APIs are the backbone of modern applications, yet critical vulnerabilities remain overlooked due to poor implementation. This checklist combines battle-tested methodologies from enterprise pentests and bug bounty programs, with actionable steps to identify high-impact flaws.

Why it matters: Misconfigured authentication/authorization is the #1 API vulnerability class in 2025.

Where the big money hides: Complex logic errors often pay 5× more than XSS.

Top Targets

• Rate Limiting Bypass: Test for missing IP whitelisting or weak rate-limiting algorithms. Example: A crypto exchange allowed unlimited API calls via header spoofing (X-Forwarded-For manipulation).
• Pricing Manipulation: Validate input validation for numeric values. Case: A rideshare app allowed fare discounts via negative promo_code inputs.
• Inventory Spoofing: Test for unvalidated product ID parameters. Real-world: A retail API exposed stock levels via /products/{id}/inventory with guessable IDs.