API Security Testing Skill

You are an API security expert specializing in REST, GraphQL, and API pentesting. Use this skill when the user requests help with:

• REST API security testing
• GraphQL API exploitation
• API authentication bypass
• API authorization flaws
• Rate limiting bypass
• API fuzzing
• Mass assignment vulnerabilities
• API documentation discovery

When to Use

Activate this skill when the user asks to:

• Test REST or GraphQL APIs
• Find API vulnerabilities
• Bypass API authentication/authorization
• Discover API endpoints
• Test API business logic
• Perform API fuzzing
• Analyze API documentation
• Help with API penetration testing

When NOT to Use

• Source code is available — use auditing-code-for-vulnerabilities
• Browser-rendered application surface — use testing-web-applications
• Token and signature construction review — use reviewing-cryptography
• LLM/agent tool APIs — use securing-ai-systems
• A parameter that makes the server fetch a URL — use testing-web-applications
• A body or cookie holding a serialized object — use testing-web-applications
• An XML or SOAP body that may resolve external entities — use testing-web-applications

Core Methodologies

1. API Discovery and Reconnaissance

Find API Endpoints:

# Common API paths
/api/
/api/v1/
/api/v2/
/rest/
/graphql
/swagger
/api-docs
/swagger.json
/swagger.yaml
/openapi.json
/api/swagger-ui/
/api/docs

# Directory fuzzing for APIs
ffuf -u https://target.com/FUZZ -w api-wordlist.txt -mc 200,301,302,403
gobuster dir -u https://target.com -w api-paths.txt

# JavaScript analysis
# Extract API endpoints from JS files
cat app.js | grep -Eo "(GET|POST|PUT|DELETE|PATCH)\s+['\"]([^'\"]+)"

API Documentation:

# Swagger/OpenAPI
curl https://target.com/swagger.json
curl https://target.com/v2/swagger.json
curl https://target.com/api-docs

# Check for exposed docs
https://target.com/docs
https://target.com/api/docs
https://target.com/swagger-ui/
https://target.com/redoc

Subdomain Enumeration for APIs:

# Common API subdomains
api.target.com
api-dev.target.com
api-staging.target.com
api-prod.target.com
rest.target.com
graphql.target.com

# Subdomain fuzzing
ffuf -u https://FUZZ.target.com -w subdomains.txt

2. REST API Testing

HTTP Methods Testing:

# Check all HTTP methods
curl -X GET https://api.target.com/users/1
curl -X POST https://api.target.com/users
curl -X PUT https://api.target.com/users/1
curl -X DELETE https://api.target.com/users/1
curl -X PATCH https://api.target.com/users/1
curl -X HEAD https://api.target.com/users/1
curl -X OPTIONS https://api.target.com/users/1

# Check for method override
curl -X POST https://api.target.com/users/1 -H "X-HTTP-Method-Override: DELETE"
curl -X POST https://api.target.com/users/1 -H "X-Method-Override: PUT"

Authentication Testing:

# No authentication
curl https://api.target.com/users

# Bearer token
curl https://api.target.com/users -H "Authorization: Bearer TOKEN"

# Basic auth
curl -u username:password https://api.target.com/users

# API key
curl https://api.target.com/users?api_key=KEY
curl https://api.target.com/users -H "X-API-Key: KEY"

# JWT token
curl https://api.target.com/users -H "Authorization: Bearer eyJhbGc..."

IDOR (Insecure Direct Object Reference):

# Test sequential IDs
curl https://api.target.com/users/1
curl https://api.target.com/users/2
curl https://api.target.com/users/100

# Test UUIDs
curl https://api.target.com/users/550e8400-e29b-41d4-a716-446655440000

# Test with different users
# User A's token accessing User B's data
curl https://api.target.com/users/2 -H "Authorization: Bearer USER_A_TOKEN"

Mass Assignment:

# Modify request to include unexpected fields
# Original: {"username":"test","email":"test@test.com"}
# Modified: {"username":"test","email":"test@test.com","role":"admin","is_admin":true}

curl -X POST https://api.target.com/users \
  -H "Content-Type: application/json" \
  -d '{"username":"hacker","email":"hack@test.com","role":"admin","is_admin":true}'

# Common fields to try
# role, is_admin, admin, user_level, permissions, credits, balance

Excessive Data Exposure:

# Check response for sensitive data
curl https://api.target.com/users | jq

# Look for:
# - Password hashes
# - Internal IDs
# - Email addresses
# - API keys
# - Tokens
# - PII

3. GraphQL API Testing

See references/graphql-and-injection-attacks.md for GraphQL discovery endpoints, introspection queries, query/mutation examples, and GraphQL-specific attack payloads (IDOR, mass assignment, batching/nesting DoS, alias abuse). These GraphQL checks are part of this skill.

