Is Open Source Software More Secure?
Russell ClarkeDavid Dorwin

russellc@microsoft.com

ddorwin@u.washington.edu

Rob Nashrundaemon@gmail.com
Homeland Security / Cyber Security

Contents
1 Introduction
1.1

1

Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2 Initial Systems Analysis

1
2

A Brief Overview of Source Theologies . . . . . . . . . . . . . . .

2

2.1.1

Open Source is More Secure . . . . . . . . . . . . . . . . .

2

2.1.2

Security Through Obscurity . . . . . . . . . . . . . . . . .

3

2.2

Metrics are Often Misleading . . . . . . . . . . . . . . . . . . . .

4

2.3

Open and Closed Source Software are Intertwined

4

2.1. . . . . . . .

3 Vulnerabilities

5

Source-Dependent Attacks . . . . . . . . . . . . . . . . . . . . . .

6

3.1.1

Buffer Overflow . . . . . . . . . . . . . . . . . . . . . . . .

8

3.1.2

SQL Injection . . . . . . . . . . . . . . . . . . . . . . . . .

8

3.1.3

Patch Reverse Engineering . . . . . . . . . . . . . . . . .

9Source-Independent Attacks . . . . . . . . . . . . . . . . . . . . .

9

3.2.1

User Participation . . . . . . . . . . . . . . . . . . . . . .

10

3.2.2

Brute Force Attacks . . . . . . . . . . . . . . . . . . . . .

11

3.2.3

Protocol Vulnerabilities. . . . . . . . . . . . . . . . . . . .

11

3.2.4

Physical Intrusion and Inside Jobs . . . . . . . . . . . . .

11

3.3

Frequency of Attacks . . . . . . . . . . . . . . . . . . . . . . . . .

12

3.4

Understanding Attack Frequency . . . . . . . . . . . . . . . . . .

13

3.1

3.2

4 Security Analysis

13

4.1

Vulnerability Discovery. . . . . . . . . . . . . . . . . . . . . . .

13

4.2

Security Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

14

4.2.1

14

Threat Modeling and Exploit Classification . . . . . . . .

4.2.2
4.3

Source Code Scanners . . . . . . . . . . . . . . . . . . . .

15

Security Reviews . . . . . . . . . . . . . . . . . . . . . . . . . . .

16

5 Socioeconomic Effects

17

5.1

Business Decisions . . . . . . . . . . . . . . . . . . . . . . . . . .

18

5.2

Accountability and Support . . . . . . . . . . . . . . . . . . . . .

19

5.3

Patch and Fix Distribution . . . . . . . . . . . . . . . . . . . . .

20

5.4

Availability of Security Fixes . . . . . . . . . . . . . . . . . . . .

21

5.5

Custom Software and Configurations . . . . . . . . . . . . . . . .

22

5.6

Security Fixes for End-of-Lifed Software . . . . . . . . . . . . . .

24

6 Conclusion

25

Abstract
In an attempt to contribute to the current state of understanding with respect tosystems security, this paper inspects one software source distribution philosophy
that underlies the operation of a representative class of networked computers
today. Establishing whether open source leads to more secure software will
have serious implications for organizations utilizing or constructing open source
software, the trust established between a user and a program (irrespective ofsource visibility), and provide valuable observations for proprietary software
vendors as well. It is the intent of this paper to advance forward the stateof understanding with respect to source philosophies, initially and critically
explore the vulnerability differences caused by the source visibility differences,and propose an answer to a modern and relevant question: Does open source
development lead to more secure code?

1

INTRODUCTION

1

1Introduction

In this paper, we seek to measure the security value (or lack thereof) of theopen source software distribution philosophy. While many groups treat this discussion as a religious debate between open source and proprietary software, we
seek to empirically describe the issues and factors in support of or against the
security of open source software and avoid as best we can the issues we cannot
measure. Further, this debate is often partitioned along the axis of the dominantoperating system on each side, and may be colored by opinions just as much
as fact. However, we believe the security analysis of, and differences between,
the open source and proprietary software design philosophies dont reduce to acontest between Microsoft Windows (proprietary) and Apple Macintosh OS X
(where the kernel is primarily open source) or Linux (completely open source).
Rather, in this paper we seek to observe the theoretical and practical differences in security between representative classes of software systems, regardlessof the corporations or organizations responsible for the products under scrutiny.
Hence, the examples presented in this paper are chosen as practical, representative, and timely. We feel it is appropriate, then, to recognize and understandthe tussles that exist in this area of research, to then formulate our reasoning
and research further. Also, since there is both fiction and fact on each side of thedebate, we initially advocate for both sides, only to later analyze and explainthese observations.

