API Pentesting: The Complete Checklist
- Douglas Leal
- May 13
- 2 min read
APIs are at the core of modern systems, connecting applications, databases, cloud services, and much more. That’s precisely why they’ve become prime targets for attackers. API Pentesting is essential to uncover vulnerabilities that could lead to data leaks, operation tampering, or unauthorized system control.
In this post, you’ll find a practical and technical checklist to help secure your APIs—whether REST, WebSocket, GraphQL, or SOAP—from end to end.
What Is API Pentesting?
API Pentesting (Application Programming Interface) is an offensive security assessment focused on identifying vulnerabilities in system communication interfaces. It simulates real-world attacks, examining everything from exposed endpoints to authorization flaws and business logic issues.
It’s crucial for mobile app backends, SaaS platforms, third-party integrations, IoT ecosystems, and financial services.
Full API Pentest Checklist
✅ 1. Endpoint Discovery
Identify all available endpoints—documented and undocumented.
Watch out for Shadow APIs: legacy or undocumented APIs still active in production.
✅ 2. Authentication
Is authentication enforced? (Bearer Token, OAuth2, JWT, etc.)
Are tokens exposed in headers or URLs?
Do tokens expire properly? Is there token rotation?
Test common weaknesses: brute force, token replay, use of expired/invalid tokens.
✅ 3. Authorization
Can users access resources that belong to others?
Test for IDOR (Insecure Direct Object Reference) by changing object IDs.
Confirm users with different roles receive properly scoped responses.
✅ 4. Input Validation
Are inputs validated server-side?
Test malicious inputs: SQL Injection, XSS, XXE, NoSQL Injection.
Try unexpected or tampered payload fields.
✅ 5. Rate Limiting & Abuse Protection
Is brute-force protection in place?
Are requests limited by IP, user, or token?
Try bypassing rate limits with proxy rotation or custom headers.
✅ 6. Sensitive Data Exposure
Are responses minimizing exposed data?
Are fields like passwords, tokens, CPF, or financial info properly masked?
Check error logs and debug messages—they often leak sensitive data.
✅ 7. Versioning & Legacy Security
Is there clear API versioning? (e.g., /v1/, /v2/)
Are outdated versions still active and exploitable?
Test for fallback behaviors in deprecated endpoints.
✅ 8. Secure Communication (HTTPS)
Is all traffic forced over HTTPS?
Does HTTP redirect properly, or does it return content?
✅ 9. Headers & CORS Security
Analyze CORS policy: is it overly permissive?
HeAre security headers configured?
Content-Security-Policy
X-Frame-Options
Strict-Transport-Security
✅ 10. Documentation & Swagger Exposure
Is API documentation publicly exposed?
Are real user data or tokens shown in examples?
Can you use the Swagger/OpenAPI docs to fuzz endpoints?
Best Practices During the Pentest
Test as both authenticated and unauthenticated users.
Emulate API calls from various contexts (mobile app, browser, scripts).
Document all unexpected or inconsistent responses.
Go beyond technical flaws—test for logical vulnerabilities too.
Conclusion
An API Pentest is far more than just endpoint testing—it's a deep audit of how your system communicates, protects, and exposes information.
With the checklist above, your tech team can build an efficient API security process, proactively addressing critical flaws and staying ahead of increasingly sophisticated threats.
