ + Server.GetLastError().Message+ “\n\nSTACK TRACE:\n” + Server.GetLastError().StackTrace;// Insert into Event Log
EventLog Log = new EventLog();
Log.Source = LogName;
Log.WriteEntry(Message, EventLogEntryType.Error);Server.Redirect(Error.htm) // this shall also clear the error}</script>
Above is an example of code in Global.asax and the Application_Error method. The error is logged and then the user
is redirected. Non-validated parameters are being logged here in the form of Request.Path. Care must be taken
not to log or display non-validated input from any external source. ‘’<link to XSS>’’Web.config has custom error tags which can be used to handle errors. This is called last and if Page_error or
Application_error is called and has functionality, that functionality shall be executed first. If the previous two
handling mechanisms do not redirect or clear (Response.Redirect or a Server.ClearError), this will be called andyou shall be forwarded to the page defined in web.config in the customErrors section, which is configured asfollows:
Sample 20.8
<customErrors mode=”<On|Off|RemoteOnly>” defaultRedirect=”<default redirect page>”><error statusCode=”<HTTP status code>” redirect=”<specific redirect page for listed status code>”/></customErrors>

The “mode” attribute value of “On” means that custom errors are enabled whilst the “Off” value means that
custom errors are disabled. The “mode” attribute can also be set to “RemoteOnly” which specifies that custom
errors are shown only to remote clients and ASP.NET errors are shown to requests coming from the the local
host. If the “mode” attribute is not set then it defaults to “RemoteOnly”.
When an error occurs, if the status code of the response matches one of the error elements, then the relevent‘redirect’ value is returned as the error page. If the status code does not match then the error page from the‘defaultRedirect’ attribute will be displayed. If no value is set for ‘defaultRedirect’ then a generic IIS error pageis returned.
An example of the customErrors section completed for an application is as follows:

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Sample 20.9
<customErrors mode=”On” defaultRedirect=”error.html”><error statusCode=”500” redirect=”err500.aspx”/><error statusCode=”404” redirect=”notHere.aspx”/><error statusCode=”403” redirect=”notAuthz.aspx”/></customErrors>

What to Review: Error Handling in Apache
In Apache you have two choices in how to return error messages to the client:
1. You can write the error status code into the req object and write the response to appear the way you want,then have you handler return ‘DONE’ (which means the Apache framework will not allow any further handlers/filters to process the request and will send the response to the client.
2. Your handler or filter code can return pre-defined values which will tell the Apache framework the result ofyour codes processsing (essentially the HTTP status code). You can then configure what error pages should be
returned for each error code.
In the interest of centralizing all error code handling, option 2 can make more sense. To return a specific predefined value from your handler, refer to the Apache documentation for the list of values to use, and thenreturn from the handler function as shown in the following example:
Sample 20.10
static int my_handler(request_rec *r){if ( problem_processing() ){return HTTP_INTERNAL_SERVER_ERROR;}... continue processing request ...
}In the httpd.conf file you can then specify which page should be returned for each error code using the‘ErrorDocument’ directive. The format of this directive is as follows:• ErrorDocument <3-digit-code> <action>... where the 3 digit code is the HTTP response code set by the handler, and the action is a local or external URL
to be returned, or specific text to display. The following examples are taken from the Apache ErrorDocument
documentation (https://httpd.apache.org/docs/2.4/custom-error.html) which contains more informationand options on ErrorDocument directives:

Error Handling

Sample 20.11
ErrorDocument 500 “Sorry, our script crashed. Oh dear”
ErrorDocument 500 /cgi-bin/crash-recover
ErrorDocument 500 http://error.example.com/server_error.html
ErrorDocument 404 /errors/not_found.html
ErrorDocument 401 /subscription/how_to_subscribe.html

What to Review: Leading Practice for Error Handling
Code that might throw exceptions should be in a try block and code that handles exceptions in a catch block.
The catch block is a series of statements beginning with the keyword catch, followed by an exception type andan action to be taken.
Example: Java Try-Catch:
Sample 20.12
public class DoStuff {public static void Main() {try {StreamReader sr = File.OpenText(“stuff.txt”);
Console.WriteLine(“Reading line {0}”, sr.ReadLine());}
catch(MyClassExtendedFromException e) {Console.WriteLine(“An error occurred. Please leave to room”);
logerror(“Error: “, e);}}}.NET Try–CatchSample 20.13
public void run() {while (!stop) {try {// Perform work here
} catch (Throwable t) {// Log the exception and continue
WriteToUser(“An Error has occurred, put the kettle on”);
logger.log(Level.SEVERE, “Unexception exception”, t);

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Error Handling}}}

C++ Try–CatchSample 20.14
void perform_fn() {try {// Perform work here
} catch ( const MyClassExtendedFromStdException& e) {// Log the exception and continue
WriteToUser(“An Error has occurred, put the kettle on”);
logger.log(Level.SEVERE, “Unexception exception”, e);}}In general, it is best practice to catch a specific type of exception rather than use the basic catch(Exception) or
catch(Throwable) statement in the case of Java.
What to Review: The Order of Catching Exceptions
Keep in mind that many languages will attempt to match the thrown exception to the catch clause even if it means
matching the thrown exception to a parent class. Also remember that catch clauses are checked in the order they arecoded on the page. This could leave you in the situation where a certain type of exception might never be handledcorrectly, take the following example where ‘non_even_argument’ is a subclass of ‘std::invalid_argument’:Sample 20.15
class non_even_argument : public std::invalid_argument {public:explicit non_even_argument (const string& what_arg);};
void do_fn(){try{// Perform work that could throw}
catch ( const std::invalid_argument& e ){Error Handling// Perform generic invalid argument processing and return failure}
catch ( const non_even_argument& e ){// Perform specific processing to make argument even and continue processing}}
The problem with this code is that when a ‘non_even_argument is thrown, the catch branch handling‘std::invalid_argument’ will always be executed as it is a parent of ‘non_even_argument’ and thus the runtime
system will consider it a match (this could also lead to slicing). Thus you need to be aware of the hierarchy ofyour exception objects and ensure that you list the catch for the more specific exceptions first in your code.
If the language in question has a finally method, use it. The finally method is guaranteed to always be called.
The finally method can be used to release resources referenced by the method that threw the exception. This isvery important. An example would be if a method gained a database connection from a pool of connections,and an exception occurred without finally, the connection object shall not be returned to the pool for sometime (until the timeout). This can lead to pool exhaustion. finally() is called even if no exception is thrown.
Sample 20.16
void perform_fn() {try {// Perform work here}

} catch ( const MyClassExtendedFromStdException& e) {// Log the exception and continue
WriteToUser(“An Error has occurred, put the kettle on”);
logger.log(Level.SEVERE, “Unexception exception”, e);}

A Java example showing finally() being used to release system resources.
What to Review: Releasing resources and good housekeeping
RAII is Resource Acquisition Is Initialization, which is a way of saying that when you first create an instance ofa type, it should be fully setup (or as much as possible) so that it’s in a good state. Another advantage of RAII
is how objects are disposed of, effectively when an object instance is no longer needed then it resources areautomatically returned when the object goes out of scope (C++) or when it’s ‘using’ block is finished (C# ‘using’
directive which calls the Dispose method, or Java 7’s try-with-resources feature)
RAII has the advantage that programmers (and users to libraries) don’t need to explicitly delete objects, theobjects will be removed themselves, and in the process of removing themselves (destructor or Dispose)
For Classic ASP pages it is recommended to enclose all cleaning in a function and call it into an error handling

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Error Handlingstatement after an “On Error Resume Next”.
Building an infrastructure for consistent error reporting proves more difficult than error handling. Struts
provides the ActionMessages and ActionErrors classes for maintaining a stack of error messages to be reported,which can be used with JSP tags like <html: error> to display these error messages to the user.
To report a different severity of a message in a different manner (like error, warning, or information) the
following tasks are required:
1. Register, instantiate the errors under the appropriate severity
2. Identify these messages and show them in a consistent manner.
Struts ActionErrors class makes error handling quite easy:
Sample 20.17
ActionErrors errors = new ActionErrors()
errors.add(“fatal”, new ActionError(“....”));
errors.add(“error”, new ActionError(“....”));
errors.add(“warning”, new ActionError(“....”));
errors.add(“information”, new ActionError(“....”));
saveErrors(request,errors); // Important to do thisNow that we have added the errors, we display them by using tags in the HTML page.
Sample 20.18<logic:messagePresent property=”error”><html:messages property=”error” id=”errMsg” ><bean:write name=”errMsg”/></html:messages></logic:messagePresent >References• For classic ASP pages you need to do some IIS configuration, follow http://support.microsoft.com/
kb/299981 for more information.
• For default HTTP error page handling in struts (web.xml) see https://software-security.sans.org/
blog/2010/08/11/security-misconfigurations-java-webxml-files

