What Is API Security Testing? A Practical Guide

API security testing is the practice of evaluating application programming interfaces (REST, GraphQL, gRPC) for security weaknesses, access control bypasses, and data leakage. Unlike traditional web applications that return rendered HTML, APIs return raw structured data (JSON/XML), exposing backend object references directly to client applications.

KEY VECTORS IN API SECURITY TESTING

1. AUTHENTICATION & JWT SECURITY

APIs frequently rely on JSON Web Tokens (JWT) or OAuth 2.0 bearer tokens. Security testing verifies whether signature verification can be bypassed (e.g. `alg: none` attack), whether token expiration is enforced, and whether secret keys can be brute-forced.

2. BROKEN FUNCTION LEVEL AUTHORIZATION (BFLA)

BFLA occurs when regular API clients can invoke administrative endpoints (e.g. `/api/v1/admin/users/delete`) simply by guessing endpoint URLs without role validation.

3. EXCESSIVE DATA EXPOSURE

Many API endpoints return complete database objects to the frontend, relying on the client UI to filter sensitive fields. An attacker inspecting raw API responses can extract PII, password hashes, or internal metrics.