API Pentesting

While APIs play a vital role in enabling communication and integration between systems, applications, and services essential for an application.
The sensitive act of accessing services from remote endpoints or from other Third-Party services or even transmitting data to remote servers for performing crucial action like user authentication, among others.
A simple glitch in their working can lead to the compromise of sensitive credentials, be it,calling sensitive service without authorization , leakage of user credential while in transit , etc.
Thus the need for API penetration testing to thoroughly assess the robustness of, any APIs used, to potential attacks.
API Penetration testing is the process of assessing the security posture of an applicationʼs APIs.
By scrutinizing the API endpoints, authentication mechanisms, input validation, and more to ensure theyʼre robust enough to withstand potential attacks.

What are some of the common vulnerability that you should include to your testing ?
1. Improper Authentication
     APIs often expose authentication flaws, allowing unauthorized users to access sensitive data or perform actions without proper validation.
     Example: An API that accepts weak credentials or does not enforce multi- factor authentication (MFA).

2. Improper Authorization
     Occurs when APIs fail to enforce proper role-based access control (RBAC) or attribute-based access control (ABAC).
     Example: A standard user can access administrative functions by modifying request parameters or tokens.

3. Improper Object-Level Authorization (IDOR - Insecure Direct Object References)
     Attackers manipulate object identifiers (e.g., User IDs, Order IDs) to gain unauthorized access to data.
     Example: Changing user_id=1234 to user_id=5678 in the request URL to access another user's account information.

4. Embedded API Keys in Code or Responses
     Hardcoded API keys within mobile apps, frontend applications, or configuration files can be extracted and exploited.
     Example: API keys found in JavaScript files can be used to gain full API access.

5. Data Validation and Input Handling Issues
     Lack of validation allows attackers to inject malicious input, leading to SQL Injection, Cross-Site Scripting (XSS), or Server-Side Request Forgery (SSRF).
     Example: Entering '; DROP TABLE users;-- in an input field can erase database records if SQL Injection is not mitigated.

6. Data Integrity and Encryption Weaknesses
     Unencrypted data transmission (e.g., using HTTP instead of HTTPS) allows data interception and tampering (Man-in-the-Middle attacks).
     Example: An API transmitting credentials over an insecure connection can expose sensitive data.
 
7. Poor Session Management
     Weak session controls enable session hijacking, replay attacks, and unauthorized access.
     Example: API tokens that never expire or lack rotation mechanisms make it easy for attackers to maintain unauthorized access.

8. Rate Limiting and Resource Exhaustion Issues
     APIs without rate limits allow attackers to perform brute-force attacks or Distributed Denial-of-Service (DDoS) attacks.
     Example: An attacker repeatedly attempts password logins without triggering an account lockout.

9. Excessive Data Exposure
     APIs return more data than necessary, exposing sensitive information (e.g., returning full customer records when only an email is required).
     Example: A payment API leaking full credit card details instead of masked numbers.

10. Security Misconfigurations
     Poorly configured security settings, such as improper CORS policies, debug endpoints, or default credentials, expose APIs to attacks.
     Example: Allowing    Access-Control-Allow-Origin: * enables any domain to access API data

API Pentesting Methodology
1. Scoping & Planning
Before testing, clearly define the scope and methodology. Keep in mind the following questions while pentesting endpoints like the following :-
     How many API endpoints are used ?
     What is their functionalities, and authentication mechanisms ?.
     Determine testing scope (Public APIs, Internal APIs, Partner APIs, etc.).

•    Public APIs: Openly available APIs that anyone can access, often with authentication and rate limits (e.g., Google Maps API, OpenWeatherAPI).
•    Internal APIs: Private APIs used within an organization to connect internal systems and services, not exposed to external users (e.g., APIs for internal HR or finance systems).
•    Partner APIs: Restricted APIs shared with specific business partners to enable integration and collaboration (e.g., payment gateway APIs like PayPal Partner API).
•      API Documentation (Swagger, OpenAPI, Postman collections) .

Studying about the functionality of the API and itʼs parameters itʼs configurations and it overall working can help identify its misconfiguration.
2. Reconnaissance & Information Gathering
Collect relevant API information to facilitate testing.

•      Review API documentation to understand its structure and functionality.
•      Intercept API traffic using tools such as Burp Suite, Postman, OWASP ZAP, or Fiddler.
•      Identify authentication methods (OAuth, JWT, API Keys).  
•      Enumerate hidden API endpoints using fuzzing tools.
•      Some common tools you should use , Burp Suite, Postman, OWASP ZAP, dirsearch, ffuf .

3. Vulnerability Assessment
Identify security weaknesses in the API through automated scanning and manual testing. Test for common vulnerablities like : -

•      Broken Authentication – Weak authentication mechanisms allow unauthorized access.
•      Broken Authorization – Users can access restricted API functions or data.
•      Insecure Data Exposure – API returns sensitive data such as emails, passwords, or tokens.
•      Injection Attacks – SQL Injection, XSS, SSRF, and XML External Entity (XXE) vulnerabilities.
•      Rate Limiting Issues – API does not restrict brute-force or excessive requests.
•      Security Misconfigurations – Issues such as CORS misconfigurations, stack traces, and exposed secrets.

     Some common tools you should use , OWASP ZAP, Burp Suite Scanner, sqlmap, Nikto, jwt_tool .

4. Exploitation & Attack Simulation
Attempt to exploit identified vulnerabilities to assess their impact.

•      Brute-force API authentication (test weak passwords or leaked API keys).      
•      Modify API requests (change object IDs, bypass access controls).
•      Token tampering (edit JWT payloads to escalate privileges).
•      Injection attacks (submit malicious SQL, JavaScript, or XML payloads).
•      Denial-of-Service (DoS) Simulation (send excessive requests to check API resilience).
•      Some common tools you should use , Burp Suite Intruder, sqlmap, jwt_tool, ffuf, wfuzz.

5. Business Logic Testing
Test if the API enforces correct workflows and prevents logical abuse.

•      Determine if a user can modify product prices in an e-commerce API.
•      Verify whether a user can skip authentication steps in a multi-step process.      
•      Check if users can overuse free API features by manipulating requests.

6. Rate Limiting & DoS Testing
Evaluate the APIʼs resilience against excessive requests. Try to : -

•      Brute-force login attempts to check if API restricts login attempts.      
•      Excessive API calls to verify request rate limitations.
•      Testing large payloads to determine if the API accepts oversized request bodies.
•      Some common tools you should use , ffuf, Burp Suite Intruder, slowloris, Python scripts.

8.    Encryption & Data Protection Testing

• Ensure sensitive data is encrypted at rest and in transit.      
• Verify if the API enforces HTTPS (TLS 1.2+).
• Check if passwords and API keys are encrypted before transmission.      
• Intercept API requests to confirm that sensitive data is not exposed.

Popular API Penetration Testing Tools
1.    Burp Suite – Intercepts, tests, and manipulates API traffic for security vulnerabilities.
2.    OWASP ZAP – Open-source API security scanner for detecting vulnerabilities.
3.    Postman – API testing tool with security testing capabilities.
4.    Fiddler – Web debugging proxy to inspect and modify API traffic.
5.    Nessus – Automated vulnerability scanner for APIs and web applications.
6.    OpenVAS – Open-source tool for API and network vulnerability assessment.
7.    ffuf – Fast web fuzzer for testing API endpoints.
8.    SQLmap – Automates SQL injection testing for APIs.
9.    JWT Toolkit – Analyzes and manipulates JWT authentication tokens.