API Security Testing: Common Vulnerabilities and How to Find Them

Why API Security Matters

APIs are the backbone of modern applications. They expose business logic and sensitive data, making them prime targets for attackers. The OWASP API Security Top 10 provides a framework for understanding the most critical API security risks.

OWASP API Security Top 10

API1: Broken Object Level Authorization (BOLA) The most common API vulnerability. Occurs when APIs don't properly verify that a user has permission to access a specific object.

Testing approach: - Manipulate object IDs in API requests - Test horizontal privilege escalation between users - Check for IDOR (Insecure Direct Object Reference) vulnerabilities

API2: Broken Authentication Weak or improperly implemented authentication mechanisms.

Testing approach: - Test for weak password policies - Check token strength and expiration - Look for credential stuffing vulnerabilities - Test password reset flows

API3: Broken Object Property Level Authorization APIs that expose or accept more object properties than necessary.

Testing approach: - Check API responses for excessive data exposure - Test mass assignment vulnerabilities - Verify that sensitive fields can't be modified through API requests

API4: Unrestricted Resource Consumption APIs without proper rate limiting or resource constraints.

Testing approach: - Test rate limiting effectiveness - Check for pagination bypass - Test upload size limits - Verify timeout configurations

API5: Broken Function Level Authorization Improper access controls on API endpoints based on user roles.

Testing approach: - Map all API endpoints by role - Test accessing admin endpoints as a regular user - Check for hidden or undocumented endpoints

Essential API Testing Tools - **Burp Suite** — Comprehensive web security testing platform - **Postman** — API development and testing - **OWASP ZAP** — Open-source security scanner - **Insomnia** — API client with security testing features