Reviewing Security Alerts

REVIEWING SECURITY ALERTS
How will your code and applications react when something has gone wrong? Many companies that follow
secure design and coding principals do so to prevent attackers from getting into their network, however
many companies do not consider designing and coding for the scenario where an attacker may have founda vulnerability, or has already exploited it to run code within a companies firewalls (i.e. within the Intranet).
Many companies employ SIEM logging technologies to monitor network and OS logs for patterns that detectsuspicions activity, this section aims to further encourage application layers and interfaces to do the same.
21.1 Description
This section concentrates on:
1. Design and code that allows the user to react when a system is being attacked.
2. Concepts allowing applications to flag when they have been breached.
When a company implements secure design and coding, it will have the aim of preventing attackers frommisusing the software and accessing information they should not have access to. Input validation checks for
SQL injections, XSS, CSRF, etc. should prevent attackers from being able to exploit these types of vulnerabilities
against the software. However how should software react when an attacker is attempting to breach thedefenses, or the protections have been breached?
For an application to alert to security issues, it needs context on what is ‘normal’ and what constitutes asecurity issue. This will differ based on the application and the context within which it is running. In generalapplications should not attempt to log every item that occurs as the excessive logging will slow down thesystem, fill up disk or DB space, and make it very hard to filter through all the information to find the security
issue.
At the same time, if not enough information is monitored or logged, then security alerting will be very hard to
do based on the available information. To achieve this balance an application could use its own risk scoring
system, monitoring at a system level what risk triggers have been spotted (i.e. invalid inputs, failed passwords,etc.) and use different modes of logging. Take an example of normal usage, in this scenario only critical items
are logged. However if the security risk is perceived to have increased, then major or security level items
can be logged and acted upon. This higher security risk could also invoke further security functionality as
described later in this section.
Take an example where an online form (post authentication) allows a user to enter a month of the year. Herethe UI is designed to give the user a drop down list of the months (January through to December). In this casethe logged in user should only ever enter one of 12 values, since they typically should not be entering any text,
instead they are simply selecting one of the pre-defined drop down values.
If the server receiving this form has followed secure coding practices, it will typically check that the form field
matches one of the 12 allowed values, and then considers it valid. If the form field does not match, it returns
an error, and may log a message in the server. This prevents the attacker from exploiting this particular field,
however this is unlikely to deter an attacker and they would move onto other form fields.

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In this scenario we have more information available to us than we have recorded. We have returned an error
back to the user, and maybe logged an error on the server. In fact we know a lot more; an authenticated user
has entered an invalid value which they should never have been able to do (as it’s a drop down list) in normal
usage.
This could be due to a few reasons:• There’s a bug in the software and the user is not malicious.
• An attacker has stolen the users login credentials and is attempting to attack the system.
• A user has logged in but has a virus/trojan which is attempting to attack the system.
• A user has logged in but is experiencing a man-in-the-middle attack.
• A user is not intending to be malicious but has somehow changed the value with some browser plugin, etc.
If it’s the first case above, then the company should know about it to fix their system. If it’s case 2, 3 or 3 then theapplication should take some action to protect itself and the user, such as reducing the functionality available
to the user (i.e. no PII viewable, can’t change passwords, can’t perform financial transactions) or forcing further
authentication such as security questions or out-of-band authentication. The system could also alert the user
to the fact that the unexpected input was spotted and advise them to run antivirus, etc., thus stopping an
attack when it is underway.
Obviously care must be taken in limiting user functionality or alerting users encase it’s an honest mistake, sousing a risk score or noting session alerts should be used. For example, if everything has been normal in thebrowsing session and 1 character is out of place, then showing a red pop-up box stating the user has been
hacked is not reasonable, however if this is not the usual IP address for the user, they have logged in at an
unusual time, and this is the 5th malformed entry with what looks like an SQL injection string, then it would bereasonable for the application to react. This possible reaction would need to be stated in legal documentation.
In another scenario, if an attacker has got through the application defenses extracted part of the applications
customer database, would the company know? Splitting information in the database into separate tables
makes sense from an efficiency point of view, but also from a security view, even putting confidential
information into a separate partition can make it harder for the attacker. However if the attacker has theinformation it can be hard to detect and applications should make steps to aid alerting software (e.g. SIEM
systems). Many financial institutions use risk scoring systems to look at elements of the user’s session to givea risk score, if Johnny always logs in at 6pm on a Thursday from the same IP, then we have a trusted pattern.
If suddenly Johnny logs in at 2:15am from an IP address on the other side of the world, after getting the
password wrong 7 times, then maybe he’s jetlagged after a long trip, or perhaps his account has been hacked.
Either way, asking him for out-of-band authentication would be reasonable to allow Johnny to log in, or toblock an attacker from using Johnny’s account.
If the application takes this to a larger view, it can determine that on a normal day 3% of the users log onin what would be considered a riskier way, i.e. different IP address, different time, etc. If on Thursday it sees
this number rise to 23% then has something strange happened to the user base, or has the database been
hacked? This type of information can be used to enforce a blanket out-of-band authentication (and internal
investigation of the logs) for the 23% of ‘riskier’ users, thereby combining the risk score for the user with theoverall risk score for the application.

Reviewing Security Alerts

Another good option is ‘honey accounts’ which are usernames and passwords that are never given out tousers. These accounts are added just like any other user, and stored in the DB, however they are also recorded
in a special cache and checked on login. Since they are never given to any user, no user should ever logon withthem, however if one of those accounts are used, then the only way that username password combination
could be known is if an attacker got the database, and this information allows the application to move to amore secure state and alert the company that the DB has been hacked.
What to Review
When reviewing code modules from a security alerting point of view, some common issues to look out forinclude:• Will the application know if it’s being attacked? Does it ignore invalid inputs, logins, etc. or does it log themand monitor this state to capture a cumulative perception of the current risk to the system?
• Can the application automatically change its logging level to react to security threats? Is changing security
levels dynamic or does it require a restart?
• Does the SDLC requirements or design documentation capture what would constitute a security alert? Has
this determination been peer reviewed? Does the testing cycle run through these scenarios?
• Does the system employ ‘honey accounts’ such that the application will know if the DB has been compromised?
• Is there a risk based scoring system that records the normal usage of users and allows for determination orreaction if the risk increases?
• If a SIEM system is being used, have appropriate triggers been identified? Has automated tests been created
to ensure those trigger log messages are not accidentally modified by future enhancements or bug fixes?
• Does the system track how many failed login attempts a user has experienced? Does the system react to this?
• Does certain functionality (i.e. transaction initiation, changing password, etc) have different modes ofoperation based on the current risk score the application is currently operating within?
• Can the application revert back to ‘normal’ operation when the security risk score drops to normal levels?
• How are administrators alerted when security risk score rises? Or when a breach has been assumed? At an
operational level, is this tested regularly? How are changes of personnel handled?

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REVIEW FOR ACTIVE DEFENSE
Attack detection undertaken at the application layer has access to the complete context of an interactionand enhanced information about the user. If logic is applied within the code to detect suspicious activity(similar to an application level IPS) then the application will know what is a high-value issue and what is noise.
Input data are already decrypted and canonicalized within the application and therefore application-specificintrusion detection is less susceptible to advanced evasion techniques. This leads to a very low level of attack
identification false positives, providing appropriate detection points are selected.
The fundamental requirements are the ability to perform four tasks:
1. Detection of a selection of suspicious and malicious events.
2. Use of this knowledge centrally to identify attacks.
3. Selection of a predefined response.
4. Execution of the response.
22.1 Description
Applications can undertake a range of responses that may include high risk functionality such as changes toa user’s account or other changes to the application’s defensive posture. It can be difficult to detect active
defense in dynamic analysis since the responses may be invisible to the tester. Code review is the best method
to determine the existence of this defense.
Other application functionality like authentication failure counts and lock-out, or limits on rate of file uploads
are ‘localized’ protection mechanisms. This sort of standalone logic is ‘not’ active defense equivalents inthe context of this review, unless they are rigged together into an application-wide sensory network andcentralized analytical engine.
It is not a bolt-on tool or code library, but instead offers insight to an approach for organizations to specify
or develop their own implementations – specific to their own business, applications, environments, and risk
profile – building upon existing standard security controls.
What to Review
In the case where a code review is being used to detect the presence of a defense, its absence should be noted
as a weakness. Note that active defense cannot defend an application that has known vulnerabilities, andtherefore the other parts of this guide are extremely important. The code reviewer should note the absenceof active defense as a vulnerability.
The purpose of code review is not necessarily to determine the efficacy of the active defense, but could simply
be to determine if such capability exists.
Detection points can be integrated into presentation, business and data layers of the application. Applicationspecific intrusion detection does not need to identify all invalid usage, to be able to determine an attack. Thereis no need for “infinite data” or “big data” and therefore the location of “detection points” may be very sparse
within source code.

