The Ten Most Critical API Security Risks

TOC

Table of Contents

Table of Contents
TOC Table of Contents...............................................2
FW Foreword.............................................................3
I Introduction..............................................................4
RN Release Notes.......................................................5
RISK API Security Risk.............................................6
T10 OWASP API Security Top 10 - 2019..................7
API1:2019 Broken Object Level Authorization.........8
API2:2019 Broken User Authentication...................10
API3:2019 Excessive Data Exposure.......................12
API4:2019 Lack of Resources & Rate Limiting......14
API5:2019 Broken Function Level Authorization. . .16
API6:2019 Mass Assignment...................................18
API7:2019 Security Misconfiguration.....................20
API8:2019 Injection.................................................22
API9:2019 Improper Assets Management................24
API10:2019 Insufficient Logging & Monitoring.....26+D What’s Next for Developers...............................28+DSO What’s Next for DevSecOps.........................29+DAT Methodology and Data..................................30+ACK Acknowledgments........................................31

About OWASPThe Open Web Application Security Project(OWASP) is an open community dedicated toenabling organizations to develop, purchase, andmaintain applications and APIs that can be trusted.
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FW

Foreword

A foundational element of innovation in today’s app-driven world is the Application Programming Interface(API). From banks, retail, and transportation to IoT, autonomous vehicles, and smart cities, APIs are a criticalpart of modern mobile, SaaS, and web applications and can be found in customer-facing, partner-facing, andinternal applications.
By nature, APIs expose application logic and sensitive data such as Personally Identifiable Information (PII)
and because of this, APIs have increasingly become a target for attackers. Without secure APIs, rapid
innovation would be impossible.
Although a broader web application security risks Top 10 still makes sense, due to their particular nature, an
API-specific security risks list is required. API security focuses on strategies and solutions to understand andmitigate the unique vulnerabilities and security risks associated with APIs.
If you're familiar with the OWASP Top 10 Project, then you'll notice the similarities between both documents:
they are intended for readability and adoption. If you're new to the OWASP Top 10 series, you may be better off
reading the API Security Risks and Methodology and Data sections before jumping into the Top 10 list.
You can contribute to OWASP API Security Top 10 with your questions, comments, and ideas at our GitHub
project repository:• https://github.com/OWASP/API-Security/issues• https://github.com/OWASP/API-Security/blob/master/CONTRIBUTING.md
You can find the OWASP API Security Top 10 here:• https://www.owasp.org/index.php/OWASP_API_Security_Project• https://github.com/OWASP/API-Security
We wish to thank all the contributors who made this project possible with their effort and contributions. They
are all listed in the Acknowledgments section. Thank you!

I

Introduction

Welcome to the OWASP API Security Top 10 - 2019!
Welcome to the first edition of the OWASP API Security Top 10. If you're familiar with the OWASP Top 10
series, you'll notice the similarities: they are intended for readability and adoption. Otherwise, consider visiting
the OWASP API Security Project wiki page, before digging deeper into the most critical API security risks.
APIs play a very important role in modern applications' architecture. Since creating security awareness andinnovation have different paces, it's important to focus on common API security weaknesses.
The primary goal of the OWASP API Security Top 10 is to educate those involved in API development andmaintenance, for example, developers, designers, architects, managers, or organizations.
In the Methodology and Data section, you can read more about how this first edition was created. In futureversions, we want to involve the security industry, with a public call for data. For now, we encourage everyone
to contribute with questions, comments and ideas at our GitHub repository or Mailing list.

RN

Release Notes

This is the first OWASP API Security Top 10 edition, which we plan to be updated periodically, every three orfour years.
Unlike this version, in future versions, we want to make a public call for data, involving the security industry inthis effort. In the Methodology and Data section, you'll find more details about how this version was built. For
more details about the security risks, please refer to the API Security Risks section.
It is important to realize that over the last few years, architecture of applications has significantly changed.
Currently, APIs play a very important role in this new architecture of microservices, Single Page Applications(SPAs), mobile apps, IoT, etc.
The OWASP API Security Top 10 was a required effort to create awareness about modern API security issues. It
was only possible due to a great effort of several volunteers, all of them listed in the Acknowledgments section.
Thank you!

RISK

API Security Risk

The OWASP Risk Rating Methodology was used to do the risk analysis.
The table below summarizes the terminology associated with the risk score.
Threat Agents

API Specific

Exploitability

Weakness
Prevalence

Weakness
Detectability

Technical
ImpactEasy: 3

Widespread 3

Easy 3Severe 3

Average: 2

Common 2

Average 2

Moderate 2

Difficult: 1

Difficult 1

Difficult 1

Minor 1Business
Impacts
Business
Specific

Note: This approach does not take the likelihood of the threat agent into account. Nor does it account for any ofthe various technical details associated with your particular application. Any of these factors could significantly
affect the overall likelihood of an attacker finding and exploiting a particular vulnerability. This rating does nottake into account the actual impact on your business. Your organization will have to decide how much security
risk from applications and APIs the organization is willing to accept given your culture, industry, and regulatoryenvironment. The purpose of the OWASP API Security Top 10 is not to do this risk analysis for you.

ReferencesOWASP•

OWASP Risk Rating Methodology•

Article on Threat/Risk Modeling

External
•
•
•
•
•
•

ISO 31000: Risk Management Std
ISO 27001: ISMSNIST Cyber Framework (US)ASD Strategic Mitigations (AU)NIST CVSS 3.0
Microsoft Threat Modeling ToolT10

OWASP API Security Top 10 - 2019

API1:2019 - Broken Object Level Authorization

APIs tend to expose endpoints that handle object identifiers,
creating a wide attack surface Level Access Control issue. Object
level authorization checks should be considered in every functionthat accesses a data source using an input from the user.