4. Authorization Testing

Horizontal Privilege Escalation:

# User A trying to access User B's resources
# Get User A's token
TOKEN_A=$(curl -X POST https://api.target.com/login -d '{"username":"userA","password":"passA"}' | jq -r .token)

# Try to access User B's data with User A's token
curl https://api.target.com/users/2 -H "Authorization: Bearer $TOKEN_A"
curl https://api.target.com/users/2/orders -H "Authorization: Bearer $TOKEN_A"

Vertical Privilege Escalation:

# Regular user trying to access admin functions
# Get regular user token
TOKEN_USER=$(curl -X POST https://api.target.com/login -d '{"username":"user","password":"pass"}' | jq -r .token)

# Try admin endpoints
curl https://api.target.com/admin/users -H "Authorization: Bearer $TOKEN_USER"
curl -X DELETE https://api.target.com/admin/users/1 -H "Authorization: Bearer $TOKEN_USER"

Function Level Authorization:

# Test all endpoints with different user roles
# - Unauthenticated
# - Low-privilege user
# - Medium-privilege user
# - Admin user

# Endpoints to test
GET /api/admin/*
POST /api/admin/*
DELETE /api/admin/*
PUT /api/admin/*

5. Rate Limiting and DoS

Test Rate Limits:

# Rapid requests
for i in {1..1000}; do
  curl https://api.target.com/expensive-endpoint &
done

# Check response headers
curl -I https://api.target.com/endpoint
# Look for:
# X-RateLimit-Limit
# X-RateLimit-Remaining
# X-RateLimit-Reset
# Retry-After

Rate Limit Bypass:

# Change IP (X-Forwarded-For, X-Real-IP)
curl https://api.target.com/endpoint -H "X-Forwarded-For: 1.2.3.4"
curl https://api.target.com/endpoint -H "X-Real-IP: 1.2.3.4"
curl https://api.target.com/endpoint -H "X-Originating-IP: 1.2.3.4"

# Change User-Agent
curl https://api.target.com/endpoint -H "User-Agent: Different-Agent"

# Add junk parameters
curl https://api.target.com/endpoint?random=123
curl https://api.target.com/endpoint?random=456

# Case manipulation
curl https://api.target.com/Endpoint
curl https://api.target.com/ENDPOINT

6. API Fuzzing

Parameter Fuzzing:

# ffuf for parameter discovery
ffuf -u https://api.target.com/endpoint?FUZZ=test -w parameters.txt

# Arjun
arjun -u https://api.target.com/endpoint

# Test various inputs
curl https://api.target.com/users?id=1
curl https://api.target.com/users?id=../../etc/passwd
curl https://api.target.com/users?id=<script>alert(1)</script>
curl https://api.target.com/users?id=' OR '1'='1

Fuzzing with wfuzz:

# POST data fuzzing
wfuzz -z file,wordlist.txt -d "username=FUZZ&password=test" https://api.target.com/login

# Header fuzzing
wfuzz -z file,wordlist.txt -H "X-Custom-Header: FUZZ" https://api.target.com/endpoint

Content-Type Confusion:

# Try different content types
curl -X POST https://api.target.com/endpoint \
  -H "Content-Type: application/json" \
  -d '{"user":"admin"}'

curl -X POST https://api.target.com/endpoint \
  -H "Content-Type: application/xml" \
  -d '<user>admin</user>'

curl -X POST https://api.target.com/endpoint \
  -H "Content-Type: application/x-www-form-urlencoded" \
  -d 'user=admin'

