API Penetration Testing: Uncovering Security Vulnerabilities in APIs

The rapid expansion of APIs has brought about numerous security challenges. API penetration testing, a specialized process, is essential for identifying vulnerabilities that are unique to APIs and may not be covered by regular application pentests.

Understanding API Penetration Testing

API penetration testing is a meticulous examination aimed at pinpointing security vulnerabilities in an API. The process involves determining if an API is susceptible to various vulnerabilities, which may include:

  • Disclosure of sensitive information.
  • API Mass Assignment.
  • Bypassing access controls.
  • Broken authentication.
  • SQL Injection and other input validation flaws.

API pentests encompass a wide range of protocols and schemes, including REST, SOAP, and RPC.

How API Pentesting Differs from Application Pentesting

API penetration testing closely aligns with application penetration testing, with some key distinctions specific to API frameworks and design patterns. While an API pentest is often implicitly conducted within an application pentest, there are times when an independent API requires a tailored API penetration test. This determination is typically made during the scoping process.

Scoping an API Pentest

API pentest scoping is crucial, much like application pentesting. Proper preparation involves:

  • Deciding if a client is required to generate and send requests to the API.
  • Understanding the available API documentation and what can be provided.
  • Grasping the roles, authentication methods, and API design.
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When engaging a pentest company, it’s important to make key decisions about how the pentest will be performed and delivered.

Common API Vulnerabilities

API penetration tests often unearth unique vulnerabilities specific to APIs.

Information Disclosure

APIs often reveal more information than necessary in their responses, potentially jeopardizing security. An API pentest should address questions such as:

  • Should password hashes be disclosed to users?
  • Should users be able to see other users’ locations?

Developers frequently stumble into the pitfall of returning an entire object’s state instead of providing users with the minimum required information.

API Mass Assignment

API Mass Assignment refers to a condition where a client can overwrite server-side variables that should not be allowed. This vulnerability poses a high risk as it grants users the ability to escalate privileges and manipulate business logic.

API Authentication Vulnerabilities

API authentication schemes have distinct security requirements. A comprehensive API pentest should thoroughly review how access tokens are generated and revoked, as well as delve into specific weaknesses associated with those tokens.

API Pentesting and XSS

Cross-site Scripting (XSS) is a common vulnerability in pentest reports, and it remains relevant when dealing with APIs. However, determining if an API is vulnerable to XSS is not always straightforward and depends on the Content-Type header.

API Pentesting and CSRF

Cross-Site Request Forgery (CSRF) may also be identified during an API penetration test. To validate the finding, various behaviors must be reviewed, including enforcing a specific Content-Type request header and handling malformed JSON.

CORS Policies

Cross-Origin Resource Sharing (CORS) misconfigurations are a prevalent source of vulnerabilities. While CORS relaxes the same-origin policy enforced by browsers, careful attention must be given to ensure that an overly permissive CORS policy does not compromise API security.

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API Pentesting Tools

The core tools employed in API Pentests resemble those used in general application testing. Frameworks like Burp Suite are commonly utilized for tampering with parameters and scanning requests. Enhancing the API testing experience, integrating Postman or Swagger UI with these frameworks offers added value. Postman, for instance, can be configured to use Burp via system proxy settings.

Binding Data Securely

In many web-based applications utilizing APIs, the binding of data becomes crucial. How an API response is included in the DOM shown to users can impact security. While this relationship between web-based clients and APIs is rarely viewed as an API vulnerability, it should be carefully analyzed during the pentest.

API Rate Limiting

APIs, particularly those intended for public use, are frequent targets of abuse and automated attacks. Rate limiting has become a vital defense mechanism for large API providers to deter bots and other malicious activities. During an API pentest, vulnerable endpoints that may be susceptible to automated attacks are identified, and appropriate rate limiting measures are recommended.


API penetration testing is a complex and subjective process that depends on the design of each API. The topics covered here provide valuable guidance for performing or contracting a thorough API pentest. If you’re seeking assistance in assessing an API, explore our pentest services and reach out to ProgramMatek for expert support.