API2:2019 - Broken User Authentication

Authentication mechanisms are often implemented incorrectly,
allowing attackers to compromise authentication tokens or to
exploit implementation flaws to assume other user's identities
temporarily or permanently. Compromising system's ability toidentify the client/user, compromises API security overall.

API3:2019 - Excessive Data Exposure

Looking forward to generic implementations, developers tend toexpose all object properties without considering their individual
sensitivity, relying on clients to perform the data filtering before
displaying it to the user.

API4:2019 - Lack of Resources & Rate Limiting Quite often, APIs do not impose any restrictions on the size or
number of resources that can be requested by the client/user. Not
only can this impact the API server performance, leading to
Denial of Service (DoS), but also leaves the door open toauthentication flaws such as brute force.
API5:2019 - Broken Function Level
Authorization

Complex access control policies with different hierarchies,groups, and roles, and an unclear separation between
administrative and regular functions, tend to lead to authorization
flaws. By exploiting these issues, attackers gain access to otherusers’ resources and/or administrative functions.

API6:2019 - Mass Assignment

Binding client provided data (e.g., JSON) to data models, without
proper properties filtering based on a whitelist, usually lead toMass Assignment. Either guessing objects properties, exploringother API endpoints, reading the documentation, or providing
additional object properties in request payloads, allows attackers
to modify object properties they are not supposed to.

API7:2019 - Security Misconfiguration

Security misconfiguration is commonly a result of unsecuredefault configurations, incomplete or ad-hoc configurations, open
cloud storage, misconfigured HTTP headers, unnecessary HTTP
methods, permissive Cross-Origin resource sharing (CORS), andverbose error messages containing sensitive information.

API8:2019 - Injection

Injection flaws, such as SQL, NoSQL, Command Injection, etc.,
occur when untrusted data is sent to an interpreter as part of acommand or query. The attacker's malicious data can trick theinterpreter into executing unintended commands or accessing data
without proper authorization.

API9:2019 - Improper Assets Management

APIs tend to expose more endpoints than traditional web
applications, making proper and updated documentation highly
important. Proper hosts and deployed API versions inventory also
play an important role to mitigate issues such as deprecated API
versions and exposed debug endpoints.

API10:2019 - Insufficient Logging & Monitoring

Insufficient logging and monitoring, coupled with missing or
ineffective integration with incident response, allows attackers tofurther attack systems, maintain persistence, pivot to more
systems to tamper with, extract, or destroy data. Most breachstudies demonstrate the time to detect a breach is over 200 days,
typically detected by external parties rather than internal
processes or monitoring.

API1:2019 Broken Object Level Authorization
ThreatAgentsAttackVectors

Security
Weakness

Impacts

API Specific Exploitability: 3
Prevalence: 3
Detectability: 2
Technical: 3 Business SpecificAttackers can exploit API
This has been the most common andUnauthorized access can result
endpoints that are vulnerable to impactful attack on APIs. Authorization and in data disclosure to
broken object level authorization access control mechanisms in modern
unauthorized parties, data loss,
by manipulating the ID of an
applications are complex and wide-spread. or data manipulation.
object that is sent within the
Even if the application implements a proper Unauthorized access to objects
request. This may lead to
infrastructure for authorization checks,
can also lead to full account
unauthorized access to sensitive developers might forget to use these checks takeover.
data. This issue is extremely
before accessing a sensitive object. Access
common in API-based
control detection is not typically amenableapplications because the server to automated static or dynamic testing.
component usually does not fully
track the client’s state, andinstead, relies more on
parameters like object IDs, thatare sent from the client to decide
which objects to access.

Is The API Vulnerable?
Object level authorization is an access control mechanism that is usually implemented at the code level tovalidate that one user can only access objects that they should have access to.
Every API endpoint that receives an ID of an object, and performs any type of action on the object, should
implement object level authorization checks. The checks should validate that the logged-in user does haveaccess to perform the requested action on the requested object.
Failures in this mechanism typically leads to unauthorized information disclosure, modification, or destructionof all data.

Example Attack Scenarios
Scenario #1
An e-commerce platform for online stores (shops) provides a listing page with the revenue charts for their
hosted shops. Inspecting the browser requests, an attacker can identify the API endpoints used as a data source
for those charts and their pattern /shops/{shopName}/revenue_data.json. Using another API
endpoint, the attacker can get the list of all hosted shop names. With a simple script to manipulate the names in
the list, replacing {shopName} in the URL, the attacker gains access to the sales data of thousands of ecommerce stores.

Scenario #2
While monitoring the network traffic of a wearable device, the following HTTP PATCH request gets theattention of an attacker due to the presence of a custom HTTP request header X-User-Id: 54796.
Replacing the X-User-Id value with 54795, the attacker receives a successful HTTP response, and is able tomodify other users' account data.

API1:2019 Broken Object Level Authorization
How To Prevent• Implement a proper authorization mechanism that relies on the user policies and hierarchy.
• Use an authorization mechanism to check if the logged-in user has access to perform the requested
action on the record in every function that uses an input from the client to access a record in the
database.
• Prefer to use random and unpredictable values as GUIDs for records’ IDs.
• Write tests to evaluate the authorization mechanism. Do not deploy vulnerable changes that break thetests.

