API Security Testing Guide

Why API Security Matters

APIs are the backbone of modern applications. They're also a primary attack vector. This guide covers how to test and secure your APIs.

OWASP API Security Top 10

1. Broken Object Level Authorization (BOLA)

What it is: Users can access resources belonging to other users.

Example:

# Attacker accesses user 123's data
GET /api/users/123/profile

# What if they could access ANY user?
GET /api/users/456/profile

How to test:

1. Enumerate user IDs
2. Attempt horizontal privilege escalation
3. Test for IDOR vulnerabilities

Fix: Implement authorization checks on every endpoint.

2. Broken Authentication

What it is: Flaws in authentication mechanisms allow attackers to compromise tokens or exploit implementation flaws.

Testing:

# Test for weak passwords
POST /api/auth/login
{"email":"admin@company.com","password":"password123"}

# Test for password reset
POST /api/auth/reset
{"email":"attacker@malicious.com"}

# Test for JWT weaknesses
{"alg":"none"}
{"alg":"HS256","kid":"..."}

Fix: Implement MFA, rate limiting, and secure token handling.

3. Excessive Data Exposure

What it is: API returns sensitive data that clients shouldn't see.

Testing:

# Test response for sensitive data
GET /api/users/me
# Check for: passwords, tokens, PII, internal IDs

Fix: Use explicit whitelisting, not blacklisting.

4. Lack of Rate Limiting

What it is: No protection against automated attacks.

Testing:

# Execute 1000 requests in 10 seconds
for i in {1..1000}; do
  curl -s https://api.company.com/endpoint
done

Fix: Implement per-user, per-IP rate limits.

5. Broken Function Level Authorization

What it is: Complex access control allowing privilege escalation.

Testing:

# Try admin endpoints as regular user
GET /api/admin/users
POST /api/admin/settings
DELETE /api/admin/cache

Fix: Implement role-based access control with explicit checks.

6. Mass Assignment

What it is: Binding client parameters to internal objects.

Testing:

# Try to set admin flag
PUT /api/users/me
{"name":"John","role":"admin","verified":true}

Fix: Use explicit field allowlists.

7. Security Misconfiguration

What it is: Improperly configured security settings.

Testing:

# Check for verbose errors
GET /api/debug/trace

# Check for exposed configs
GET /api/config
GET /api/.env

Fix: Disable debug mode, use generic errors.

8. Injection

What it is: SQL, NoSQL, or command injection through APIs.

Testing:

# SQL Injection
GET /api/users?name=' OR 1=1--

# NoSQL Injection  
GET /api/users?email[$ne]=null

# Command Injection
GET /api/ping?host=google.com;ls

Fix: Use parameterized queries, input validation.

9. Improper Assets Management

What it is: Outdated API versions, unpatched systems.

Testing:

# Check API versions
GET /api/v1/users
GET /api/v2/users

# Look for debug endpoints
GET /api/debug/health
GET /api/actuator/info

Fix: Maintain API versioning, deprecate old versions.

10. Insufficient Logging & Monitoring

What it is: No visibility into attacks.

Fix: Implement:

• Failed authentication logging
• Access control failures
• Input validation failures
• Rate limit violations
• Suspicious pattern detection

API Testing Tools

1. Burp Suite - Comprehensive testing
2. Postman - Manual testing
3. OWASP ZAP - Automated scanning
4. nmap - Port scanning
5. SQLMap - SQL injection

Conclusion

API security requires continuous testing and monitoring. Regular penetration testing helps identify vulnerabilities before attackers do.

Get your API security audit to identify vulnerabilities in your APIs.