1.1

Scope

This section defines the extent and limits of our discussion. We have chosen
many issues as topics for future work, and identified issues not bearing significant relevance for their inclusion, understanding that time, trials, and experience
will validate or discredit these simplifying assumptions. We are not interested in

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INITIAL SYSTEMS ANALYSIS

2

the dissection of the corporate ecosystems, nor in comparisons of one company’s
product versus another (indeed, such reports are already plentiful). We are notarguing which operating system is superior overall, which has more features, or
which is more economical. We do not even argue that one system is more securethan another, but rather, we seek out data in an attempt to qualitatively (moredata here) and quantitatively (less data here) confirm or deny the security ofopen source software due to its inherent code visibility. Topics peripheral to ourstudy of cyber security, such as licensing for open source and closed source systems, have been ommitted. Finally, we recognize that significant changes to thestate of the art, such as stronger tools, breakthroughs in quantum computing,types of attacks not previously known, underlying network protocol alterations(via deployment of Internet2 or IPv6, for example) will have significant ramifications to what is researched and presented here.

2

Initial Systems Analysis

2.1

A Brief Overview of Source Theologies

In this section we present the two opposing views on the security of open source.
The intent is to summarize the view of each side of the argument, regardless ofwhether it can be proven. In later sections we address these claims with facts
and data.

2.1.1

Open Source is More SecureWith open source, any who wish to see the source code for any part of project can
do so. Bugs including security vulnerabilities may be spotted by the many eyes
– both the experts and novices alike – on the code. Open source code is subject
to security reviews (as in any professional software developing enterprise), but

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3

in addition to “in-house” reviews by those engineers tied to the project, is also
subject to unsolicited security reviews that may be conducted by anyone in theworld. One could argue that, in general, more code walkthroughs are likely tooccur in open source projects, since usually an entity responsible for a projectwill test it to some amount and outside sources will also test your source someamount, potentially resulting in more code scrutiny in this paradigm. This isespecially true when large corporations have a vested interest in or sell products
based on an open source project. In this case, they have the same interest in
the project being secure as if it were proprietary. Finally, while security is oft
cited as a primary reason for not opening source code to the public, there seems
to be little conclusive evidence to support this.

2.1.2

Security Through Obscurity

This argument is as follows: If I hide my code from my adversaries, it will be
more difficult for them to identify vulnerabilities in my software. Only negative
results can come from offering visibility at the source level. Writing good software is difficult, and writing nontrivial, flawless software is probably impossible.
Hence, if software is complex to generate, and at least somewhat complex to reverse engineer, why sacrifice protection afforded to us by obscurity? Certainly,
the technique is not mutually beneficial; it simply provides the blueprints ofyour construction to your adversary. As tools for identifying security flaws improve, these will also benefit the adversary as they can identify vulnerabilities
in code bodies before fixes can be deployed to customers.
If I could slow or prevent the propagation of my source, then my organization
can capitalize on the gains that we are exclusively responsible for.
As with any battle, giving your plans to your adversaries or competitors is
not a good strategy for security or business in general.

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INITIAL SYSTEMS ANALYSIS

2.2

4

Metrics are Often Misleading

Deciding this debate would be much simpler if we could just look at the numberof attacks against or reported vulnerabilities in comparable open source andproprietary software products and declare a winner. However, there are many
variables besides development philosophy that affect such metrics. The size ofthe deployed base, code size, features provided, business decisions, effectivenessof customer maintenance and patching, and adversaries’ preferences all affectthese statistics. It is in an adversary’s interest to exploit vulnerabilities present
in as many systems as possible. Thus, she may choose to identify and exploit
security vulnerabilities in the most popular operating systems or applications.
Likewise, it is unlikely that security organizations and “white hats” put equalamounts of effort into identifying vulnerabilities in all software (even the leading
products). As such, we are unable to conclude directly that a software productwith more reported attacks or vulnerabilities is, in fact, more or less secure thananother product from this information alone.

2.3

Open and Closed Source Software are Intertwined

In one example[1], a vulnerability was identified, reported, and fixed in theLinux operating system. Upon release of the security update, attackers learned
the exact nature of the vulnerability and conjectured a similar vulnerability
might exist in other resource managers and operating systems. That groupwas successful in constructing an attack exploiting the same vulnerability thatwas indeed present in the closed source operating system, Windows NT. So,
we see that the existence of an open source program (and its constituent security updates) can reveal the vulnerabilities of both itself and any isometricsystem. This is implicit, obvious, and important in that it could suggest, forexample, that open source software and closed source software are intertwined

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in a way that makes investigation difficult. While it is the case that these systems do not operate in isolation of one another, and have each been constructedwith many engineering standards and similarities, we conjecture that it is not
impossible to separate the two systems for analysis, and further, that this evidence might suggest that we can leverage the similarity of systems in our study
by observing wide classes of vulnerabilities in a platform-independent fashion.
Further, we should consider the effects of releasing patches out in to “the wild”
that reveal high-level operational vulnerabilities that might be shared amongst
similar platforms, providing attackers with a so-called day zero attack that iscross-platform.
It is important to understand the broad overlap in the general constructionof the major resource managers studied here, and expect that as vulnerabilities
are found, reported, and fixed, that there should be overlap in the vulnerability
space as well. This offers incentives for major operating systems to share findings with each other, to offer mutually beneficial security enhancements, but islikely to be complex to navigate in a commercial or competitive environment.
However, for this study, it is valuable to observe that software systems in general
overlap a great deal in the design patterns employed, and hence can corporately
suffer from classes of vulnerabilities that affect all platforms.

