Fuzz Api

Automated fuzz testing for REST APIs to discover vulnerabilities, crashes, and unexpected behavior through malformed inputs, boundary values, and random payloads. This command generates comprehensive fuzz test suites targeting injection attacks, input validation failures, and edge cases.

Design Decisions

Why fuzz testing matters:

• Security: Discovers SQL injection, XSS, command injection vulnerabilities
• Robustness: Finds crashes from unexpected inputs before users do
• Edge cases: Uncovers boundary conditions developers didn't consider
• Compliance: Validates input sanitization meets security standards

Alternatives considered:

• Manual testing: Too slow, can't cover mutation space
• Property-based testing: Good for unit tests, less suited for API integration
• Penetration testing tools: Expensive, requires security expertise
• Static analysis: Misses runtime-only issues

This approach balances: Automation, coverage, security focus, and integration with CI/CD.

When to Use

Use API fuzzing when:

• Testing security-critical APIs (auth, payment, admin endpoints)
• Validating input sanitization and validation logic
• Finding edge cases before production incidents
• Meeting security compliance requirements (PCI-DSS, SOC 2)
• Testing third-party API integration error handling
• Preparing for penetration testing or security audits

Don't use when:

• API has no user input (static data endpoints)
• Building proof-of-concept with no security requirements
• Input validation is already exhaustively tested
• Time-sensitive release without CI/CD integration

Prerequisites

• Existing REST API with endpoints to test
• Node.js 16+ or Python 3.8+ for fuzzing scripts
• API documentation (OpenAPI/Swagger or manual endpoint list)
• (Optional) Authentication credentials or test accounts
• (Optional) CI/CD pipeline for automated fuzzing
• (Optional) Security monitoring tools (SIEM, IDS)

Process

1. Identify Attack Surface

• List all API endpoints accepting user input
• Identify input types (strings, numbers, JSON, files)
• Prioritize high-risk endpoints (auth, admin, payment)

1. Generate Fuzz Inputs

• Malformed data (null, undefined, empty, overflow)
• Injection payloads (SQL, XSS, command injection)
• Boundary values (max int, negative, infinity)
• Type confusion (string as int, object as array)

1. Execute Fuzz Tests

• Send fuzz inputs to all endpoints
• Monitor responses for crashes, 500 errors, timeouts
• Capture stack traces and error details

1. Analyze Results

• Categorize vulnerabilities by severity (critical, high, medium)
• Create reproducible test cases for failures
• Generate security report with findings

1. Remediate and Retest

• Fix discovered vulnerabilities
• Add regression tests for fixed issues
• Rerun fuzzer to verify fixes

Output Format

Jest Fuzz Test Suite (Node.js)

// tests/api-fuzzer.test.js
const axios = require('axios');

const API_BASE = process.env.API_URL || 'http://localhost:3000';

// Fuzz input generators
const fuzzInputs = {
  // String mutations
  strings: [
    '', // Empty
    null,
    undefined,
    ' ', // Whitespace
    'A'.repeat(10000), // Very long
    'A'.repeat(1000000), // Extremely long
    '<script>alert(1)</script>', // XSS
    '<img src=x onerror=alert(1)>', // XSS variant
    '${7*7}', // Template injection
…

Python REST-Assured Fuzzer

# tests/test_api_fuzzer.py
import pytest
import requests
from typing import Any, List
import string
import random

API_BASE = "http://localhost:3000"

class FuzzInputGenerator:
    """Generate various fuzz inputs for API testing"""

    @staticmethod
    def string_mutations() -> List[Any]:
        return [
            "",  # Empty
            None,
            " ",  # Whitespace
…

Example Usage

Example 1: Automated Fuzzing in CI/CD

// .github/workflows/fuzz-tests.yml
name: API Fuzz Testing

on: [push, pull_request]

jobs:
  fuzz:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
      - name: Start API server
        run: npm run start:test &
      - name: Wait for API
        run: npx wait-on http://localhost:3000/health
      - name: Run fuzz tests
        run: npm run test:fuzz
      - name: Upload fuzz report
        if: failure()
…