Reviewing for Active Defense

A useful approach for identifying such code is to find the name of a dedicated module for detecting suspicious
activity (such as OWASP AppSensor). Additionally a company can implement a policy of tagging active defense
detection points based on Mitre’s Common Attack Pattern Enumeration and Classifcation (CAPEC), strings
such as CAPEC-212, CAPEC-213, etc.
The OWASP AppSensor detection point type identifiers and CAPEC codes will often have been used inconfiguration values (e.g. [https://code.google.com/p/appsensor/source/browse/trunk/AppSensor/src/test/
resources/.esapi/ESAPI.properties?r=53 in ESAPI for Java]), parameter names and security event classification.
Also, examine error logging and security event logging mechanisms as these may be being used to collectdata that can then be used for attack detection. Identify the code or services called that perform this logging
and examine the event properties recorded/sent. Then identify all places where these are called from.
An examination of error handling code relating to input and output validation is very likely to reveal thepresence of detection points. For example, in a whitelist type of detection point, additional code may havebeen added adjacent, or within error handling code flow:
In some situations attack detection points are looking for blacklisted input, and the test may not exist otherwise,
if ( var !Match this ) {Error handling
Record event for attack detection}

so brand new code is needed. Identification of detection points should also have found the locations where
events are recorded (the “event store”). If detection points cannot be found, continue to review the code for
execution of response, as this may provide insight into the existence of active defense.
The event store has to be analysed in real time or very frequently, in order to identify attacks based on
predefined criteria. The criteria should be defined in configuration settings (e.g. in configuration files, or readfrom another source such as a database). A process will examine the event store to determine if an attack is inprogress, typically this will be attempting to identify an authenticated user, but it may also consider a single
IP address, range of IP addresses, or groups of users such as one or more roles, users with a particular privilege
or even all users.
Once an attack has been identified, the response will be selected based on predefined criteria. Again an
examination of configuration data should reveal the thresholds related to each detection point, groups ofdetection points or overall thresholds.
The most common response actions are user warning messages, log out, account lockout and administrator
notification. However, as this approach is connected into the application, the possibilities of response actions
are limited only by the coded capabilities of the application.
Search code for any global includes that poll attack identification/response identified above. Response actions(again a user, IP address, group of users, etc) will usually be initiated by the application, but in some cases otherapplications (e.g. alter a fraud setting) or infrastructure components (e.g. block an IP address range) may also
be involved.
Examine configuration files and any external communication the application performs.

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The following types of responses may have been coded:• Logging increased• Administrator notification• Other notification (e.g. other system)• Proxy• User status change• User notification• Timing change• Process terminated (same as traditional defenses)• Function disabled• Account log out• Account lock out• Collect data from user.
Other capabilities of the application and related system components can be repurposed or extended, toprovide the selected response actions. Therefore review the code associated with any localised security
measures such as account lock out.
References• The guidance for adding active response to applications given in theOWASP_AppSensor_Project• Category: OWASP Enterprise Security API• https://code.google.com/p/appsensor/ AppSensor demonstration code

Race Conditions

RACE CONDITIONS
Race Conditions occur when a piece of code does not work as it is supposed to (like many security issues). They
are the result of an unexpected ordering of events, which can result in the finite state machine of the code totransition to a undefined state, and also give rise to contention of more than one thread of execution over thesame resource. Multiple threads of execution acting or manipulating the same area in memory or persisted
data which gives rise to integrity issues.
23.1 Description
With competing tasks manipulating the same resource, we can easily get a race condition as the resource is
not in step-lock or utilises a token based system such as semaphores. For example if there are two processes(Thread 1, T1) and (Thread 2, T2). The code in question adds 10 to an integer X. The initial value of X is 5.
X = X + 10
With no controls surrounding this code in a multithreaded environment, the code could experience thefollowing problem:
T1 places X into a register in thread 1
T2 places X into a register in thread 2
T1 adds 10 to the value in T1’s register resulting in 15
T2 adds 10 to the value in T2’s register resulting in 15
T1 saves the register value (15) into X.
T1 saves the register value (15) into X.
The value should actually be 25, as each thread added 10 to the initial value of 5. But the actual value is 15 due
to T2 not letting T1 save into X before it takes a value of X for its addition.
This leads to undefined behavior, where the application is in an unsure state and therefore security cannot be
accurately enforced.
What to Review• In C#.NET look for code which used multithreaded environments:o Threado System.Threadingo ThreadPoolo System.Threading.Interlocked• In Java code look foro java.lang.Threado start()o stop()o destroy()o init()o synchronized

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o wait()o notify()
o notifyAll()• For classic ASP multithreading is not a directly supported feature, so this kind of race condition could bepresent only when using COM objects.
• Static methods and variables (one per class, not one per object) are an issue particularly if there is a sharedstate among multiple threads. For example, in Apache, struts static members should not be used to store
information relating to a particular request. The same instance of a class can be used by multiple threads, andthe value of the static member cannot be guaranteed.
• Instances of classes do not need to be thread safe as one is made per operation/request. Static states must
be thread safe.
o References to static variables, these must be thread locked.
o Releasing a lock in places other then finally{} may cause issues.
o Static methods that alter static state.

References• http://msdn2.microsoft.com/en-us/library/f857xew0(vs.71).aspx

Buffer Overruns

BUFFER OVERRUNS
A buffer is an amount of contiguous memory set aside for storing information. For example if a program
has to remember certain things, such as what your shopping cart contains or what data was inputted prior
to the current operation. This information is stored in memory in a buffer. Languages like C, C++ (which
many operating systems are written in), and Objective-C are extremely efficient, however they allow code toaccess process memory directly (through memory allocation and pointers) and intermingle data and control
information (e.g. in the process stack). If a programmer makes a mistake with a buffer and allows user input
to run past the allocated memory, the user input can overwrite program control information and allow theuser to modify the execution of the code.
Note that Java, C#.NET, Python and Ruby are not vulnerable to buffer overflows due to the way they store theirstrings in char arrays, of which the bounds are automatically checked by the frameworks, and the fact thatthey do not allow the programmer direct access to the memory (the virtual machine layer handles memory
instead). Therefore this section does not apply to those languages. Note however that native code called
within those languages (e.g. assembly, C, C++) through interfaces such as JNI or ‘unsafe’ C# sections can besusceptible to buffer overflows.
24.1 Description
To allocate a buffer the code declares a variable of a particular size:• char myBuffer[100]; // large enough to hold 100 char variables• int myIntBuf[5]; // large enough to hold 5 integers• Widget myWidgetArray[17]; // large enough to hold 17 Widget objects
As there is no automatic bounds checking code can attempt to add a Widget at array location 23 (which does
not exist). When the code does this, the complier will calculate where the 23rd Widget should be placed inmemory (by multiplying 23 x sizeof(Widget) and adding this to the location of the ‘myWidgetArray’ pointer).
Any other object, or program control variable/register, that existed at this location will be overwritten.
Arrays, vectors, etc. are indexed starting from 0, meaning the first element in the container is at ‘myBuffer[0]’,this means the last element in the container is not at array index 100, but at array index 99. This can often leadto mistakes and the ‘off by one’ error, when loops or programming logic assume objects can be written to thelast index without corrupting memory.
In C, and before the C++ STL became popular, strings were held as arrays of characters:• char nameString[10];This means that the ‘nameString’ array of characters is vulnerable to array indexing problems described above,
and when many of the string manipulation functions (such as strcpy, strcat, described later) are used, thepossibility of writing beyond the 10th element allows a buffer overrun and thus memory corruption.
As an example, a program might want to keep track of the days of the week. The programmer tells thecomputer to store a space for 7 numbers. This is an example of a buffer. But what happens if an attempt to add

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8 numbers is performed? Languages such as C and C++ do not perform bounds checking, and therefore if the
program is written in such a language, the 8th piece of data would overwrite the program space of the next
program in memory, and would result in data corruption. This can cause the program to crash at a minimum
or a carefully crafted overflow can cause malicious code to be executed, as the overflow payload is actual code.
What to Review: Buffer Overruns
Sample 24.1
void copyData(char *userId) {char smallBuffer[10]; // size of 10
strcpy (smallBuffer, userId);}
int main(int argc, char *argv[]) {char *userId = “01234567890”; // Payload of 12 when you include the ‘\n’ string termination// automatically added by the “01234567890” literal
copyData (userId); // this shall cause a buffer overload}