3

Vulnerabilities

Not all attacks rely on vulnerabilities in the source code and as such not all
attacks directly relate to open source software security. Many attacks rely onhumans as the weak link, or at least rely in part on humans to help. In addition,
there are other types of cybersecurity attacks that can be executed even on
perfectly secure software. We classify a variety of popular attacks into sourcedependent and source-independent and examine them in the following sections.

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VULNERABILITIES

3.1

6

Source-Dependent Attacks

Open source software has not been a major target of security attacks, and thisis generally thought to be due to the predominance of Windows as a platform.
The number of Linux security attacks is increasing, however, due to increased
popularity of Linux. There are currently more than 100 viruses that target
Linux [2] and this number is constantly rising. There are also other attacks
against Linux, including worms, Trojans, denial of service (DOS) attacks androotkits.
Many Linux servers have been attacked and attacks are increasing [3], because many servers on the Internet run Linux. The main reason cited for thevulnerability of Linux servers is due to poor server administration, and not due
to inherent security vulnerabilities in the software (Ibid). The complexity of administering a Linux server is also cited as one of the reasons to use a Windows
server instead of Linux [4].
This raises another source of insecurities, which is the complexity of administrating the server. Poor administration leads to insecurities. Different
distributions of Linux leave multiple ports open, increasing the vulnerability ofthe system to attacks. A default Windows installation may leave ports open,
but because it is generally easier to administer, it is easier to close unwanted
services than on Linux. Systems that offer better ease of administration are
therefore preferred, but this issue is not inherent to source distribution philosophies, and so is beyond the scope of this work. It is important to note that easeof administration does not mean that one OS (whether open or closed source)
is more or less secure than another. An system administrator expert in any OS
will know how to best reduce vulnerabilities and secure the server.
There is one major way in which Linux and Windows are administered, whichis that Linux administrators generally don’t run as “root”, instead running as a

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7

normal user account, and use “setuid” or “sudo” to execute commands as root.
Windows administrators, on the other hand, typically run using Administrator
mode all the time. While this is not exactly an open vs. closed source issue, it
can help explain why there might be more successful attacks on Windows thanLinux.
Linux viruses often infect ELF (Executable and Linkable Format) files, which
are common on Linux systems. Other attacks use shell scripts, which are often
compatible across platforms and distributions, allowing them to spread more
widely than a binary file. The sophistication of Linux viruses has also increased.
For example, a recent virus, W32/Etap.d is a polymorphic virus, making it
difficult to detect. A variant of this virus is able to infect Windows portable
executable files in addition to Linux files.
Because open source code is often shared among different open source software (e.g. different Linux distributions or applications), some vulnerabilities
can exist on multiple open source software.

Apache is one example.

The

Linux/Slapper worm makes use of a known vulnerability in the Open SSL library to infect Apache web servers. In addition, the Linux/Adore.worm uses a
random port scan to identify systems that contain a root access vulnerability on
Linux servers. Even though there is only one vulnerability, multiple operating
systems are affected.
Because open source software comes in many different flavors (e.g. the manyvarieties and distributions of Linux), it is more difficult to coordinate a simultaneous patch for all distributions. Since most attacks are made after both thevulnerability and patch have been made known publicly, but not all distributions
have patches available, some distributions will be left vulnerable to attacks whileothers can be patched. This is a clear disadvantage of the distributed nature ofopen source software development. However, by the same argument, it is more

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VULNERABILITIES

8

difficult for attackers to target multiple distributions simultaneously, so there
are both positive and negative aspects to open source software distributions.
3.1.1

Buffer Overflow

Buffer overflows are easily found via automated tools that perform static codeanalysis; since this type of attack is specifically related to the structure andsymbols in a given program’s construction, we state that buffer overflows are
source level attacks. As such, they are quickly detectable from any body ofvisible source code. Hence, any open source project with buffer overflows vulnerabilities in its code base will be susceptible to such an attack, and adversaries
could use automated tools to perform a vulnerability analysis. However, thesesame tools are available during the lifecycle of the software under construction,and should be a routine part of any security analysis and walkthrough the code
receives. In fact, many modern development environments will automaticallywarn developers if they have used legacy code constructs that could be subject tooverflow attacks, and newer programming languages seek to disallow the typesof memory structure declarations that allow programmers to overrun memory
spaces.
An example of a buffer overflow exploit is the infamous Blaster worm released
in 2003 exploited a vulnerability in the Windows DCOM service. The Blasterworm spreads without any user participation, such as opening an e-mail orvisiting a web site. [5]3.1.2

SQL Injection

SQL injection attacks generally follow the same strategy – find a web server thatadvertises database services and look for an interface, such as a username andpassword login webpage, that accepts client-side input. By providing carefully
constructed input to the interface, the SQL database or even the server it is3

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9

running on can can be compromised. This attack is analogous to the buffer
overflow attack for remote code execution in that it exploits failure to properly
verify and limit input data. As such, it is a source-dependent attack.