References
External• CWE-284: Improper Access Control• CWE-285: Improper Authorization• CWE-639: Authorization Bypass Through User-Controlled Key

API2:2019 Broken User Authentication
ThreatAgentsAttackVectors

Security
Weakness

Impacts

API Specific Exploitability: 3
Prevalence: 2
Detectability: 2
Technical: 3 Business Specific
Authentication in APIs is a
There are two sub-issues: 1. Lack ofAttackers can gain control tocomplex and confusing
protection mechanisms: APIs endpoints that other users’ accounts in themechanism. Software andare responsible for authentication must be system, read their personal
security engineers might havetreated differently from regular endpoints
data, and perform sensitive
misconceptions about what are and implement extra layers of protection 2. actions on their behalf, likethe boundaries of authentication Misimplementation of the mechanism: The money transactions andand how to implement it
mechanism is used / implemented without sending personal messages.
correctly. In addition, theconsidering the attack vectors, or it’s theauthentication mechanism is an wrong use case (e.g., an authenticationeasy target for attackers, since it’s mechanism designed for IoT clients might
exposed to everyone. These two not be the right choice for web
points makes the authentication applications).
component potentially vulnerable
to many exploits.

Is the API Vulnerable?
Authentication endpoints and flows are assets that need to be protected. “Forgot password / reset password”
should be treated the same way as authentication mechanisms.
An API is vulnerable if it:• Permits credential stuffing whereby the attacker has a list of valid usernames and passwords.
• Permits attackers to perform a brute force attack on the same user account, without presenting
captcha/account lockout mechanism.
• Permits weak passwords.
• Sends sensitive authentication details, such as auth tokens and passwords in the URL.
• Doesn’t validate the authenticity of tokens.
• Accepts unsigned/weakly signed JWT tokens ("alg":"none")/doesn’t validate their expiration date.
• Uses plain text, non-encrypted, or weakly hashed passwords.
• Uses weak encryption keys.

Example Attack Scenarios
Scenario #1
Credential stuffing (using lists of known usernames/passwords), is a common attack. If an application does not
implement automated threat or credential stuffing protections, the application can be used as a password oracle(tester) to determine if the credentials are valid.

Scenario #2
An attacker starts the password recovery workflow by issuing a POST request to/api/system/verification-codes and by providing the username in the request body. Next an SMS
token with 6 digits is sent to the victim’s phone. Because the API does not implement a rate limiting policy, theattacker can test all possible combinations using a multi-threaded script, against the/api/system/verification-codes/{smsToken} endpoint to discover the right token within a few
minutes.

API2:2019 Broken User Authentication
How To Prevent•

Make sure you know all the possible flows to authenticate to the API (mobile/ web/deep links that
implement one-click authentication/etc.)• Ask your engineers what flows you missed.
• Read about your authentication mechanisms. Make sure you understand what and how they are used.
OAuth is not authentication, and neither is API keys.
• Don't reinvent the wheel in authentication, token generation, password storage. Use the standards.
• Credential recovery/forget password endpoints should be treated as login endpoints in terms of brute
force, rate limiting, and lockout protections.
• Use the OWASP Authentication Cheatsheet.
• Where possible, implement multi-factor authentication.
• Implement anti brute force mechanisms to mitigate credential stuffing, dictionary attack, and brute force
attacks on your authentication endpoints. This mechanism should be stricter than the regular rate
limiting mechanism on your API.
• Implement account lockout / captcha mechanism to prevent brute force against specific users.
Implement weak-password checks.
• API keys should not be used for user authentication, but for client app/project authentication.

References
OWASP• OWASP Key Management Cheat Sheet• OWASP Authentication Cheatsheet• Credential Stuffing

External• CWE-798: Use of Hard-coded Credentials

API3:2019 Excessive Data Exposure
ThreatAgentsAttackVectors

Security
Weakness

Impacts

API Specific Exploitability: 3
Prevalence: 2
Detectability: 2
Technical: 2 Business Specific
Exploitation of Excessive Data APIs rely on clients to perform the data
Excessive Data Exposure
Exposure is simple, and is usually filtering. Since APIs are used as data
commonly leads to exposure of
performed by sniffing the traffic sources, sometimes developers try tosensitive data.
to analyze the API responses,
implement them in a generic way withoutlooking for sensitive data
thinking about the sensitivity of the
exposure that should not be
exposed data. Automatic tools usually can’t
returned to the user.
detect this type of vulnerability because it’s
hard to differentiate between legitimate
data returned from the API, and sensitive
data that should not be returned without adeep understanding of the application.

Is the API Vulnerable?
The API returns sensitive data to the client by design. This data is usually filtered on the client side before being
presented to the user. An attacker can easily sniff the traffic and see the sensitive data.

Example Attack Scenarios
Scenario #1
The mobile team uses the /api/articles/{articleId}/comments/{commentId} endpoint in thearticles view to render comments metadata. Sniffing the mobile application traffic, an attacker finds out that
other sensitive data related to comment’s author is also returned. The endpoint implementation uses a generic
toJSON() method on the User model, which contains PII, to serialize the object.

Scenario #2
An IOT-based surveillance system allows administrators to create users with different permissions. An admin
created a user account for a new security guard that should only have access to specific buildings on the site.
Once the security guard uses his mobile app, an API call is triggered to: /api/sites/111/cameras inorder to receive data about the available cameras and show them on the dashboard. The response contains a list
with details about cameras in the following format: {"id":"xxx","live_access_token":"xxxxbbbbb","building_id":"yyy"}. While the client GUI shows only cameras which the security guard
should have access to, the actual API response contains a full list of all the cameras in the site.

API3:2019 Excessive Data Exposure
How To Prevent•

Never rely on the client side to filter sensitive data.

•

Review the responses from the API to make sure they contain only legitimate data.

