Building Secure APIs: Lessons from 500+ Vulnerability Reports

Key patterns and anti-patterns I've observed across hundreds of API security assessments, with practical remediation advice.

Authentication Failures

The most common vulnerability pattern I see is broken authentication. APIs that accept tokens without proper validation, session IDs in URLs, and password reset tokens that never expire. After reviewing 500+ vulnerability reports, I found that 34% involved authentication issues — making it the #1 category by far.

Authorization Gaps

Horizontal privilege escalation (accessing other users' data) accounts for 28% of findings. The root cause is almost always missing object-level authorization checks. The fix is straightforward: every endpoint must verify that the authenticated user owns or has access to the requested resource.

Input Validation

SQL injection still exists in modern APIs — 12% of findings involved injection flaws. Parameterized queries prevent SQLi, but NoSQL injection, GraphQL injection, and command injection are often overlooked. Validate and sanitize all inputs, even from authenticated users.

Rate Limiting & Abuse

15% of findings involved missing rate limits or weak anti-automation controls. APIs without rate limiting are vulnerable to credential stuffing, data enumeration, and denial of service. Implement rate limiting at the API gateway level, not just at the application level.

Security Headers & CORS

Missing security headers (CORS misconfiguration, missing CSP, no HSTS) accounted for 11% of findings. A permissive CORS policy that allows any origin to make authenticated requests is essentially an open door for cross-origin attacks.