3.1.3

Patch Reverse Engineering

A so-called “day zero” attack is the result of reverse engineering a recently
released software update, in an effort to discover and exploit the problem thepatch is supposed to fix. The patch itself serves as a blueprint describing a realvulnerability that most computers (unpatched thus unprotected) are susceptible
to. The “day zero” moniker is derived from the fact that the best time to exploit
the greatest number of computers is immediately following the patch releasebecause the exploit becomes less valuable as the patch is applied to an increasing
number of systems. In preparation for this style of time sensitive attack, tools
are generated by attackers that can automatically generate a working exploit,
given a patch as input.
Day zero attacks are code level attacks, and a disadvantage to open source
software. Interesting to note here is that “closed” source software suffer fromthis exact class of attacks, too. The subtlety lies in the fact that the differences
in machine code between patched and unpatched files can be reverse engineered
to give an attacker visibility into the original security flaw, regardless of whether
the source code is available. So, in this class of attacks, both open and closed
source software are at risk.

3.2

Source-Independent Attacks

A large set of attacks do not rely at all on security flaws in source code, and thus
neither open source or proprietary software is safer with respect to such attacks.
In this section we describe some of these attacks to demonstrate that for these

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10

classes, systems are equally vulnerable regardless of the software development
philosophy employed when creating the operating system and other software on
it.
3.2.1

User Participation

Many types of data theft (both corporate data and phishing for consumer data),
spyware, viruses, and other attacks require some participation by a system user.
Often these attacks just rely on the user executing a program or providing data
to a party they should not. Often such attacks exploit the ignorance or inattention of the user. For example, many users will click “Yes” on any dialog thatpops up just to make it go away.[6] Traditional viruses propogated when users
transferred files between computers, often by floppy disk, and ran an infected
application. Such viruses do not rely on security flaws in the applications thatthey infect.
If a user is logged into a system with root or administrator priveleges, any
application that is executed during that session can also run with such privileges.
This includes installing drivers, patching the kernel, etc. Other attacks may
rely on user participation to exploit vulnerabilities, such as those that permit
elevation of privileges. For example if there is such a security flaw in a part of an
operating system that is not accessible via the network, an attack may require
an unprivileged user to launch a program that implements the exploit. The
vulnerability exploited as part of the Red Team exercise is one such example.
The target application was not accessible via the network, but once the Red
Team logged on with a limited user account it was able to execute the programand elevate the accounts privileges.
Attacks sent via email attachment are a common example of those that
require user participation. An adversary creates a script or program that generates “socially-engineered” emails that entice the user to open the attached file.

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VULNERABILITIES11

The infamous “Love Letter” (also known as “I Love You”)[7] and MyDoom[8]worms are examples of such attacks. While these worms attacked a specific operating system, they were dependent on user participation rather than exploits
to spread.

3.2.2

Brute Force Attacks

Brute force attacks, such as dictionary password attacks on systems that permit
remote access, are also independent of source bugs. In the case of dictionary
password attacks, adversaries repeatedly try to log into a system (often as rootor administrator) until they are successful. No code vulnerability is necessary
to execute such an attack and therefore software based on both open and proprietary source is susceptible.

3.2.3

Protocol Vulnerabilities.

Some industry standard protocols have vulnerabilities inherent in the protocol itself or its dependencies. Since all software that supports the protocol isimplementing the same protocol, all such software is equally susceptible. The
Kerberos “Man-in-the-Middle” vulnerability is one example. Any standard implementaiton of Kerberos could be exploited regardless of source code availability. Such protocol vulnerabilities may be more worrisome than software
vulnerabilities because the potential set of targets is larger as it includes manydifferent platforms.

3.2.4

Physical Intrusion and Inside Jobs

While clearly the most pedestrian of the techniques surveyed thus far, no lesseffective is the physical intruder or an “inside job” executed by someone withaccess. In addition to offering immediate access to any system on the Internet

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12

remote attacks also offer an additional level of anonymity and security for adversaries. In contrast, the simplicity of a physical attack is alluring for some,
especially when network security is tight and a specific target is desired. Whenan adversary has physical access, the software implementation philosophy rarely
matters.