•

Backend engineers should always ask themselves "who is the consumer of the data?" before exposing a
new API endpoint.

•

Avoid using generic methods such as to_json() and to_string(). Instead, cherry-pick specificproperties you really want to return.

•Classify sensitive and personally identifiable information (PII) that your application stores and works
with, reviewing all API calls returning such information to see if these responses pose a security issue.

•

Implement a schema-based response validation mechanism as an extra layer of security. As part of thismechanism define and enforce data returned by all API methods, including errors.

References
External•

CWE-213: Intentional Information ExposureAPI4:2019 Lack of Resources & Rate LimitingThreatAgentsAttackVectors

Security
Weakness

API Specific Exploitability: 2
Prevalence: 3
Detectability: 3
Exploitation requires simple API It’s common to find APIs that do not
requests. No authentication is
implement rate limiting or APIs whererequired. Multiple concurrent
limits are not properly set.
requests can be performed from asingle local computer or by using
cloud computing resources.

Impacts

Technical: 2 Business Specific
Exploitation may lead to DoS,
making the API unresponsiveor even unavailable.

Is the API Vulnerable?
API requests consume resources such as network, CPU, memory, and storage. The amount of resources
required to satisfy a request greatly depends on the user input and endpoint business logic. Also, consider thefact that requests from multiple API clients compete for resources. An API is vulnerable if at least one of thefollowing limits is missing or set inappropriately (e.g., too low/high):
•
•
•
•
•
•
•

Execution timeouts
Max allocable memory
Number of file descriptors
Number of processes
Request payload size (e.g., uploads)
Number of requests per client/resource
Number of records per page to return in a single request response

Example Attack Scenarios
Scenario #1
An attacker uploads a large image by issuing a POST request to /api/v1/images. When the upload iscomplete, the API creates multiple thumbnails with different sizes. Due to the size of the uploaded image,available memory is exhausted during the creation of thumbnails and the API becomes unresponsive.

Scenario #2
We have an application that contains the users' list on a UI with a limit of 200 users per page. The users' list is
retrieved from the server using the following query: /api/users?page=1&size=200. An attacker
changes the size parameter to 200 000, causing performance issues on the database. Meanwhile, the API
becomes unresponsive and is unable to handle further requests from this or any other clients (aka DoS).
The same scenario might be used to provoke Integer Overflow or Buffer Overflow errors.

API4:2019 Lack of Resources & Rate Limiting
How To Prevent• Docker makes it easy to limit memory, CPU, number of restarts, file descriptors, and processes.
• Implement a limit on how often a client can call the API within a defined timeframe.
• Notify the client when the limit is exceeded by providing the limit number and the time at which thelimit will be reset.
• Add proper server-side validation for query string and request body parameters, specifically the one that
controls the number of records to be returned in the response.
• Define and enforce maximum size of data on all incoming parameters and payloads such as maximum
length for strings and maximum number of elements in arrays.

References
OWASP• Blocking Brute Force Attacks• Docker Cheat Sheet - Limit resources (memory, CPU, file descriptors, processes, restarts)• REST Assessment Cheat Sheet

External• CWE-307: Improper Restriction of Excessive Authentication Attempts• CWE-770: Allocation of Resources Without Limits or Throttling• “Rate Limiting (Throttling)” - Security Strategies for Microservices-based Application Systems, NISTAPI5:2019 Broken Function Level Authorization
ThreatAgentsAttackVectors

Security
Weakness

Impacts

API Specific Exploitability: 3
Prevalence: 2
Detectability: 1
Technical: 2 Business Specific
Exploitation requires the attacker Authorization checks for a function or
Such flaws allow attackers toto send legitimate API calls to the resource are usually managed via
access unauthorized
API endpoint that they should not configuration, and sometimes at the code
functionality. Administrative
have access to. These endpoints level. Implementing proper checks can be a functions are key targets formight be exposed to anonymous confusing task, since modern applications this type of attack.
users or regular, non-privileged can contain many types of roles or groups
users. It’s easier to discover these and complex user hierarchy (e.g., sub-users,
flaws in APIs since APIs areusers with more than one role).
more structured, and the way toaccess certain functions is morepredictable (e.g., replacing the
HTTP method from GET to PUT,or changing the “users” string in
the URL to "admins").

Is the API Vulnerable?
The best way to find broken function level authorization issues is to perform deep analysis of the authorization
mechanism, while keeping in mind the user hierarchy, different roles or groups in the application, and asking
the following questions:• Can a regular user access administrative endpoints?
• Can a user perform sensitive actions (e.g., creation, modification, or erasure) that they should not haveaccess to by simply changing the HTTP method (e.g., from GET to DELETE)?
• Can a user from group X access a function that should be exposed only to users from group Y, by simply
guessing the endpoint URL and parameters (e.g., /api/v1/users/export_all)?
Don’t assume that an API endpoint is regular or administrative only based on the URL path.
While developers might choose to expose most of the administrative endpoints under a specific relative path,like api/admins, it’s very common to find these administrative endpoints under other relative paths together
with regular endpoints, like api/users.

Example Attack Scenarios
Scenario #1
During the registration process to an application that allows only invited users to join, the mobile application
triggers an API call to GET /api/invites/{invite_guid}. The response contains a JSON with details
about the invite, including the user’s role and the user’s email.
An attacker duplicated the request and manipulated the HTTP method and endpoint to POST
/api/invites/new. This endpoint should only be accessed by administrators using the admin console,which does not implement function level authorization checks.
The attacker exploits the issue and sends himself an invite to create an admin account:
POST /api/invites/new{“email”:”hugo@malicious.com”,”role”:”admin”}

API5:2019 Broken Function Level Authorization
Scenario #2
An API contains an endpoint that should be exposed only to administrators - GET/api/admin/v1/users/all. This endpoint returns the details of all the users of the application and does
not implement function-level authorization checks. An attacker who learned the API structure takes an educatedguess and manages to access this endpoint, which exposes sensitive details of the users of the application.