C library functions such as strcpy (), strcat (), sprintf () and vsprintf () operate on null terminated strings and performno bounds checking. gets() is another function that reads input (into a buffer) from stdin until a terminating newline
or EOF (End of File) is found. The scanf () family of functions also may result in buffer overflows.
Using strncpy(), strncat() and snprintf() functions allows a third ‘length’ parameter to be passed which determines the
maximum length of data that will be copied/etc. into the destination buffer. If this is correctly set to the size of thebuffer being written to, it will prevent the target buffer being overflowed. Also note fgets() is a replacement for gets().
Always check the bounds of an array before writing it to a buffer. The Microsoft C runtime also provides additionalversions of many functions with an ‘_s’ suffix (strcpy_s, strcat_s, sprintf_s). These functions perform additional checks
for error conditions and call an error handler on failure.
The C code below is not vulnerable to buffer overflow as the copy functionality is performed by ‘strncpy’ which
specifies the third argument of the length of the character array to be copied, 10.
Sample 24.2
void copyData(char *userId) {char smallBuffer[10]; // size of 10
strncpy(smallBuffer, userId, sizeof(smallBuffer)); // only copy first 10 elements
smallBuffer[10] = 0; // Make sure it is terminated.
}
int main(int argc, char *argv[]) {char *userId = “01234567890”; // Payload of 11
copyData (userId);}

Buffer Overruns

Modern day C++ (C++11) programs have access to many STL objects and templates that help prevent security
vulnerabilities. The std::string object does not require the calling code have any access to underlying pointers,
and automatically grows the underlying string representation (character buffer on the heap) to accommodate
the operations being performed. Therefore code is unable to cause a buffer overflow on a std::string object.
Regarding pointers (which can be used in other ways to cause overflows), C++11 has smart pointers which
again take away any necessity for the calling code to user the underlying pointer, these types of pointers areautomatically allocated and destroyed when the variable goes out of scope. This helps to prevent memory leaks anddouble delete errors. Also the STL containers such as std::vector, std::list, etc., all allocate their memory dynamicallymeaning normal usage will not result in buffer overflows. Note that it is still possible to access these containers
underlying raw pointers, or reinterpret_cast the objects, thus buffer overflows are possible, however they are moredifficult to cause.
Compliers also help with memory issues, in modern compilers there are ‘stack canaries’ which are subtle elements
placed in the complied code which check for out-of-bound memory accesses. These can be enabled when compilingthe code, or they could be enabled automatically. There are many examples of these stack canaries, and for somesystem many choices of stack canaries depending on an organizations appetite for security versus performance.
Apple also have stack canaries for iOS code as Objective-C is also susceptible to buffer overflows.
In general, there are obvious examples of code where a manual code reviewer can spot the potential for overflows
and off-by-one errors, however other memory leaks (or issues) can be harder to spot. Therefore manual code review
should be backed up by memory checking programs available on the market.
What to Review: Format Function Overruns
A format function is a function within the ANSI C specification that can be used to tailor primitive C data types tohuman readable form. They are used in nearly all C programs to output information, print error messages, or processstrings.
Table 23: Format Function Overruns
Format String

Relevant Input%x

Hexadecimal values (unsigned int)%s

Strings ((const) (unsigned) char*)%nInteger%d

Decimal%uUnsigned decimal (unsigned int)

Some format parameters:
The %s in this case ensures that value pointed to by the parameter ‘abc’ is printed as an array of characters.
For example:
char* myString = “abc”;
printf (“Hello: %s\n”, abc);

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Through supplying the format string to the format function we are able to control the behaviour of it. So
supplying input as a format string makes our application do things it’s not meant to. What exactly are we able
to make the application do?
If we supply %x (hex unsigned int) as the input, the ‘printf’ function shall expect to find an integer relating tothat format string, but no argument exists. This cannot be detected at compile time. At runtime this issue shallsurface.
For every % in the argument the printf function finds it assumes that there is an associated value on the stack.
In this way the function walks the stack downwards reading the corresponding values from the stack andprinting them to the user.
Using format strings we can execute some invalid pointer access by using a format string such as:• printf (“%s%s%s%s%s%s%s%s%s%s%s%s”);
Worse again is using the ‘%n’ directive in ‘printf()’. This directive takes an ‘int*’ and ‘writes’ the number of bytes
so far to that location.
Where to look for this potential vulnerability. This issue is prevalent with the ‘printf()’ family of functions,‘’printf(),fprintf(), sprintf(), snprintf().’ Also ‘syslog()’ (writes system log information) and setproctitle(const char*fmt, ...); (which sets the string used to display process identifier information).
What to Review: Integer Overflows
Data representation for integers will have a finite amount of space, for example a short in many languages is
16 bits twos complement number, which means it can hold a maximum number of 32,767 and a minimum
number of -32,768. Twos complement means that the very first bit (of the 16) is a representation of whether
the number of positive or negative. If the first bit is ‘1’, then it is a negative number.
The representation of some boundary numbers are given in table 24.
Table 24: Integer Overflows
NumberRepresentation

32,7660111111111111110

32,7670111111111111111-32,768

1000000000000000-11111111111111111

If you add 1 to 32,766, it adds 1 to the representation giving the representation for 32,767 shown above.
However if you add one more again, it sets the first bit (a.k.a. most significant bit), which is then interpreted by
the system as -32,768.
If you have a loop (or other logic) which is adding or counting values in a short, then the application could
experience this overflow. Note also that subtracting values below -32,768 also means the number will wrap
around to a high positive, which is called underflow.

Buffer Overruns

Sample 24.3
#include <stdio.h>
int main(void){int val;val = 0x7fffffff;/* 2147483647*/
printf(“val = %d (0x%x)\n”, val, val);printf(“val + 1 = %d (0x%x)\n”, val + 1 , val + 1); /*Overflow the int*/
return 0;}

The binary representation of 0x7fffffff is 1111111111111111111111111111111; this integer is initialized withthe highest positive value a signed long integer can hold.
Here when we add 1 to the hex value of 0x7fffffff the value of the integer overflows and goes to a negative
number (0x7fffffff + 1 = 80000000) In decimal this is (-2147483648). Think of the problems this may cause.
Compilers will not detect this and the application will not notice this issue.
We get these issues when we use signed integers in comparisons or in arithmetic and also when comparing
signed integers with unsigned integers.
Sample 24.4
int myArray[100];
int fillArray(int v1, int v2){
if(v2 > sizeof(myArray) / sizeof(int) -1 ){return -1; /* Too Big */}myArray [v2] = v1;
return 0;}

Here if v2 is a massive negative number the “if” condition shall pass. This condition checks to see if v2 is bigger
than the array size.
If the bounds check was not performed the line “myArray[v2] = v1” could have assigned the value v1 to alocation out of the bounds of the array causing unexpected results.
References• See the OWASP article on buffer overflow attacks.

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Buffer Overruns• See the OWASP Testing Guide on how to test for buffer overflow vulnerabilities.
• See Security Enhancements in the CRT: http://msdn2.microsoft.com/en-us/library/8ef0s5kh(VS.80).aspx

JavaScript has several known security vulnerabilities, with HTML5 and JavaScript becoming more prevalent in
web sites today and with more web sites moving to responsive web design with its dependence on JavaScript

CLIENT SIDE JavaScript
the code reviewer needs to understand what vulnerabilities to look for. JavaScript is fast becoming a significant point of entry of hackers to web application. For that reason we have included in the A1 Injection sub
section.
The most significant vulnerabilities in JavaScript are cross-site scripting (XSS) and Document Object Model,
DOM-based XSS.
Detection of DOM-based XSS can be challenging. This is caused by the following reasons.
• JavaScript is often obfuscated to protect intellectual property.
• JavaScript is often compressed out of concerned for bandwidth.
In both of these cases it is strongly recommended the code review be able to review the JavaScript before it
has been obfuscated and or compressed. This is a huge point of contention with QA software professionals
because you are reviewing code that is not in its production state.
Another aspect that makes code review of JavaScript challenging is its reliance of large frameworks such as
Microsoft .NET and Java Server Faces and the use of JavaScript frameworks, such as JQuery, Knockout, Angular,
Backbone. These frameworks aggravate the problem because the code can only be fully analyzed given thesource code of the framework itself. These frameworks are usually several orders of magnitude larger then thecode the code reviewer needs to review.
Because of time and money most companies simple accept that these frameworks are secure or the risks are
low and acceptable to the organization.
Because of these challenges we recommend a hybrid analysis for JavaScript. Manual source to sink validation
when necessary, static analysis with black-box testing and taint testing.
First use a static analysis. Code Reviewer and the organization needs to understand that because of event-driven behaviors, complex dependencies between HTML DOM and JavaScript code, and asynchronous communication with the server side static analysis will always fall short and may show both positive, false, false –positive, and positive-false findings.
Black-box traditional methods detection of reflected or stored XSS needs to be preformed. However this approach will not work for DOM-based XSS vulnerabilities.
Taint analysis needs to be incorporated into static analysis engine. Taint Analysis attempts to identify variables
that have been ‘tainted’ with user controllable input and traces them to possible vulnerable functions also knownBuffer Overruns

as a ‘sink’. If the tainted variable gets passed to a sink without first being sanitized it is flagged as vulnerability.
Second the code reviewer needs to be certain the code was tested with JavaScript was turned off to make sure
all client sided data validation was also validated on the server side.
Sample 25.1
<html><script type=”text/javascript”>
var pos=document.URL.indexOf(“name=”)+5;
document.write( document.URL.substring(pos,document.URL.length));</script><html>