3.3

Frequency of Attacks

The reported numbers of cyber attacks for all operating systems has grownsteadily and, during some outbreaks, explosively, in the past decade and beyond. CERT, the coordination center for the Computer Emergency Response
Team at Carnegie Mellon, has reported more than three hundred thousand incidents (where each incident could involve whole networks) since 1990. CERT
reports that, given the growing use of automated attack tools, attacks against
networked systems have become so frequent that “counts of the number of incidents reported provide little information with regard to assessing the scopeand impact of attacks. Therefore, as of 2004, [CERT] will no longer publish
the number of incidents reported” [9] and in place of sheer numbers, CERT
will work to develop a more meaningful metric. However, we are able, at thevery least, to note an increase in interest in and use of cyber attacks over the
past decade and a half, so we include figure one. The growth in the numberof attacks might seem to suggest multiple things, but we must take care not toderive too many conclusions; these statistics are for all operating systems andeach incident could represent a completely compromised network or simply one
computer. The increase in automation of attacks would help account for someof the explosive growth, and the increase in personal computer sales from 1988
to today provides for significantly more possible targets to attack (all sharing asimilar base of operating systems and applications).

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3.4

13

Understanding Attack Frequency

Many observers agree that a significant reason why Windows has been subject tomore attacks than Linux is because it is a much more popular operating system[10]. In 2004, Linux market share globally was only about 3% [11], about thesame as Mac OS, leaving Windows with most of the remaining 94%. Given
this data, replacing closed source software with open source would not solve
the problem – hackers would simply move to the next major target. As we will
shortly see, Linux viruses exist, but in much fewer numbers than for Windows.
In fact, some vulnerabilities have been left unpatched in Debian Linux because
they did not feel the risk of an attack was significant enough to fix (Ibid).

4

Security Analysis

4.1

Vulnerability Discovery

Currently, it is mainly left up to the open source community to find and fix
security issues in open source software. However, with the growing endorsement
and investment of open source software (e.g. Linux) from large corporations suchas IBM and Novell, more people who are security experts (who are employedby those corporations) will be finding and fixing security issues. Thus, we can
expect the security of open source software to increase in the next few years.
This will reduce any difference that exists in the security of open and closed
source software.
For closed source software, security processes are now commonplace (e.g.
at Microsoft) to attempt to reduce the number of vulnerabilities that exist
in the software. These companies employ software security experts and evenhave security teams set up such as Microsoft’s MSRC [12]. These processes
enable companies to fix problems before they are discovered by customers, or a

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14

hacker. As with open source software, security bug reports also come from thecommunity.

4.2

Security Tools

4.2.1

Threat Modeling and Exploit Classification

Threat modeling is a method commonly used to find and address security threatsin software. It is basically a brainstorming process that can be summarized asfollows: A list of all inputs into the program is created. A list of all vulnerabilities in the program is created. These may be external dependencies (forexample, APIs which the programmer has no control over) or other vulnerabilities which are not easy to eliminate.
Vulnerabilities are typically classified according to the STRIDE classification
system. STRIDE is an acronym which stands for: Spoofing identity, Tamperingwith data, Repudiation, Information disclosure, Denial of service, and Elevation of privileges. They each refer to different classes of threats. “Spoofing
identity” refers to obtaining access and use of another person’s authentication
information, such as username and password. “Tampering with data” refers tomodifying data without the owner’s permission. “Repudiation” refers to theability to keep track what actions were performed, and who performed them.
“Information” disclosure refers to obtaining information without having permission to access it. “Denial of service” refers to attempts to prevent legitimate
users from using a service or system. “Elevation of privileges” refers to wherean underprivileged user obtains higher-privileged access.
The STRIDE model was developed by Microsoft and is used by developers
to identify security issues in their code.
After categorizing each of the vulnerabilities; for each of the inputs into theprogram, ways in which the inputs could threaten the vulnerabilities are listed.

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15

For example, a program that sends data across a network might have a threatthat the data could get tampered with as it is being sent across the network.
Finally, mitigations for each threat are listed. A mitigation for the network
example above could be to encrypt the data before sending it using a strong
encryption method, or to use checksums. If a program is too big for all inputs
to be listed at once, it can be broken down into features and subfeatures, and athreat model performed on each of these.
4.2.2

Source Code Scanners

Source code scanners (such as Flawfinder, RATS and ITS4) exist for both open
and closed source software, and help create more secure code by finding common
security issues in source code and often suggesting more secure code that could
be used instead [13].
Both Flawfinder and RATS (Rough Auditing Tool for Security) are free,
while ITS4 (It’s The Software Stupid Source Scanner) charges a fee for its use.
All three tools find problems with using functions such as printf() with variable
length strings, gettext libraries and their use in internationalization. All three
are configurable, allowing different levels out output (depending on the priorityof the issue found) and ignorable lines. Flawfinder is the fastest of the three,while RATS finds the most errors (Ibid). One problem with all three tools isthat they do not do any preprocessing, so some mistakes can go unnoticed (forexample, if you create a macro for the printf function). ITS4 does not give as
useful output as the other two tools, as it does not suggest secure alternatives.
A commercial tool, Microsoft Visual Studio 2005, includes two source code
security scanners, one called FXCop, which scans managed, .NET Framework
code; and one called PREfast, which scans regular C/C++ code [14]. These
tools were used internally and have been tested within Microsoft before becoming part of Visual Studio. They both scan for a large number of potential errors,

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16

which can be selected individually from a settings dialog. Suggestions on how
to fix the code are also given.
The main limitation of source code scanners is that as a general-purpose
automated tool, it will never be as thorough as a manual code review. A goodsuggestion is to use a code scanner as a pre-pass before the programmer does amanual audit of the code. It is also important for the programmer to understandwhat each API used does. For example, some libraries may be using unsafe
APIs in them, which a code scanner would not find (unless the source for themis scanned as well).
Internally, Microsoft uses another tool, called PREfix (Ibid) which takes
much longer but searches for even more errors. Because the tool is internal, Microsoft here has an advantage over open source code. But as mentioned earlier,as more large corporations invest in open source software, these advantages are
expected to become less relevant in the near future.