How To Prevent
Your application should have a consistent and easy to analyze authorization module that is invoked from all
your business functions. Frequently, such protection is provided by one or more components external to theapplication code.
• The enforcement mechanism(s) should deny all access by default, requiring explicit grants to specificroles for access to every function.
• Review your API endpoints against function level authorization flaws, while keeping in mind thebusiness logic of the application and groups hierarchy.
• Make sure that all of your administrative controllers inherit from an administrative abstract controllerthat implements authorization checks based on the user’s group/role.
• Make sure that administrative functions inside a regular controller implements authorization checks
based on the user’s group and role.

ReferencesOWASP•

OWASP Article on Forced Browsing•

OWASP Top 10 2013-A7-Missing Function Level Access Control•

OWASP Development Guide: Chapter on Authorization

External•

CWE-285: Improper Authorization

API6:2019 Mass Assignment
ThreatAgentsAttackVectors

Security
Weakness

Impacts

API Specific Exploitability: 2
Prevalence: 2
Detectability: 2
Technical: 2 Business Specific
Exploitation usually requires an Modern frameworks encourage developers Exploitation may lead tounderstanding of the business
to use functions that automatically bind
privilege escalation, datalogic, objects' relations, and the input from the client into code variables and tampering, bypass of security
API structure. Exploitation ofinternal objects. Attackers can use thismechanisms, and more.
mass assignment is easier inmethodology to update or overwrite
APIs, since by design they
sensitive object’s properties that the
expose the underlying
developers never intended to expose.
implementation of the application
along with the properties’ names.

Is the API Vulnerable?
Objects in modern applications might contain many properties. Some of these properties should be updated
directly by the client (e.g., user.first_name or user.address) and some of them should not (e.g.,user.is_vip flag).
An API endpoint is vulnerable if it automatically converts client parameters into internal object properties,
without considering the sensitivity and the exposure level of these properties. This could allow an attacker toupdate object properties that they should not have access to.
Examples for sensitive properties:• Permission-related properties: user.is_admin, user.is_vip should only be set by admins.
• Process-dependent properties: user.cash should only be set internally after payment verification.
• Internal properties: article.created_time should only be set internally by the application.

Example Attack Scenarios
Scenario #1
A ride sharing application provides a user the option to edit basic information for their profile. During thisprocess, an API call is sent to PUT /api/v1/users/me with the following legitimate JSON object:{"user_name":"inons","age":24}

The request GET /api/v1/users/me includes an additional credit_balance property:{"user_name":"inons","age":24,"credit_balance":10}

The attacker replays the first request with the following payload:{"user_name":"attacker","age":60,"credit_balance":99999}

Since the endpoint is vulnerable to mass assignment, the attacker receives credits without paying.

API6:2019 Mass Assignment
Scenario #2
A video sharing portal allows users to upload content and download content in different formats. An attacker
who explores the API found that the endpoint GET /api/v1/videos/{video_id}/meta_data returns
a JSON object with the video’s properties. One of the properties is "mp4_conversion_params":"-v
codec h264", which indicates that the application uses a shell command to convert the video.
The attacker also found the endpoint POST /api/v1/videos/new is vulnerable to mass assignment andallows the client to set any property of the video object. The attacker sets a malicious value as follows:"mp4_conversion_params":"-v codec h264 && format C:/". This value will cause a shellcommand injection once the attacker downloads the video as MP4.

How To Prevent•

If possible, avoid using functions that automatically bind a client’s input into code variables or internal
objects.

•

Whitelist only the properties that should be updated by the client.

•

Use built-in features to blacklist properties that should not be accessed by clients.

•

If applicable, explicitly define and enforce schemas for the input data payloads.

References
External•

CWE-915: Improperly Controlled Modification of Dynamically-Determined Object Attributes

API7:2019 Security Misconfiguration
ThreatAgentsAttackVectors

Security
Weakness

Impacts

API Specific Exploitability: 3
Prevalence: 3
Detectability: 3
Technical: 2 Business SpecificAttackers will often attempt toSecurity misconfiguration can happen at
Security misconfigurations canfind unpatched flaws, common
any level of the API stack, from the network not only expose sensitive user
endpoints, or unprotected files
level to the application level. Automateddata, but also system details
and directories to gain
tools are available to detect and exploitthat may lead to full server
unauthorized access or
misconfigurations such as unnecessary
compromise.
knowledge of the system.
services or legacy options.

Is the API Vulnerable?
The API might be vulnerable if:• Appropriate security hardening is missing across any part of the application stack, or if it has improperly
configured permissions on cloud services.
• The latest security patches are missing, or the systems are out of date.
• Unnecessary features are enabled (e.g., HTTP verbs).
• Transport Layer Security (TLS) is missing.
• Security directives are not sent to clients (e.g., Security Headers).
• A Cross-Origin Resource Sharing (CORS) policy is missing or improperly set.
• Error messages include stack traces, or other sensitive information is exposed.

Example Attack Scenarios
Scenario #1
An attacker finds the .bash_history file under the root directory of the server, which contains commands
used by the DevOps team to access the API:$ curl -X GET 'https://api.server/endpoint/' -H 'authorization: Basic Zm9vOmJhcg=='

An attacker could also find new endpoints on the API that are used only by the DevOps team and are notdocumented.

