API Security Testing: A Comprehensive Guide

APIs are the backbone of modern applications, making them prime targets for attackers. This guide covers essential API security testing techniques and common vulnerabilities.

Why API Security Matters

APIs often expose sensitive data and business logic directly. Unlike traditional web applications, APIs:

Lack visual interfaces that might hint at security boundaries
Are designed for programmatic access, making automation attacks easier
Often handle authentication tokens and sensitive data
May expose internal services and data structures

OWASP API Security Top 10

API1: Broken Object Level Authorization (BOLA)

Attackers substitute the ID of their own resource with an ID of a resource belonging to another user.

Capture requests with resource IDs
Substitute IDs with those of other users
Check if unauthorized access is granted

API2: Broken Authentication

Weak authentication mechanisms allow attackers to compromise tokens or exploit implementation flaws.

Test for weak password policies
Check token security (JWT vulnerabilities)
Test session management
Look for credential stuffing vulnerabilities

API3: Broken Object Property Level Authorization

APIs expose endpoints that return all object properties, including sensitive ones.

Analyze response objects for sensitive data
Test if users can modify protected properties
Check for mass assignment vulnerabilities

API4: Unrestricted Resource Consumption

APIs don't limit the size or number of resources that can be requested.

Test rate limiting
Check for pagination limits
Test file upload size limits
Look for DoS vulnerabilities

API5: Broken Function Level Authorization

Complex access control policies with different roles and groups can lead to authorization flaws.

Map all API endpoints
Test admin endpoints with regular user tokens
Check for privilege escalation

API Security Testing Methodology

1. Reconnaissance

Identify all API endpoints
Analyze API documentation (OpenAPI/Swagger)
Map authentication mechanisms
Understand data flow

2. Authentication Testing

Test token generation and validation
Check for insecure token storage
Test OAuth/OIDC implementations
Verify MFA implementation

3. Authorization Testing

Test horizontal privilege escalation
Test vertical privilege escalation
Check object-level access controls
Verify function-level access controls

4. Input Validation Testing

Test for SQL injection
Check for NoSQL injection
Test command injection
Verify input sanitization

5. Business Logic Testing

Test for race conditions
Check for logic flaws
Test workflow bypasses
Verify transaction integrity

Tools for API Security Testing

Burp Suite - Comprehensive web security testing
OWASP ZAP - Open-source security scanner
Postman - API testing and automation
Insomnia - REST and GraphQL client
SQLMap - SQL injection testing

GraphQL-Specific Testing

GraphQL APIs require additional testing:

Introspection query exposure
Deeply nested query attacks
Batch query attacks
Field suggestion exploitation

Conclusion

API security testing requires a methodical approach covering authentication, authorization, input validation, and business logic. At CyberSecify, we provide thorough API security assessments as part of our web application testing services.