4.3

Security Reviews

Closed source development is generally more organized than open source development, and thus there are more processes involved. One such process isthe “security review”, which most major software development companies use.
These typically include Threat Modeling and running security tools staticallyon the code or during runtime. For example, Microsoft does a security pass atleast once during each product development cycle. During a security pass, relevant source code is reviewed for security issues, code analysis tools are run on
the source code and the bugs fixed, and tests, which try to find various security
issues, are run on the software.
Open source development, being more distributed and less conventional, does
not always go through a security process. Security review methods that can be

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17

used effectively in open source projects include running code scanners (such as
Flawfinder, RATS, and ITS4) and the fact that there are a large number ofpeople looking at the code. However, the people looking at the code may not
be experts or understand the code fully, which could let more security bugs gounnoticed. In addition, the tools available to large corporations are often moreadvanced than those available to the open source community. Furthermore,
corporations - as with individuals - have access to all the tools in the open
source community. The open source community may also be limited as to theAPIs that can be used, where more secure APIs exist commercially.
While open source may have traditionally been a less organized “grass roots”
effort, that is not always the case, especially for the major open source projects.
Over the past decade, corporations have invested billions of dollars in open
source software - mostly Linux. [15][16] IBM, Intel, Red Hat, and many othercompanies have full time employees contributing to Linux. In addition, Open
Source Development Labs (OSDL) acts as “the central body dedicated to accelerating the use of Linux for enterprise computing.”[17]It is worth noting that around 95% of software bugs are caused by 19 ‘common, well-understood’ programming errors” [18].

5Socioeconomic Effects

The availability of the cource code for open source software has additional effects
on the ecosystem of such software beyond just source code availability. These
effects include the speed at which security fixes are available and the ability tocustomize security, to name a few.

5

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5.1

18

Business Decisions

Some security vulnerabilities are worsened by business decisions that are unrelated to the product’s source code availability. For example, Microsoft made thedecision to enable the DCOM service by default in Windows XP despite the low
utilization of this future in the user base. [19] While the vulnerabilities discovered in DCOM was a code flaw, the Blaster worm that exploited it would nothave spread nearly as quickly or broadly as it did if DCOM had been disabled
by default.
The Code Red and Nimda worms took advantage of a similar business decision that left vulnerable services enabled by default. Default installations for
Microsoft’s Internet Information Services (IIS) web server (versions 4.0 and 5.0)
included the Microsoft Indexing Service. As a result, a vulnerability in the
Indexing Service could be exploited on most systems using the IIS web server
regardless of whether the system was actually running the Indexing Service. [20]Code Red alone is estimated to have caused billions of dollars in lost productivity. [21] The impact would have likely been reduced had only systems thatneeded it had the Indexing Service not been installed on servers that were notusing it.
The decisions or lack of awareness of customers can also have an impact
on the success of exploits. Once a patch is available, it is important for customers to apply them to their systems. A patch for the DCOM vulnerability
was available for nearly a month before Blaster was released [22], and Nimda
exploited the same vulnerability (for which a patch was available) as Code Red
had a few weeks earlier. [21] (Note: The average delay between the publishingof a vulnerability or patch and the release of malicious exploits has decreased
significantly since these worms were released.)
If different decisions - both by software vendors and customers - had been

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19

made, some of the most damaging and infamous worms in the Internet’s history
may have been merely minor issues or not created at all because of the low
potential return on investment for the author. Both open source and proprietary
software products are susceptible to business decisions worsening the impact ofor exposing code flaws. For example, one need not see the source code for
DCOM to understand that this service was a potential vulnerability. They
are also both susceptible to customer ignorance, apathy, or delay in applying
patches for known vulnerabilities.

5.2

Accountability and Support

Accountability is an issue that often comes up when large organizations are
considering adopting open source software. For example, a recent editorial inFederal Computer Week [23] says “CIOs considering a move to open-source
software need someone to hold accountable – someone who has the resources
to address any problems that occur.” They want to know that they will havesupport available when they have problems. This is especially important for
security issues, because they can be very costly to corporations, as shown by
the numerous worms and viruses that have spread recently.
When a company buys software from Microsoft (as an example of a closed
source system), they expect Microsoft has a level of accountability. The EULA
(End User License Agreement) users agree to in order to use the software does
not actually provide any accountability, but the reputation of software gives acompany a level or accountability. If the software does not work, people will
not use it, so it is in Microsoft’s interests to fix security problems as they are
discovered. In fact, this is what happens, as attested to by the numerous security
updates that Microsoft releases.
When a company buys a Linux solution from IBM (as an example of an open

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20

source system), there is also no accountability provided by IBM, but because
they do not own the software (Linux software is distributed under the GNU
Public License), there is less loss of reputation for any security issues that arefound. IBM will provide setup support, but will not fix any bugs or security
issues. These issues are left up to the Linux distribution’s open source community to fix. Because there is no company that can be immediately contacted
or responsible for fixing the issue, some corporations will not use open source
software.