Scenario #2
To target a specific service, an attacker uses a popular search engine to search for computers directly accessible
from the Internet. The attacker found a host running a popular database management system, listening on thedefault port. The host was using the default configuration, which has authentication disabled by default, and theattacker gained access to millions of records with PII, personal preferences, and authentication data.

Scenario #3
Inspecting traffic of a mobile application an attacker finds out that not all HTTP traffic is performed on a secure
protocol (e.g., TLS). The attacker finds this to be true, specifically for the download of profile images. As userinteraction is binary, despite the fact that API traffic is performed on a secure protocol, the attacker finds apattern on API responses size, which he uses to track user preferences over the rendered content (e.g., profile
images).

API7:2019 Security Misconfiguration
How To Prevent
The API life cycle should include:• A repeatable hardening process leading to fast and easy deployment of a properly locked down
environment.
• A task to review and update configurations across the entire API stack. The review should include:
orchestration files, API components, and cloud services (e.g., S3 bucket permissions).
• A secure communication channel for all API interactions access to static assets (e.g., images).
• An automated process to continuously assess the effectiveness of the configuration and settings in allenvironments.
Furthermore:• To prevent exception traces and other valuable information from being sent back to attackers, if
applicable, define and enforce all API response payload schemas including error responses.
• Ensure API can only be accessed by the specified HTTP verbs. All other HTTP verbs should be disabled(e.g. HEAD).
• APIs expecting to be accessed from browser-based clients (e.g., WebApp front-end) should implement a
proper Cross-Origin Resource Sharing (CORS) policy.

ReferencesOWASP•

OWASP Secure Headers Project•

OWASP Testing Guide: Configuration Management•

OWASP Testing Guide: Testing for Error Codes•

OWASP Testing Guide: Test Cross Origin Resource Sharing

External•

CWE-2: Environmental Security Flaws•

CWE-16: Configuration•

CWE-388: Error Handling•

Guide to General Server Security, NIST•

Let’s Encrypt: a free, automated, and open Certificate Authority

API8:2019 Injection
ThreatAgentsAttackVectors

Security
Weakness

API Specific Exploitability: 3
Prevalence: 2
Detectability: 3
Attackers will feed the API with Injection flaws are very common and are
malicious data through whatever often found in SQL, LDAP, or NoSQL
injection vectors are available
queries, OS commands, XML parsers, and(e.g., direct input, parameters,
ORM. These flaws are easy to discoverintegrated services, etc.),
when reviewing the source code. Attackers
expecting it to be sent to an
can use scanners and fuzzers.
interpreter.

Impacts

Technical: 3 Business SpecificInjection can lead to
information disclosure and data
loss. It may also lead to DoS,
or complete host takeover.

Is the API Vulnerable?
The API is vulnerable to injection flaws if:• Client-supplied data is not validated, filtered, or sanitized by the API.
• Client-supplied data is directly used or concatenated to SQL/NoSQL/LDAP queries, OS commands,
XML parsers, and Object Relational Mapping (ORM)/Object Document Mapper (ODM).
• Data coming from external systems (e.g., integrated systems) is not validated, filtered, or sanitized by
the API.

Example Attack Scenarios
Scenario #1
Firmware of a parental control device provides the endpoint /api/CONFIG/restore which expects an
appId to be sent as a multipart parameter. Using a decompiler, an attacker finds out that the appId is passed
directly into a system call without any sanitization:
snprintf(cmd, 128, "%srestore_backup.sh /tmp/postfile.bin %s %d","/mnt/shares/usr/bin/scripts/", appid, 66);
system(cmd);

The following command allows the attacker to shut down any device with the same vulnerable firmware:$ curl -k "https://${deviceIP}:4567/api/CONFIG/restore" -F'appid=$(/etc/pod/power_down.sh)'Scenario #2
We have an application with basic CRUD functionality for operations with bookings. An attacker managed to
identify that NoSQL injection might be possible through bookingId query string parameter in the delete
booking request. This is how the request looks like: DELETE /api/bookings?bookingId=678.
The API server uses the following function to handle delete requests:
router.delete('/bookings', async function (req, res, next) {try {const deletedBooking = await Bookings.findOneAndRemove({_id' : req.query.bookingId});
res.status(200);} catch (err) {res.status(400).json({error: 'Unexpected error occured while processing a request'
});
}
});

API8:2019 Injection
The attacker intercepted the request and changed bookingId query string parameter as shown below. In thiscase, the attacker managed to delete another user's booking:
DELETE /api/bookings?bookingId[$ne]=678

How To Prevent
Preventing injection requires keeping data separate from commands and queries.
•
•
•
•
•
•
•

Perform data validation using a single, trustworthy, and actively maintained library.
Validate, filter, and sanitize all client-provided data, or other data coming from integrated systems.
Special characters should be escaped using the specific syntax for the target interpreter.
Prefer a safe API that provides a parameterized interface.
Always limit the number of returned records to prevent mass disclosure in case of injection.
Validate incoming data using sufficient filters to only allow valid values for each input parameter.
Define data types and strict patterns for all string parameters.

