The Ultimate API Penetration Testing Checklist [ Free Excel File]

7 min readApr 10, 2024


Click here to get a Free API Pen-Testing Checklist

When was the last time your organization conducted an API security assessment? And did you have the framework and resources to do so?

Now more than ever, companies need to know where their APIs are vulnerable to malicious actors. Check out the API Penetration Testing checklist, which outlines how to conduct an effective API security assessment for your organization.

What is API Pen Testing?

API penetration testing enables organizations to evaluate the effectiveness of API security by simulating an attack in secure conditions.

By simulating an attack against an application’s API, a certified penetration tester can identify weaknesses in the design and implementation of the API, such as insufficient authentication and authorization controls, input validation vulnerabilities, and other security weaknesses. This allows the organization to address these issues before attackers can exploit them.

Why is API Penetration Testing Essential?

APIs enable communication and data exchange between software components, apps, etc., offering programmatic access to business logic and digital assets. As such, they are a frequent target for attackers looking to exploit vulnerabilities for financial gain, data theft, or to gain access to other systems. Through pen testing, organizations can find and fix unknown vulnerabilities and flaws that automated tools miss.

Furthermore, API pen testing is important because APIs often expose critical business functionality, such as payment processing or customer data management. A successful attack against an API can result in losing sensitive data or disrupting essential business processes, leading to financial loss and reputational damage.

Overall, API pen testing is a critical component of a comprehensive security program and is necessary to ensure that APIs are secure and can resist attacks from malicious actors.

The Complete API Penetration Testing Checklist

1. Planning & Goal Setting

The most important item in any API penetration testing checklist is planning and goal setting, as they help set the direction for the testing. To achieve this,

  • Specify the pen test’s goals, objectives, and scope
  • Identify the API to be tested
  • Identify a trustworthy pen testing expert to perform the testing
  • Discuss and finalize the scope, timelines, and budgetary details

2. Scoping

Define the scope of the API pen test, including the API endpoints and associated systems. This information includes but is not limited to the following:

  • IP addresses and URLs
  • Postman, swagger and other documentation files for all API endpoints and related details
  • User roles
  • Authentication credentials or API tokens (at least two sets of credentials for each role being tested)
  • List of sensitive or restricted endpoints that should be left out of the scope of API pen testing
  • Examples of API calls and valid responses
  • All available API documentation

While the tester is responsible for gathering information and scoping, organizations can reduce time and resources by preparing all necessary information and documentation. This way, the tester can simply review the available information and request additional information, getting straight into testing.

3. Reconnaissance

Every good API pen testing checklist will tell you how vital the reconnaissance stage of testing is. This is when the pen tester will gather open-source intelligence by reviewing publicly available information and resources.

The idea is to find any sensitive information, like usernames, passwords, user manuals, forum posts, etc., that may be useful in the different stages of testing. They should also look for sensitive information that should not be publicly available such as employee information, internal communications, etc.

They should review available API documentation to understand API functionality, uses, parameters required, etc. It is equally important to determine the attack surface.

The tester must list all API inputs and outputs, API calls, cookies, headers, web responses, file uploads, etc. They can leverage automated tools to help them with reconnaissance and determine the attack surface.

4. Threat Modelling

The threat modeling phase is crucial in assessing potential threats to the target APIs within the scope. This involves evaluating the types of attacks and their likelihood of occurring, which helps assign risk priorities to vulnerabilities identified during the assessment.

The testing perspective (external/internal, authenticated/unauthenticated, black box/crystal box, etc.) is also considered to ensure the accuracy of the identified vulnerabilities. During this phase, the exposed endpoints are manually reviewed to identify the business functionality and the attack surface of unauthenticated/authenticated endpoints.

5. Detect and Analyze API Vulnerabilities

Once the pen-tester has identified and enumerated all targets and resources present in the API, they must scan for potential vulnerabilities at both the network and application layers.

The tester must identify and other known vulnerabilities in its different layers using automated tools. However, it is important to note that manual identification and confirmation of vulnerabilities for each tested endpoint should also be conducted.

Finally, vulnerability identification based on identified software versions should be attempted to ensure that known vulnerabilities are properly addressed.