5.3

Patch and Fix Distribution

Microsoft releases regular security patches which they call “critical updates”.
Apple also releases monthly (or, as necessary) security updates for Mac OS X.
Users have the option to be alerted when security patches are available, and thenlet them download and install the patch manually; or download automaticallyand let them install manually; or automatically both download and install thepatch. One problem is that users typically need to reboot after installing asecurity patch, which may delay installation of the patch since many users do
not wish to be forced to reboot their computer, instead preferring to keep theirapplications and documents open for days at a time. In this situation, thecomputer is vulnerable until it is rebooted, even though the patch has been
downloaded and installed. This problem exists less on Linux, where reboots are
not required as frequently [24].
Several major Linux distributions (e.g. Debian, Red Hat) also have automated security updates, which have similar features to Microsoft’s security
updates. While the patch mechanism is not an open source vs. closed source
issue, it shows that the open source community has the resources to adequately
issue patches and notify users of patches.

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5.4

21

Availability of Security Fixes

When proprietary software vendors fix security flaws, they must validate it and
get it into a release or patch. This process could take up to a year or more in
the worst case. In some cases, the company may decide not to fix the flaw atall. In the open source community, however, end users could apply a fix as soon
as it is implemented. This process is complicated by the fact that vendors must
avoid incompatibilities with their customer base. Microsoft says the following
regarding this issue.
Microsoft products run on thousands of different manufacturers’
hardware, in millions of different configurations, and in conjunction with countless other applications. Our patches must operatecorrectly on every single machine. This is a significant engineering
challenge under any conditions, but it is even more difficult when
details of a vulnerability have been made public before a patch canbe developed. In such cases, speed must become our primary consideration, in order to protect our customers against malicious users
who would exploit the vulnerability. [25]Furthermore, significant new security features may not be available to users
until the next major release or service pack. Software is often only released
every couple or few years, so significant new security features may be withheld
from users for years regardless of how long the feature took to implement.
There are also vendors that package open source software and must follow
the same steps and be just as careful when releasing patches or other software
updates. However, if an end user really wants the fix and is willing to accept
the risk or test their configuration (i.e. in an IT department), they can get thefix immediately. Even if an open source vendor does not fix a security flaw, if
it is important enough to end users, someone will likely implement a fix. In

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22

addition, its possible that some group or vendor will release a distribution withthe feature. Because there are many different distributions of Linux, there iscompetition between the makers of these distributions which are based mostly
on the same code base. This means that once one distribution has a feature,
there could be pressure on other distributions to add it as well for fear of losing
customers. Regardless, users could always switch to another distribution –while switching distributions can be a hassle, it is much simpler than changing
operating systems all together (i.e. Windows to Linux).

5.5

Custom Software and Configurations

In addition to permitting end users to apply security fixes as early as they wish,
open source software also allows end users to make their own modifications
to improve security. Anyone with the right knowledge and skills can make aderivative or custom version of open source software. One might choose to do
so to limit exposure or increase security. Linux and Windows each containtens of millions of lines of code.[26] Security vulnerabilities could be waiting innearly any portion of that code, yet most users only need a minority of the code.
As noted earlier, the Blaster, Code Red, and Nimda worms took advantage ofvulnerabilities in features that many Windows users did not need. By compilingand installing only features that one really needs, there is less code with potential
bugs and less code to review for security flaws if one chooses to. Custom builds
do create their own potential vulnerabilities since the changes have not beenreviewed by or tested by the wider community. However, as mentioned earlier,adversaries are more likely to go after more widely distributed software, unlessof course you are a high value target. Custom builds also require that themaintaner merge applicable security fixes into the custom build. One solution
to both issues is to submit the changes or derivative back to the community.