ReferencesOWASP•

OWASP Injection Flaws•

SQL Injection•

NoSQL Injection Fun with Objects and Arrays•

Command Injection

External•

CWE-77: Command Injection•

CWE-89: SQL Injection

API9:2019 Improper Assets Management
ThreatAgentsAttackVectors

Security
Weakness

Impacts

API Specific Exploitability: 3
Prevalence: 3
Detectability: 2
Technical: 2 Business Specific
Old API versions are usually
Outdated documentation makes it more
Attackers may gain access to
unpatched and are an easy way to difficult to find and/or fix vulnerabilities.
sensitive data, or even takeovercompromise systems without
Lack of assets inventory and retire
the server through old,having to fight state-of-the-art
strategies leads to running unpatched
unpatched API versions
security mechanisms, which
systems, resulting in leakage of sensitive
connected to the samemight be in place to protect the data. It’s common to find unnecessarily
database.
most recent API versions.
exposed API hosts because of modern
concepts like microservices, which makeapplications easy to deploy and independent(e.g., cloud computing, k8s).

Is the API Vulnerable?
The API might be vulnerable if:• The purpose of an API host is unclear, and there are no explicit answers to the following questions:• Which environment is the API running in (e.g., production, staging, test, development)?
• Who should have network access to the API (e.g., public, internal, partners)?
• Which API version is running?
• What data is gathered and processed by the API (e.g., PII)?
• What's the data flow?
• There is no documentation, or the existing documentation is not updated.
• There is no retirement plan for each API version.
• Hosts inventory is missing or outdated.
• Integrated services inventory, either first- or third-party, is missing or outdated.
• Old or previous API versions are running unpatched.

Example Attack Scenarios
Scenario #1
After redesigning their applications, a local search service left an old API version(api.someservice.com/v1) running, unprotected, and with access to the user database. While targeting
one of the latest released applications, an attacker found the API address (api.someservice.com/v2).
Replacing v2 with v1 in the URL gave the attacker access to the old, unprotected API, exposing the personal
identifiable information (PII) of over 100 Million users.

Scenario #2
A social network implemented a rate-limiting mechanism that blocks attackers from using brute-force to guess
reset password tokens. This mechanism wasn’t implemented as part of the API code itself, but in a separate
component between the client and the official API (www.socialnetwork.com). A researcher found a beta
API host (www.mbasic.beta.socialnetwork.com) that runs the same API, including the reset
password mechanism, but the rate limiting mechanism was not in place. The researcher was able to reset the
password of any user by using a simple brute-force to guess the 6 digits token.

API9:2019 Improper Assets Management
How To Prevent• Inventory all API hosts and document important aspects of each one of them, focusing on the API
environment (e.g., production, staging, test, development), who should have network access to the host(e.g., public, internal, partners) and the API version.
• Inventory integrated services and document important aspects such as their role in the system, what data
is exchanged (data flow), and its sensitivity.
• Document all aspects of your API such as authentication, errors, redirects, rate limiting, cross-origin
resource sharing (CORS) policy and endpoints, including their parameters, requests, and responses.
• Generate documentation automatically by adopting open standards. Include the documentation build inyour CI/CD pipeline.
• Make API documentation available to those authorized to use the API.
• Use external protection measures such as API security firewalls for all exposed versions of your APIs,
not just for the current production version.
• Avoid using production data with non-production API deployments. If this is unavoidable, these
endpoints should get the same security treatment as the production ones.
• When newer versions of APIs include security improvements, perform risk analysis to make thedecision of the mitigation actions required for the older version: for example, whether it is possible tobackport the improvements without breaking API compatibility or you need to take the older version out
quickly and force all clients to move to the latest version.

References
External•

CWE-1059: Incomplete Documentation•

OpenAPI InitiativeAPI10:2019 Insufficient Logging & MonitoringThreatAgentsAttackVectors

Security
Weakness

API Specific Exploitability: 2
Prevalence: 3
Detectability: 1
Attackers take advantage of lack Without logging and monitoring, or withof logging and monitoring to
insufficient logging and monitoring, it isabuse systems without being
almost impossible to track suspicious
noticed.
activities and respond to them in a timely
fashion.

Impacts

Technical: 2 Business SpecificWithout visibility over ongoing malicious activities,
attackers have plenty of time tofully compromise systems.

Is the API Vulnerable?
The API is vulnerable if:• It does not produce any logs, the logging level is not set correctly, or log messages do not include
enough detail.
• Log integrity is not guaranteed (e.g., Log Injection).
• Logs are not continuously monitored.
• API infrastructure is not continuously monitored.

Example Attack Scenarios
Scenario #1
Access keys of an administrative API were leaked on a public repository. The repository owner was notified byemail about the potential leak, but took more than 48 hours to act upon the incident, and access keys exposure
may have allowed access to sensitive data. Due to insufficient logging, the company is not able to assess what
data was accessed by malicious actors.

Scenario #2
A video-sharing platform was hit by a “large-scale” credential stuffing attack. Despite failed logins being
logged, no alerts were triggered during the timespan of the attack. As a reaction to user complaints, API logs
were analyzed and the attack was detected. The company had to make a public announcement asking users toreset their passwords, and report the incident to regulatory authorities.

API10:2019 Insufficient Logging & Monitoring
How To Prevent• Log all failed authentication attempts, denied access, and input validation errors.
• Logs should be written using a format suited to be consumed by a log management solution, and should
include enough detail to identify the malicious actor.
• Logs should be handled as sensitive data, and their integrity should be guaranteed at rest and transit.
• Configure a monitoring system to continuously monitor the infrastructure, network, and the APIfunctioning.
• Use a Security Information and Event Management (SIEM) system to aggregate and manage logs fromall components of the API stack and hosts.
• Configure custom dashboards and alerts, enabling suspicious activities to be detected and responded toearlier.