Code examples of JavaScript vulnerabilities.
Sample 25.2
var url = document.location.url;
var loginIdx = url.indexOf(‘login’);var loginSuffix = url.substring(loginIdx);
url = ‘http://mySite/html/sso/’ + loginSuffix;
document.location.url = url;
Explanation: An attacker can send a link such as “http://hostname/welcome.html#name=<script>alert(1)</script>” to the victim resulting in the victim’s browser executing the injected client-side code.
Line 5 may be a false-positive and prove to be safe code or it may be open to “Open redirect attack” with taintanalysis the static analysis should be able to correctly identified if this vulnerability exists. If static analysis
relies only on black-box component this code will have flagged as vulnerable requiring the code reviewer to
do a complete source to sink review.
Additional examples and potential security risks
Source: document.url
Sink: document.write()
Results: Results:document.write(“<script>malicious code</script>”);Cybercriminal may controlled the following DOM elements including
document.url,document.location,document.referrer,window.location
Source: document.location
Sink: windon.location.href
Results: windon.location.href = http://www.BadGuysSite; - Client code open redirect.
Source: document.url
Storage: windows.localstorage.name

189

190

Buffer Overruns

Sink: elem.innerHTML
Results: elem.innerHTML = <value> =Stored DOM-based Cross-site Scriptingeval() is prone to security threats, and thus not recommended to be used.
Consider these points:
1. Code passed to the eval is executed with the privileges of the executer. So, if the code passed can be affected by some malicious intentions, it leads to running malicious code in a user’s machine with your website’s
privileges.
2. A malicious code can understand the scope with which the code passed to the eval was called.
3. You also shouldn’t use eval() or new Function() to parse JSON data.
The above if used may raise security threats. JavaScript when used to dynamically evaluate code will create apotential security risk.
eval(‘alert(“Query String ‘ + unescape(document.location.search) + ‘”);’);eval(untrusted string); Can lead to code injection or client-side open redirect.
JavaScripts “new function” also may create a potential security risk.
Three points of validity are required for JavaScript
1. Have all the logic server-side, JavaScript validation be turned off on the client
2. Check for all sorts of XSS DOM Attacks, never trust user data, know your source and sinks (i.e. look at all
variables that contain user supplied input).
3. Check for insecure JavaScript libraries and update them frequently.
References:• http://docstore.mik.ua/orelly/web/jscript/ch20_04.html• https://www.owasp.org/index.php/Static_Code_Analysis• http://www.cs.tau.ac.il/~omertrip/fse11/paper.pdf• http://www.jshint.com

191

APENDIX

192

Code Review Do’s And Dont’s

CODE REVIEW DO’S AND DONT’S
At work we are professions. But we need to make sure that even as professionals that when we do code reviews
besides the technical aspects of the code reviews we need to make sure we consider the human side of code
reviews. Here is a list of discussion points that code reviewers; peer developers need to take into consideration.
This list is not comprehensive but a suggestion starting point for an enterprise to make sure code reviews are
effective and not disruptive and a source of discourse. If code reviews become a source of discourse within
an organization the effectives of finding security, functional bugs will decline and developers will find a way
around the process. Being a good code reviewer requires good social skills, and is a skill that requires practicejust like learning to code.
• You don’t have to find fault in the code to do a code review. If you always fine something to criticize yourcomments
will loose credibility.
• Do not rush a code review. Finding security and functionality bugs is important but other developers or teammembers are waiting on you so you need to temper your do not rush with the proper amount urgency.
• When reviewing code you need to know what is expected. Are you reviewing for security, functionality, maintainability, and/or style? Does your organization have tools and documents on code style or are you using yourown coding style? Does your organization give tools to developers to mark unacceptable coding standards
per the organizations own coding standards?
• Before beginning a code review does your organization have a defined way to resolve any conflicts that may
comeup in the code review by the developer and code reviewer?
• Does the code reviewer have a define set of artifacts that need to be produce as the result of the code review?
• What is the process of the code review when code during the code review needs to be changed?
• Is the code reviewer knowledgeable about the domain knowledge of the code that is being reviewed? Ample
evidence abounds that code reviews are most effective if the code reviewer is knowledgeable about the domain of the code I.e. Compliance regularizations for industry and government, business functionality, risks, etc.
Agile Software Development Lifecycle

Code Review Do’s And Dont’s

AGILE
SOFTWARE
DEVELOPMENT
LIFECYCLE
INTERACTIONFASE

1DESIGNCODE
TEST
DEPLOYINTERACTIONFASE

INTERACTION
FASE

4

2

DESIGNCODE
TEST
DEPLOYDESIGNCODE
TEST
DEPLOYINTERACTIONFASE

3DESIGNCODE
TEST
DEPLOY193

194

Code Review Do’s And Dont’s

Integrating security into the agile sdlc process flow is difficult. The organization will need constant involvement from security team and or a dedication to security with well-trained coders on every team.

SAST - AdHoc Static Analysis - Developer Initiated

6

DEVELOPMENTYES:FIX CODESASTPASS OR
FAIL?

5
YES:
FAIL

1
CHECK-IN

START

7CHECK-IN CODE

NO/FALSE POSITIVESASTPASS ORFAIL?

DEVCHECKMARX / SAST

TUNED RULES
CONTROLLED
BY APPSEC

2

11

INVOKE SASTNO:
REQUEST FIX/ESCALATE

SAST

3

4

PASS/FAIL

NO PASSSECURITY

8ADJUST RULEBASE
YES

NO

FALSE
POSITIVEYES

FINISH/
SIGNEDOFF

SIGNOFF

SIGN/OFF?

RECORD METRICS
ISR

Continuous Integration and Test Driven Development
The term ‘Continuous Integration’ originated with the Extreme Programming development process. Today it is
one of the best practices of SDLC Agile. CI requires developers to check code into source control management

Code Review Do’s And Dont’s

application (scm) several times a day. An automated build server and application verify each check-in. The
advantage is team members can quickly detect build problems early in the software development process.
The disadvantage of CI for the Code Reviewer is while code may build properly; software security vulnerabilities may still exist. Code review may be part of the check-in process but review may only to make sure thecode only meets the minimum standards of the organization. Code review is not a secure code review with risk
assessment approach on what needs to have additional time spent doing a code review.
The second disadvantage for the code review is because the organization is moving quickly with Agile processa design flaw may be introduced allowing a security vulnerabilities and the vulnerabilities may get deployed.
A red flag for the code reviewer is…
1. No user stories talk about security vulnerabilities based on risk.
2. User stories do not openly describe source and sinks.
3. No risk assessment for the application has been done.
Breaking Down Process Areas
The term ‘Test Driven Development’ like CI originated with Extreme Programming development process. Today like CI it is one of the best practices of SDLC Agile. TDD requires developers to rely on repetition of a very
short development cycle. First the step is the developer writes an automated test case that defines a needed
improvement or new functionality. This first step TDD initial fails the test. Subsequent steps the developers
create a minimum amount of code to pass. Finally the developer refactors the new code to the organization
acceptable coding standard.
BREAKING DOWN PROCESS AREAS
SPRINT

1

30

AGILE

2

CONTINUOUS INTEGRATION

3TEST DRIVEN DEVELOPMENTFUNCTIONINGPRODUCT

DAY
(MAX)

STORY

INTEGRATION:
AUTOMATEDBUILD AND TESTSIMMEDIATEFEEDBACK ON
ISSUES

TDD:
CREATE TESTFIRST CODE
UNTIL IT PASSES

1SPRINTBACKLOGCONTINOUSINTEGRATIONPERIOD

STORY

STORY

1

2
PRODUCTBACKLOG

24

HOUR
(MAX)

195

196

Code Review Checklist

CODE REVIEW CHECKLISTCATEGORY
GeneralDESCRIPTION
Are there backdoor/unexposed business logic classes?

Business Logic and
Design

Are there unused configurations related to business logic?

Business Logic and
DesignIf request parameters are used to identify business logic methods, is there a proper mapping ofuser privileges and methods/actions allowed to them?

Business Logic and
DesignCheck if unexposed instance variables are present in form objects that get bound to user inputs.
If present, check if they have default values.

Business Logic and
DesignCheck if unexposed instance variables present in form objects that get bound to user inputs. If
present, check if they get initialized before form binding.

Authorization

Is the placement of authentication and authorization check correct?

Authorization

Is there execution stopped/terminated after for invalid request? I.e. when authentication/authorization check fails?

Authorization

Are the checks correct implemented? Is there any backdoor parameter?

Authorization

Is the check applied on all the required files and folder within web root directory?

Authorization

Are security checks placed before processing inputs?

Business Logic and
DesignCheck if unexposed instance variables are present in form objects that get bound to user inputs.
If present, check if they have default values.

Business Logic and
DesignCheck if unexposed instance variables present in form objects that get bound to user inputs. If
present, check if they get initialized before form binding.

Authorization

Is there execution stopped/terminated after for invalid request? I.e. when authentication/authorization check fails?