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23

Other benefits of the ability for end users to customize software include theability to add features or security updates to an existing version of software
without being forced to upgrade to a new version, which may contain newvulnerabilities. The opposite can happen as well – features or support may
be removed from a newer more secure version of software so users continue
to run an older less secure version. This often occurs when hardware vendors
make a business decision not to provide support (drivers) for older hardwareon new versions of Windows. Consumers must then decide whether to pay toreplace their hardware so that they can upgrade Windows or continue to run anolder version of Windows. If the driver were open source and there was enoughinterest, someone may update the driver and provide it to the general public.
In addition, if a open source software vendor does not think implementing acustomer’s request makes business sense, the customer or an organization ofcustomers can modify the software themselves to suite there needs.
Security Enhanced Linux (SE Linux) is one such example that grew out of
the United States National Security Agency’s (NSA) frustration with computersecurity weaknesses in the late 1990s. When proprietary software vendors rejected the idea of investing large amounts in extra-secure systems, the NSA
began implementing its own derivative of Linux with an advanced security architecture. The agency then submitted SE Linux for inclusion in the Linuxkernel and SE Linux eventually made its way into commercial products.[27][28]
Rootkits, such as the recent one found on some Sony music CDs, provide a
us with a specific example of a modification a user may wish to make. The XCP
rootkit on these CDs relies on the ability to patch functions in the system calltable. [29] While Microsoft acknowledges that allowing patching of the kernel
is undesirable, they have decided to continue to permit it in 32-bit versionsof Windows because preventing it “would break compatibility for a significant

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24

amount of released software.” [30] If Windows were open source, end users
would be able to decide for themselves whether protection from such rootkits orcompatibility with software that “can interfere with other software and affectthe stability of the operating system” [30] is more important to them. The
answer would vary among end users, but at least they would have the option.
Instead, end users that are concerned and knowledgeable enough are left in an
ongoing battle to detect whether a rootkit has been installed on their system.

5.6

Security Fixes for End-of-Lifed Software

As with many other industries, software vendors like to end-of-life support for
older software. For various reasons, customers may wish to or have businessneeds to continue using software beyond its supported timeframe. When proprietary software is no longer supported, these customers may be left vulnerable
to new exploits. With open source, however, customers could make their own
security fixes for security flaws indefinitely even if the original author or vendor
is no longer releasing security updates. The same benefits apply if a vendor goesout of business.
Although not directly related to cyber security, the Y2K phenomenon demonstrates the need for source code. Countless software applications had to be
replaced or updated to support four-digit years. Many companies decided to
update their decades-old software rather than migrate to new hardware andsoftware. Fortunately for many of them, the old software had been developed
in-house, in which case they had the source code, or by vendors that provided
year 2000-complian updates. Companies with commercial software for which nosource or updates were available may have had to make different - potentiallymore costly or risky - decisions.
While open source software can help alleviate such scenarios, proprietary6

CONCLUSION

25

software can also offer such cyber security blankets. Potential solutions for
proprietary software include licensing the source code for specific use or putting
it into escrow. Microsoft has even provided Windows source code to somecustomers, such as large governments.[31]6Conclusion

While open source makes code visible, we have found that source visibility does
little to increase the security risks posed to a given project, in most cases.
Security analysts report that, in some situations, source level visibility can evenassists in the convergence of projects to stable and secure states.[32]The current scale and prevalence of closed source systems relative to open
source makes discerning meaningful conclusions from attack frequency alonedifficult. It is expected that hackers will attack those targets offering superior
economies of scale, but this does not preclude the possibility of closed source
systems being more secure based on the sheer number of assaults mounted
against such a system. If the operating system a hacker chooses to attack
is rarely deployed, then the development of a corresponding exploit for thatmachine is of relatively little value. However, an exploit that can successfullycompromise a wide base of systems is valuable indeed; we conclude only that
popular systems are popular for attackers.
Closed source advocates will frequently cite the structure and adaptive remote development techniques as being too ad hoc and thus insufficient, not capable of uniform and coordinated evaluations or reviews by professionals. Open
source is not a community of hobbyists, but rather a diverse grouping of professionals and full time industry experts from international corporations such asIntel, IBM, and RedHat. The closed source software lifecycle, while necessarily
structurally different due to topology differences between contributers, has not

6

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26

to date been shown in any quantative fashion to be inherently less secure.
Open source advocates frequently note their system to be more secure basedon attack frequencies, as well as other observations that frequently are supported by strong opinions more than available data. In this research, we havebeen unable to demonstrate greater security through obsecurity or cloaking, andthus unable to assert that closed source offers additional protections or tangible
security features we can leverage in pursuit of a more secure system.
The ability to read the source in and of itself offers little additional incentive for hackers to specifically target open source projects. If the secrets insoftware were completely containable (possibly through encryption), then perhaps a “security through obscurity” argument would apply more concretely;
instead, however, since source code can be reverse engineered (to various levelsof readability or success), the value of hiding the implementation is diminished. Further, exposing the code has side-effects that can be beneficial (suchas unsolicited security reviews or software reuse), while cloaked source has nosuch counterparts. Finally, code leaks are somewhat common and difficult toprevent[33], so it is likely that any closed source project has been “opened” atsome point, diminishing the value of the information hiding further. In addition,
as described in this paper, many security attacks are independent of the source
code, so neither open source or proprietary software is less secure.
In conclusion, open source does not pose any significant barriers to security, but rather reinforces sound security practices by involving many people
that expose bugs quickly, and offers side-effects that provide customers and thecommunity with concrete examples of reusable, secure, and working code.

REFERENCES

27

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