ReferencesOWASP•

OWASP Logging Cheat Sheet•OWASP Proactive Controls: Implement Logging and Intrusion Detection•

OWASP Application Security Verification Standard: V7: Error Handling and Logging Verification
Requirements

External•

CWE-223: Omission of Security-relevant Information•

CWE-778: Insufficient Logging

+D

What’s Next for Developers

The task to create and maintain secure software, or fixing existing sofware, can be difficult. APIs are nodifferent.
We believe that education and awareness are key factors to write secure software. Everything else required toaccomplish the goal, depends on establishing and using repeatable security processes and standard
security controls.
OWASP has numerous free and open resources to address security since the very beginning of the project.
Please visit the OWASP Projects page for a comprehensive list of available projects.
You can start reading OWASP Education Project materials according to yourprofession and interest. For hands-on learning, we added crAPI - Completely
Ridiculous API on our roadmap. Meanwhile, you can practice WebAppSec using
Education
the OWASP DevSlop Pixi Module, a vulnerable WebApp and API service intent
to teach users how to test modern web applications and API's for security issues,and how to write more secure API's in the future. You can also attend OWASPAppSec Conference training sessions, or join your local chapter.
Security should be part of every project from the beginning. When doing
requirements elicitation, it is important to define what "secure" means for thatproject. OWASP recommends you use the OWASP Application Security
Security Requirements
Verification Standard (ASVS) as a guide for setting the security requirements. If
you're outsourcing, consider the OWASP Secure Software Contract Annex, whichshould be adapted according to local law and regulations.
Security should remain a concern during all the project stages. The OWASPPrevention Cheat Sheets are a good starting point for guidance on how to design
Security Architecture
security in during the architecture phase. Among many others, you'll find the
REST Security Cheat Sheet and the REST Assessment Cheat Sheet.
Adopting Standard Security Controls reduces the risk of introducing security
weaknesses while writing your own logic. Despite the fact that many modern
Standard Security
frameworks now come with built-in standard effective controls, OWASPControls
Proactive Controls gives you a good overview of what security controls you
should look to include in your project. OWASP also provides some libraries andtools you may find valuable, such as validation controls.
You can use the OWASP Software Assurance Maturity Model (SAMM) to
Secure Software
improve the process when building APIs. Several other OWASP projects are
Development Life Cycle available to help you during the different API development phases e.g., the
OWASP Code Review Project.

+DSO

What’s Next for DevSecOps

Due to their importance in modern application architectures, building secure APIs is crucial. Security cannot be
neglected, and it should be part of the whole development life cycle. Scanning and penetration testing yearly
are no longer enough.
DevSecOps should join the development effort, facilitating continuous security testing across the entire
software development life cycle. Their goal is to enhance the development pipeline with security automation,
and without impacting the speed of development.
In case of doubt, stay informed, and review, the DevSecOps Manifesto often.

Understand the Threat
Model

Testing priorities come from a threat model. If you don't have one, consider
using OWASP Application Security Verification Standard (ASVS), and the
OWASP Testing Guide as an input. Involving the development team may help tomake them more security-aware.

Understand the SDLC

Join the development team to better understand the Software Development Life
Cycle. Your contribution on continuous security testing should be compatible
with people, processes, and tools. Everyone should agree with the process, sothat there's no unnecessary friction or resistance.

Testing Strategies

As your work should not impact the development speed, you should wisely
choose the best (simple, fastest, most accurate) technique to verify the security
requirements. The OWASP Security Knowledge Framework and OWASPApplication Security Verification Standard can be great sources of functional andnonfunctional security requirements. There are other great sources for projects
and tools similar to the one offered by the DevSecOps community.

You're the bridge between developers and operations teams. To achieve
coverage, not only should you focus on the functionality, but also the
Achieving Coverage andorchestration. Work close to both development and operations teams from the
Accuracy
beginning so you can optimize your time and effort. You should aim for a state
where the essential security is verified continuously.
Clearly Communicate
Findings

Contribute value with less or no friction. Deliver findings in a timely fashion,
within the tools development teams are using (not PDF files). Join thedevelopment team to address the findings. Take the opportunity to educate them,clearly describing the weakness and how it can be abused, including an attack
scenario to make it real.

+DAT

Methodology and Data

Overview
Since the AppSec industry has not been specifically focused on the most recent architecture of applications, inwhich APIs play an important role, compiling a list of the ten most critical API security risks, based on a public
call for data, would have been a hard task. Despite there being no public data call, the resulting Top 10 list is
still based on publicly available data, security experts' contributions, and open discussion with the security
community.

Methodology and Data
In the first phase, publicly available data about APIs security incidents were collected, reviewed, andcategorized by a group of security experts. Such data was collected from bug bounty platforms andvulnerability databases, within a one-year-old time frame. It was used for statistical purposes.
In the next phase, security practitioners with penetration testing experience were asked to compile their own
Top 10 list.
The OWASP Risk Rating Methodology was used to perform he Risk Analysis. The scores were discussed andreviewed between the security practitioners. For considerations on these matters, please refer to the API
Security Risks section.
The first draft of the OWASP API Security Top 10 2019 resulted from a consensus between statistical resultsfrom phase one, and the security practitioners' lists. This draft was then submitted for appreciation and review
by another group of security practitioners, with relevant experience in the API security fields.
The OWASP API Security Top 10 2019 was first presented in the OWASP Global AppSec Tel Aviv event (May2019). Since then, it has been available on GitHub for public discussion and contributions.
The list of contributors is available in the Acknowledgments section.

+ACK

Acknowledgments

Acknowledgments to Contributors
We’d like to thank the following contributors who contributed publicly on GitHub or via other means:
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Abid Khan
Adam Fisher
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Chris Westphal
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Guillaume Benats
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Keith Casey
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