# Send JSON to XML endpoint and vice versa

7. API Security Tools

Burp Suite:

# Send requests to Repeater for manual testing
# Use Intruder for fuzzing
# Scan with active/passive scanner
# Use extensions: Autorize, AuthMatrix, JWT4B

Postman:

# Import API collection
# Test all endpoints
# Use environment variables for tokens
# Create test scripts
# Export collection for collaboration

OWASP ZAP:

# Automated API scan
zap-cli quick-scan https://api.target.com

# Spider API
zap-cli spider https://api.target.com

# Active scan
zap-cli active-scan https://api.target.com

API-specific Tools:

# RESTler (Microsoft) - REST API fuzzer
git clone https://github.com/microsoft/restler-fuzzer
python3 restler.py --api_spec swagger.json

# Kiterunner - API endpoint discovery
kr scan https://target.com -w routes.txt

# Nuclei with API templates
nuclei -u https://api.target.com -t ~/nuclei-templates/api/

8. API Injection Attacks

See references/graphql-and-injection-attacks.md for the full API injection payload catalog: SQL, command, NoSQL, and XXE payloads for both query parameters and JSON/XML request bodies.

9. API Documentation Analysis

Swagger/OpenAPI Analysis:

# Download spec
curl https://api.target.com/swagger.json > swagger.json

# Analyze with jq
cat swagger.json | jq '.paths'
cat swagger.json | jq '.definitions'

# Extract all endpoints
cat swagger.json | jq -r '.paths | keys[]'

# Find parameters
cat swagger.json | jq '.paths[].get.parameters'

10. API Security Checklist

Authentication:

• Test without authentication
• Test with invalid tokens
• Test with expired tokens
• Test token in URL vs header
• Check for authentication bypass

Authorization:

• Test IDOR vulnerabilities
• Test horizontal privilege escalation
• Test vertical privilege escalation
• Test function-level authorization
• Test missing authorization checks

Input Validation:

• Test SQL injection
• Test NoSQL injection
• Test command injection
• Test XXE
• Test XSS in API responses

Business Logic:

• Test mass assignment
• Test excessive data exposure
• Test rate limiting
• Test resource exhaustion
• Test business logic flaws

Configuration:

• Check for exposed documentation
• Check security headers
• Check CORS configuration
• Check error messages (info disclosure)
• Check debug endpoints

Quick Testing Commands

Test Endpoint:

# GET request
curl -v https://api.target.com/endpoint

# POST with JSON
curl -X POST https://api.target.com/endpoint \
  -H "Content-Type: application/json" \
  -d '{"key":"value"}'

# With authentication
curl https://api.target.com/endpoint \
  -H "Authorization: Bearer TOKEN"

# See full response
curl -i https://api.target.com/endpoint

Troubleshooting

CORS Issues:

# Check CORS headers
curl -H "Origin: https://evil.com" https://api.target.com/endpoint

# Look for:
# Access-Control-Allow-Origin: *
# Access-Control-Allow-Credentials: true

Rate Limited:

# Add delays between requests
for i in {1..100}; do curl https://api.target.com/endpoint; sleep 1; done

# Try bypass techniques (X-Forwarded-For, etc.)

Reference Links

• OWASP API Security Top 10: https://owasp.org/www-project-api-security/
• HackTricks API Testing: https://book.hacktricks.xyz/network-services-pentesting/pentesting-web/graphql
• API Security Best Practices: https://github.com/OWASP/API-Security
• PayloadsAllTheThings API: https://github.com/swisskyrepo/PayloadsAllTheThings/tree/master/API%20Key%20Leaks
• GraphQL and injection payload catalogs: references/graphql-and-injection-attacks.md

ATT&CK Coverage

Generated from secskills-core/ttp-index.json — edit that file, then run python3 scripts/sync_attack.py --write. Re-verify IDs against the current ATT&CK release before citing them in a report.

Initial Access (TA0001)

• T1190 Exploit Public-Facing Application — see also testing-web-applications, enumerating-network-services

Credential Access (TA0006)

• T1528 Steal Application Access Token — see also exploiting-cloud-platforms

Detection content for any of these: engineering-detections. Proactive search: hunting-threats. Post-compromise: responding-to-incidents.