Extensive API Pentesting Guide for Bug Bounty Hunters

Introduction

APIs (Application Programming Interfaces) are the glue that holds modern applications together. However, they are also a prime target for attackers. A successful API pentest can reveal critical vulnerabilities that could be generously rewarded in Bug Bounty programs.

Understanding APIs

● What is an API? An interface that allows different applications to communicate with each other.
● Types of APIs: SOAP, REST, GraphQL, etc.
● Key components: Endpoints, methods (GET, POST, PUT, DELETE), parameters, authentication, and authorization.

API Pentesting Methodology

1. Reconnaissance:
    ○ Mapping the attack surface: **Tools like Burp Suite, Postman, Insomnia, etc.

○ Document analysis: Swagger, OpenAPI, etc.
○ Traffic analysis: Intercept and analyze requests/responses.

2. Vulnerability Assessment:
○ Injection: SQLi, NoSQLi, Command Injection, etc.
○ Authentication and Authorization: Broken Authentication, Broken Object Level Authorization (BOLA), IDOR, etc.
○ Sensitive Data Exposure: PII, credentials, tokens, etc.
○ Security Misconfiguration: CORS misconfigurations, etc.
○ Business Logic Attacks: Parameter manipulation to gain unfair advantages.

3. Exploitation:
○ Demonstrate impact: Modify data, access restricted information, etc.

4. Reporting:
○ Detailed documentation: Steps to reproduce, impact, recommendations, etc.

Examples of Vulnerabilities and How to Find Them

● SQL Injection:
○ Example: GET /api/users?id=1' OR 1=1--
○ How to find: Test with single quotes, SQL comments, etc.
● BOLA:
○ Example: Change the ID of an object in a request to access another object you shouldn't have access to.
○ How to find: Test with different IDs, user roles, etc.
● IDOR:
○ Example: GET /api/invoices/123 (where 123 is your invoice ID) -> Change the ID to 124 to try to access another user's invoice.
○ How to find: Test with sequential, predictable IDs, etc.
● Sensitive Data Exposure:
○ Example: API responses that include password hashes, access tokens, etc.
○ How to find: Carefully analyze API responses.

Useful Tools

● Burp Suite: For intercepting and modifying API traffic.
● Postman: For sending requests and analyzing responses.
● Insomnia: Similar to Postman, but with some additional features.
● Kiterunner: For automating API tests.
● Nuclei: For scanning for known vulnerabilities.

Additional Tips

● Understand the business logic: This will help you find business logic attacks.
● Test different user roles: To discover authorization-related vulnerabilities.
● Stay up-to-date: New API vulnerabilities are emerging constantly.

API pentesting is an exciting and rewarding field in the Bug Bounty field. With the right methodology, tools, and tips, you can uncover critical vulnerabilities and earn significant rewards.

Happy Hacking

Author: Ayush khatkar is a cybersecurity researcher, technical writer and an enthusiastic pen-tester at Asecurity. Contact here.

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