Business Logic and
Design

Are the checks correct implemented? Is there any backdoor parameter?

Business Logic and
DesignIs the check applied on all the required files and folder within web root directory?

Business Logic and
DesignIs there any default configuration like Access- ALL?

Business Logic and
Design

Does the configuration get applied to all files and users?

Authorization

Incase of container-managed authentication - Is the authentication based on web methods only?

Authorization

Incase of container-managed authentication - Does the authentication get applied on all
resources?

SessionManagement

Does the design handle sessions securely?

Authorization

Incase of container-managed authentication - Is the authentication based on web methods only?

Authorization

Is Password Complexity Check enforced on the password?

Cryptography

Is password stored in an encrypted format?

Authorization

Is password disclosed to user/written to a file/logs/console?

PASS

FAIL

Code Review Checklist

CATEGORYDESCRIPTION

Cryptography

Are database credentials stored in an encrypted format

Business Logic and
Design

Does the design support weak data stores like flat files

Business Logic and
Design

Does the centralized validation get applied to all requests and all the inputs?

Business Logic and
Design

Does the centralized validation check block all the special characters?

Business Logic and
Design

Does are there any special kind of request skipped from validation?

Business Logic and
Design

Does the design maintain any exclusion list for parameters or features from being validated?

Imput Validation

Are all the untrusted inputs validated?
Input data is constrained and validated for type, length, format, and range.

Cryptography

Is the data sent on encrypted channel? Does the application use HTTPClient for making external
connections?

Cryptography
SessionManagement

Does
Is
the thedatadesign
sent oninvolveencrypted
sessionchannel?
sharingDoes
betweenthe application
components/modules?
use HTTPClient
Is session
for making
validated
external
connections?
correctly
on both ends?

Business Logic and
Design

Does the design use any elevated OS/system privileges for external connections/commands?

Business Logic and
DesignIs there any known flaw(s) in API’s/Technology used? For eg: DWRBusiness Logic and
Design

Does the design framework provide any inbuilt security control? Like <%: %> in ASP.NET MVC? Is
the application taking advantage of these controls?

Business Logic and
DesignAre privileges reduce whenever possible?

Business Logic and
DesignIs the program designed to fail gracefully?

Logging and
Auditing

Are logs logging personal information, passwords or other sensitive information?

Logging and
Auditing

Do audit logs log connection attempts (both successful and failures)?

Logging and
Auditing

Is there a process(s) in place to read audit logs for unintended/malicious behaviors?

Cryptography

Is all PI and sensitive information being sent over the network encrypted form.

Authorization

Does application design call for server authentication (anti-spoofing measure)?

Authorization

Does application support password expiration?

Cryptography

Does application use custom schemes for hashing and or cryptographic?

PASS

FAIL

197

198

Code Review Checklist

CATEGORYDESCRIPTION

Cryptography

Are cryptographic functions used by the application the most recent version of these protocols,
patched and process in place to keep them updated?

General

Are external libraries, tools, plugins used by the application functions the most recent version ofthese protocols, patched and process in place to keep them updated?

GeneralClasses that contain security secrets (like passwords) are only accessible through protected API’s

Cryptography
General
Cryptography

Does are there any special kind of request skipped from validation?
Classes that contain security secrets (like passwords) are only accessible through protected API’s
Keys are not held in code.

GeneralPlain text secrets are not stored in memory for extended periods of time.

GeneralArray bounds are checked.

User Management
and Authentication
GeneralUser and role based privileges are documented
All sensitive information used by application has been identified

User Management
and Authentication

Authentication cookies are not persisted

User Management
and Authentication

Authentication cookies are encrypted

User Management
and Authentication

Authentication credentials are not passed by HTTP GETUser Management
and Authentication

Authorization checks are granular (page and directory level)

User Management
and Authentication

Authorization based on clearly defined roles

User Management
and Authentication

Authorization works properly and cannot be circumvented by parameter manipulation

User Management
and Authentication

Authorization cannot be bypassed by cookie manipulation

SessionManagement

No session parameters are passed in URLs

SessionManagement

Session cookies expire in a reasonable short time

SessionManagement

Session cookies are encrypted

SessionManagement

Session data is validated

SessionManagement

Session id is complex

SessionManagement

Session storage is secure

PASS

FAIL

Code Review Checklist

CATEGORY
SessionManagement

DESCRIPTION
Session inactivity timeouts are enforced

Data Management

Data is validated on server side

Data Management

HTTP headers are validated for each request

Business Logic and
Design

Are all of the entry points and trust boundaries identified by the design and are in risk analysis
report?

Data Management

Is all XML input data validated against an agreed schema?

Data Management

Is output that contains untrusted data supplied input have the correct type of encoding (URL
encoding, HTML encoding)?

Data Management

Has the correct encoding been applied to all data being output by the application

Web Services

Web service has documentation protocol is disable if the application does not need dynamic
generation of WSDL.

Web Services

Web service endpoints address in Web Services Description Language (WSDL) is checked for validity

Web Services

Web service protocols that are unnecessary are disable (HTTP GET and HTTP POST

PASS

FAIL

199

200

Threat Modeling Example

THREAT MODELING EXAMPLE
Step 1 Decompose the Application
The goal of decomposing the application is to gain an understanding of the application and how it interacts
with external entities. Information gathering and documentation achieve this goal. The information gathering
process is carried out using a clearly defined structure, which ensures the correct information is collected. This
structure also defines how the information should be documented to produce the threat model.
Figure 13
General Information
The first item in the threat model is the general information relating to the threat model. This must include
the following:
1. Application Name: The name of the application
2. Application Version: The version of the application
3. Description: A high level description of the application
4. Document Owner: The owner of the threat modeling document
5. Participants: The participants involved in the threat modeling process for this application
6. Reviewer: The reviewer(s) of the threat model

Figure 14

OWNER - DAVID LOWRY
THREAT MODELINFORMATIONPARTICIPANTS - DAVID ROOKREVIEWER - EOIN KEARY

V2.0

Description
The college library website is the first implementation of a website to provide librarians and library patrons(students and college staff ) with online services. As this is the first implementation of the website, the functionality will be limited. There will be three users of the application:
1. Students
2. StaffThreat Modeling Example

3. Librarians Staff and students will be able to log in and search for books, and staff members can request
books. Librarians will be able to log in, add books, add users, and search for books.

Figure 15
Entry Points
Entry points should be documented as follows:
1. ID
A unique ID assigned to the entry point. This will be used to cross reference the entry point with any threats orvulnerabilities that are identified. In the case of layer entry points, a major, minor notation should be used.
2. NameA descriptive name identifying the entry point and its purpose.
3. Description
A textual description detailing the interaction or processing that occurs at the entry point.
4. Trust Levels
The level of access required at the entry point is documented here. These will be cross-referenced with the trusts
levels defined later in the document.

Figure 16
AssetsAssets are documented in the threat model as follows:
1. ID
A unique ID is assigned to identify each asset. This will be used to cross reference the asset with any threats orvulnerabilities that are identified.
2. NameA descriptive name that clearly identifies the asset.
3. Description
A textual description of what the asset is and why it needs to be protected.
4. Trust Levels
The level of access required to access the entry point is documented here. These will be cross-referenced with thetrust levels defined in the next step.

201

202

Threat Modeling Example

Figure 17
Trust Levels
Trust levels are documented in the threat model as follows:
1. ID
A unique number is assigned to each trust level. This is used to cross reference the trust level with the entry points
and assets.
2. NameA descriptive name that allows identification of the external entities that have been granted this trust level.
3. Description
A textual description of the trust level detailing the external entity who has been granted the trust level.

By using the understanding learned on the college library website architecture and design, the data flow diagram can be created as shown in figure X.

Figure 18

USERS

LIBRARIANSREQUEST

REQUEST
USER / WEBSERVER BOUNDARYRESPONSESWEB PAGESON DISKPAGESRESPONSESCOLEGE
LIBRARYDATABASE

SQL QUERY CALLSDATAWEB SERVER / DATABASE BOUNDARYCOLEGE
LIBRARYDATABASEDATADATABASE
FILES
DATA

Threat Modeling Example

Specifically the user login data flow diagram will appear as in figure 19.

Figure 19

Exampleapplication threat
model of the user
login

USER / WEBSERVER BOUNDARY

USERS

LOGIN REQUESTLOGIN RESPONSEWEB SERVER /DATABASE BOUNDARYWEBSERVERPAGES

AUTHENTICATE USER ()AUTHENTICATE USER RESULTLOGINPROCESSAUTHENTICATEUSER SQLQUERY RESULTAUTHENTICATEUSER SQLQUERY RESULT

DATA

WEB
PAGES

COLEGE
LIBRARYDATABASEDATABASE
FILES

DATA

Threat Modeling Example: Step 2a Threat Categorization
The first step in the determination of threats is adopting a threat categorization. A threat categorization provides a set of threat categories with corresponding examples so that threats can be systematically identified in
the application in a structured and repeatable manner.