6. Exploitation

The exploitation phase of API penetration testing involves actively attempting to exploit any vulnerabilities or weaknesses identified during the reconnaissance and scanning phases.

During this phase, the penetration tester will use various tools and techniques to gain unauthorized access to the API or its underlying systems and then attempt to escalate their privileges or perform other malicious actions.

7. Re-test

It is crucial to emphasize the importance of testing the implemented fixes after generating a penetration testing report. Implementing the recommendations is not enough to guarantee that the vulnerabilities have been fully resolved.

To accurately assess the effectiveness of the fixes, it is ideal to have the same API penetration testing partner who conducted the initial testing to perform the re-testing. This is because they thoroughly understand the application and can ensure that no new security issues have arisen during the resolution process.

It is essential to note that penetration testing is not a one-time event, and it is imperative to conduct periodic security testing. As cyber threats and attacks evolve, businesses must budget for security testing, especially when introducing new or enhanced applications.

8. Report The Findings

This is perhaps the most important item on any pen test API checklist. The pen tester must prepare detailed documentation of their findings and provide recommendations for remediation of the vulnerabilities discovered during the testing.

These reports and documents help in future pen-testing and are necessary to meet regulatory compliances.

Top Penetration Testing Use Cases to Test for

Here is a non-exhaustive list of things pen testers must test for in the actual exploitation phase of pen testing.

1. Authentication and Authorization

  • Verify the strength of the authentication mechanisms used by the API, such as passwords, tokens, or API keys.
  • Check if the API uses secure transport protocols such as HTTPS.
  • Verify if the access controls are implemented correctly and if unauthorized access is restricted.
  • Ensure the API implements a proper password policy (i.e., password complexity, reset mechanism, etc.).
  • Verify if the API uses rate-limiting to prevent brute-force attacks.

2. Input Validation

  • Check if all input parameters are validated on the server and client sides.
  • Ensure the API is not vulnerable to common Injection attacks like SQL injection, NoSQL injection, or LDAP injection.
  • Verify if the API accepts only the expected data formats (i.e., JSON, XML) and rejects unexpected formats.

3. Error Handling and Logging

  • Verify that the API provides meaningful error messages to the client.
  • Ensure the API does not leak sensitive information such as system details, stack traces, or error codes.
  • Verify if the API logs all the incoming requests, errors, and responses.

4. Data Security

  • Check if sensitive data is encrypted both in transit and at rest.
  • Verify if the API follows the principle of least privilege and ensures that users can access only the necessary data.
  • Verify if the API implements proper data validation and sanitization to prevent data tampering or exposure.

5. Session Management

  • Verify if the API uses secure session management mechanisms such as tokenization or session IDs.
  • Check if the session tokens are invalidated after a certain period of inactivity or logout.

6. Business Logic Flaws

  • Verify if the API is vulnerable to common business logic flaws such as unauthorized access, privilege escalation, or transaction manipulation.
  • Check if the API performs proper access controls and validations for all user actions.

7. Information Leakage

This involves a thorough examination of the configurations and communication mechanisms of the application to detect any sensitive data that may be disclosed and pose a security threat.

They will check the following aspects of the API as these are the common areas where excessive data exposure vulnerabilities arise:

  • Error pages — investigate if any sensitive data is revealed in error messages.
  • URL strings — check if any confidential information is exposed in the URL.
  • API responses — examine the response payload for any sensitive information.
  • Data in transit — ensure that data transmitted between the client and server is encrypted.
  • Data at rest — verify that any data stored in the database is properly encrypted and secured.
  • The client — pay special attention to how the client filters data to avoid exposing confidential information.

8. Third-Party Integration

  • Check if the API is integrated with any third-party services or libraries and if they are up-to-date and secure.
  • Verify if the API performs proper input validation and sanitization for third-party data.

The Way Forward

To protect your business from cyber threats effectively, utilizing both automatic API scanner and manual API penetration testing is critical, as it provides comprehensive security coverage, identifies potential vulnerabilities, and helps you take corrective action

You can prevent unauthorized access, ultimately boosting customer trust and brand reputation. API pen testing can be daunting, but you can improve the quality of your pen test or recruit a better API pen testing service with this API penetration testing checklist.




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