StrideA threat list of generic threats organized in these categories with examples and the affected security controls is provided in the following table:

203

204

Threat Modeling Example

Threat Modeling Example: Step 2b Ranking of Threats
Threats tend to be ranked from the perspective of risk factors. By determining the risk factor posed by thevarious identified threats, it is possible to create a prioritized list of threats to support a risk mitigation strategy,
such as deciding on which threats have to be mitigated first. Different risk factors can be used to determine
which threats can be ranked as High, Medium, or Low risk. In general, threat risk models use different factors
to model risks.
Microsoft DREAD threat-risk ranking model
By referring to the college library website it is possible to document sample threats related to the use cases suchas:Threat: Malicious user views confidential information of students, faculty members and librarians.
1. Damage potential
Threat to reputation as well as financial and legal liability:8
2. Reproducibility
Fully reproducible:10
3. Exploitability
Require to be on the same subnet or have compromised a router:7
4. Affected users
Affects all users:10
5. Discoverability
Can be found out easily:10
Overall DREAD score: (8+10+7+10+10) / 5 = 9
In this case having 9 on a 10 point scale is certainly an high risk threat.

Code Crawling

CODE CRAWLINGThis appendix gives practical examples of how to carry out code crawling in the following programming languages:• .Net• Java• ASP• C++/Apache
Searching for Code in .NET
Firstly one needs to be familiar with the tools one can use in order to perform text searching, following this one needs to knowwhat to look for.
One could scan through the code looking for common patterns or keywords such as “User”, “Password”, “Pswd”, “Key”, “Http”,
etc... This can be performed using the “Find in Files” tool in VS or using findstring as follows: findstr /s /m /i /d:c:\projects\codebase\sec “http” *.*HTTP Request Strings
Requests from external sources are obviously a key area of a security code review. We need to ensure that all
HTTP requests received are data validated for composition, max and min length, and if the data falls within therealms of the parameter whitelist. Bottom-line is this is a key area to look at and ensure security is enabled.

STRING TO SEARCH
request.accesstypes

request.httpmethod

request.cookies

request.url

request.browser

request.querystring

request.certificate

request.urlreferrer

request.files

request.item

request.rawurl

request.useragent

request.headers

request.form

request.servervariables

request.userlanguages

request.TotalBytes

request.BinaryRead

OUTPUTHTML Output
Here we are looking for responses to the client. Responses which go unvalidated or which echo external input
without data validation are key areas to examine. Many client side attacks result from poor response validation.

STRING TO SEARCH
response.write

HttpUtility

HtmlEncode

innerText

innerHTML<%=UrlEncode

205

206

Code Crawling

SQL & Database
Locating where a database may be involved in the code is an important aspect of the code review. Looking at
the database code will help determine if the application is vulnerable to SQL injection. One aspect of this is
to verify that the code uses either SqlParameter, OleDbParameter, or OdbcParameter(System.Data.SqlClient).
These are typed and treat parameters as the literal value and not executable code in the database.
STRING TO SEARCH
exec sp_select from

insertupdatedelete from where

deleteexecute sp_

exec xp_

exec @

execute @executestatement

executeSQL

setfilter

executeQuery

GetQueryResultInXML

adodb

sqloledb

sql server

driverServer.CreateObject.Provider

System.Data.sql

ADODB.recordset

New OleDbConnection

ExecuteReader

DataSource

SqlCommand

Microsoft.Jet

SqlDataReader

ExecuteReader

SqlDataAdapter

StoredProcedure

Cookies
Cookie manipulation can be key to various application security exploits, such as session hijacking/fixation andparameter manipulation. One should examine any code relating to cookie functionality, as this would have abearing on session security.

STRING TO SEARCH
System.Net.Cookie

TAGS

HTTPOnly

document.cookie

HTML Tags
Many of the HTML tags below can be used for client side attacks such as cross site scripting. It is important toexamine the context in which these tags are used and to examine any relevant data validation associated withthe display and use of such tags within a web application.

STRING TO SEARCH
HtmlEncode

URLEncode<applet><frameset><embed><frame><html><iframe><img><style><layer><ilayer><meta><object><frame security<iframe security

Code Crawling

Input Controls
The input controls below are server classes used to produce and display web application form fields. Looking forsuch references helps locate entry points into the application.

STRING TO SEARCH
htmlcontrols.htmlinputhidden

webcontrols.hiddenfield

webcontrols.hyperlink

webcontrols.textbox

webcontrols.label

webcontrols.linkbutton

webcontrols.listbox

webcontrols.checkboxlist

webcontrols.dropdownlist

WEB.config
The .NET Framework relies on .config files to define configuration settings. The .config files are text-based XML
files. Many .config files can, and typically do, exist on a single system. Web applications refer to a web.config
file located in the application’s root directory. For ASP.NET applications, web.config contains information aboutmost aspects of the application’s operation.
STRING TO SEARCH
requestEncoding

responseEncodingTraceauthorizationcompilation

webcontrols.linkbutton

webcontrols.listbox

webcontrols.checkboxlist

webcontrols.dropdownlist

CustomErrors

httpCookies

httpHandlers

httpRuntime

sessionState

maxRequestLength

Debugforms protection

appSettings

ConfigurationSettings

appSettings

connectionStrings

authentication mode

Allow

Deny

Credentials

identity impersonate

timeout

remoteglobal.asax
Each application has its own global.asax file if one is required. Global.asax sets the event code and values foran application using scripts. One must ensure that application variables do not contain sensitive information,
as they are accessible to the whole application and to all users within it.

STRING TO SEARCH
Application_OnAuthenticateRequest

Application_OnAuthorizeRequest

Session_OnStart

Session_OnEnd

207

208

Code Crawling

Logging
Logging can be a source of information leakage. It is important to examine all calls to the logging subsystem and to
determine if any sensitive information is being logged. Common mistakes are logging userID in conjunction withpasswords within the authentication functionality or logging database requests which may contain sensitive data.

STRING TO SEARCH
log4netConsole.WriteLine

System.Diagnostics.Debug

System.Diagnostics.Tracemachine.config
It is important that many variables in machine.config can be overridden in the web.config file for a particularapplication.

STRING TO SEARCH
validateRequest

enableViewState

enableViewStateMac

Threads and Concurrency
Locating code that contains multithreaded functions as concurrency issues can result in race conditions,which may result in security vulnerabilities. The Thread keyword is where new threads objects are created.
Code that uses static global variables that hold sensitive security information may cause session issues. Codethat uses static constructors may also cause issues between threads. Not synchronizing the Dispose method
may cause issues if a number of threads call Dispose at the same time, this may cause resource release issues.
STRING TO SEARCH
ThreadDisposeClass Design
Public and Sealed relate to the design at class level. Classes that are not intended to be derived from should
be sealed. Make sure all class fields are Public for a reason. Don’t expose anything that is not necessary.

STRING TO SEARCH
PublicSealedCode Crawling

Reflection, Serialization
Code may be generated dynamically at runtime. Code that is generated dynamically as a function of external
input may give rise to issues. If code contains sensitive data, does it need to be serialized?

MyClass

STRING TO SEARCH
Serializable

AllowPartiallyTrustedCallersAttribute

StrongNameIdentity

StrongNameIdentityPermission

GetObjectData

System.Reflection

Exceptions & Errors
Ensure that the catch blocks do not leak information to the user in the case of an exception. Ensure when
dealing with resources that the finally block is used. Having trace enabled is not great from an information
leakage perspective. Ensure customized errors are properly implemented

STRING TO SEARCH
catch

finally

trace enabledcustomErrors mode

Cryptography
If cryptography is used then is a strong enough cipher used, i.e. AES or 3DES? What size key is used? The larger
the better. Where is hashing performed? Are passwords that are being persisted hashed? They should be. How
are random numbers generated? Is the PRNG “random enough”?

STRING TO SEARCHRNGCryptoServiceProvider

SHA

MD5

base64DES

RC2System.Random

Randomxor

System.Security.Cryptography

Storage
If storing sensitive data in memory, it is recommend to usemy the following.

STRING TO SEARCH
SecureString

ProtectedMemory

209

210

Code Crawling

Authorization, Assert & Revert
Bypassing the .Net code access security permission? Not a good idea. Below is a list of potentially dangerous
permissions such as calling unmanaged code, outside the CLR.

STRING TO SEARCH
RequestMinimum

RequestOptional

AssertDebug.Assert

CodeAccessPermission

MemberAccessControlAppDomain

UnmanagedCode

SkipVerification

ControlEvidence

SerializationFormatter

ControlPrincipal

ControlDomainPolicy

ControlPolicy

Legacy Methods
Some standard functions that should be checked in any context include the following.

STRING TO SEARCH
printfstrcpySearching for Code in Java
Input and Output Streams
These are used to read data into one’s application. They may be potential entry points into an application.
The entry points may be from an external source and must be investigated. These may also be used in pathtraversal attacks or DoS attacks.

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FileInputStream

ObjectInputStream

FilterInputStream

PipedInputStream

SequenceInputStream

StringBufferInputStream

BufferedReader

ByteArrayInputStream

java.io.FileOutputStream

File

ObjectInputStream

PipedInputStream

StreamTokenizer

getResourceAsStream

java.io.FileReader

java.io.FileWriterjava.io.RandomAccessFile

java.io.FilerenameTo

Mkdir

Code CrawlingServlets
These API calls may be avenues for parameter/header/URL/cookie tampering, HTTP Response Splitting andinformation leakage. They should be examined closely as many of such APIs obtain the parameters directlyfrom HTTP requests.

STRING TO SEARCH
javax.servlet.*getParameterNames

getParameterValues

getParameter

getParameterMap

getScheme

getProtocol

getContentType

getServerName

getRemoteAddr

getRemoteHost

getRealPath

getLocalName

getAttribute

getAttributeNames

getLocalAddr

getAuthType

getRemoteUser

getCookies

isSecure

HttpServletRequest

getQueryString

getHeaderNames

getHeaders

getPrincipal

getUserPrincipal

isUserInRole

getInputStream

getOutputStream

getWriter

addCookie

addHeader

setHeader

setAttribute

putValuejavax.servlet.http.Cookie

getNamegetPathgetDomain

getComment

getMethod

getPathgetReader

getRealPath

getRequestURI

getRequestURL

getServerName

getValue

getValueNames

getRequestedSessionId

Cross Site Scripting
These API calls should be checked in code review as they could be a source of Cross Site Scripting vulnerabilities.

STRING TO SEARCH
javax.servlet.ServletOutputStream.print

strcpyResponse Splitting
Response splitting allows an attacker to take control of the response body by adding extra CRLFs into headers.
In HTTP the headers and bodies are separated by 2 CRLF characters, and thus if an attackers input is used ina response header, and that input contained 2 CRLFs, then anything after the CRLFs would be interpreted as
the response body. In code review ensure functionality is sanitizing any information being put into headers.
STRING TO SEARCH
javax.servlet.http.HttpServletResponse.sendRedirect

strcpysetHeader

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Code Crawling

Redirection
Any time an application is sending a redirect response, ensure that the logic involved cannot be manipulatedby an attackers input. Especially when input is used to determine where the redirect goes to.

STRING TO SEARCH
sendRedirect

setStatus

addHeaderetHeaderSQL & Database
Searching for Java database related code should help pinpoint classes/methods which are involved in thepersistence layer of the application being reviewed.

STRING TO SEARCH
java.sql.Connection.prepareStatement

java.sql.ResultSet.getObject

selectinsertjava.sql.Statement.executeUpdate

java.sql.Statement.addBatch

executeexecutestatement

createStatement

java.sql.ResultSet.getString

executeQuery

jdbc

java.sql.Statement.executeQueryjava.sql.Statement.executedeleteupdatejava.sql.Connection.prepareCall

SSL
Looking for code which utilises SSL as a medium for point to point encryption. The following fragments
should indicate where SSL functionality has been developed.

STRING TO SEARCH
com.sun.net.ssl

SSLContext

SSLSocketFactory

HttpsURLConnection

KeyManagerFactory

TrustManagerFactory

Session Management
The following APIs should be checked in code review when they control session management.

STRING TO SEARCH
getSession

invalidate

getIdCode Crawling

Legacy Interaction
Here we may be vulnerable to command injection attacks or OS injection attacks. Java linking to the native
OS can cause serious issues and potentially give rise to total server compromise.

STRING TO SEARCH
java.lang.Runtime.exec

java.lang.Runtime.getRuntime

getId

Logging
We may come across some information leakage by examining code below contained in one’s application.

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java.io.PrintStream.write

log4j

jLo

Lumberjack

MonoLogqflog

just4log

log4Ant

document.cookie

window.location

JDLabAgent

Ajax and JavaScript
Look for Ajax usage, and possible JavaScript issues:

STRING TO SEARCH
document.write

eval

document.URL

document.URL

Searching for Code in Classic ASP
Input APIs in ASP are commonly used to retrieve the input from the request, therefore code review should ensure
these requests (and dependent logic) cannot be manipulated by an attacker. Output APIs are used by ASP to write
the response body that will be sent to the end user, hence code review should check these requests are used in aproper manner and no sensitive information can be returned. Cookies can also be a source of information leakage.
STRING TO SEARCH
RequestRequest.QueryString

Request.Form

Request.ServerVariables

Query_String

hiddeninclude.inc

Response.Write

Response.BinaryWrite<%=.cookies

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Code Crawling

Error Handling
Ensure errors in an application are handled properly, otherwise an attacker could use error conditions tomanipulate the application.

STRING TO SEARCH
err.

Server.GetLastError

On Error Resume Next

On Error GoTo 0

Information in URL
These APIs are used to extract information from the URL object in the request. Code review should check thatthe information extracted from the URL is sanitized.

STRING TO SEARCH
location.href

location.replacemethod=”GET”

On Error GoTo 0

Database
These APIs can be used to interact with a database, which can lead to SQL attacks. Code review can checkthese API calls use sanitized input.

STRING TO SEARCH
commandText

select fromupdateinsert into

delete from where

IRowSetexecute.execute.open

ADODB.

Commandtype

ICommandSession
These API calls can control session within ASP applications.

STRING TO SEARCH
session.timeout

session.abandon

session.removeallCode Crawling

DOS Prevention & Logging
The following ASP APIs can help prevent DOS attacks against the application. Leaking information to a log
can be of use to an attacker, hence the following API call can be checked in code review to ensure no sensitive
information is being written to logs.

STRING TO SEARCH
server.ScriptTimeout

IsClientConnected

WriteEntry

Redirection
Do not allow attacker input to control when and where rejection occurs.

STRING TO SEARCH
Response.AddHeader

Response.AppendHeader

Response.Redirect

Response.StatusCode

Server.Transfer

Server.Execute

Response.Status

Searching for Code in Javascript and AJAX
Ajax and JavaScript have brought functionality back to the client side, which has brought a number of old security
issues back to the forefront. The following keywords relate to API calls used to manipulate user state or the control
the browser. The advent of AJAX and other Web 2.0 paradigms has pushed security concerns back to the client side,
but not excluding traditional server side security concerns. Look for Ajax usage, and possible JavaScript issues.

STRING TO SEARCH
eval

document.cookie

document.referrer

document.attachEventdocument.body

document.body.innerHtml

document.body.innerText

document.close

document.create

document.execCommand

document.forms[0].action

document.location

document.open

document.URL

document.URLUnencoded

document.write

document.writeln

location.hashlocation.href

location.searchwindow.alert

window.attachEventwindow.createRequest

window.execScript

window.location

window.open

window.navigate

window.setInterval

window.setTimeout

XMLHTTP215

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Code Crawling

Searching for Code in C++ and ApacheCommonly when a C++ developer is building a web service they will build a CGI program to be invoked bya web server (though this is not efficient) or they will use the Apache httpd framework and write a handler
or filter to process HTTP requests/responses. To aid these developers, this section deals with generic C/C++
functions used when processing HTTP input and output, along with some of the common Apache APIs thatare used in handlers.
Legacy C/C++ Methods
For any C/C++ code interacting with web requests, code that handles strings and outputs should be checked
to ensure the logic does not have any flaws.
STRING TO SEARCH
exec

sprintdocument.referrer

fprintf

printf

Stdio

FILE

strcpystrncpy

Strcat

cout

cin

cerr

Systempopen

stringstream

fstringstream

Mallocfree

Request Processing
When coding within Apache, the following APIs can be used to obtain data from the HTTP request object.

STRING TO SEARCH
headers_in

ap_read_request

post_read_request

Response Processing
Depending on the type of response to be sent to the client, the following Apache APIs can be used.

STRING TO SEARCH
headers_out
ap_vprintf

ap_rprintf

ap_send_error_response

ap_send_fd

Code Crawling

Cookie Processing
Cookie can be obtained from the list of request headers, or from specialized Apache functions.

STRING TO SEARCH
headers_in

headers_out

headers_out

ap_cookie_read

ap_cookie_check_string

ap_cookie_write2

Logging
Log messages can be implemented using custom loggers included in the module (e.g. log4cxx, etc), by using
the Apache provided logging API, or by simply writing to standard out or standard error.

STRING TO SEARCH

HTML

cout

cerr

ap_open_stderr_log

ap_log_error

ap_log_perror

ap_log_rerror

ap_error_log2stderr

HTML Encoding
When the team has got a handle for the HTML input or output in the C/C++ handler, the following methods
can be used to ensure/check HTML encoding.

STRING TO SEARCH
ap_unescape_all
ap_escape_path_segment

ap_unescape_url

ap_unescape_url_keep2f

ap_unescape_urlencoded

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