Bug bounty API testing

1. Discover the API

# Mobile app reverse → APK / IPA → look for endpoints
# JS bundle inspection
katana -u https://target.com -jc -silent | grep -E "/api/|/v1/|/graphql"
LinkFinder -i https://target.com/app.js -o cli

# Common paths
ffuf -w api-paths.txt -u https://target.com/FUZZ -mc 200,401,403
# /api, /api/v1, /v1, /graphql, /rest, /rpc, /swagger.json, /openapi.json,
# /api-docs, /redoc, /docs, /.well-known/

# Schema endpoints (huge if found)
curl https://target.com/swagger.json
curl https://target.com/openapi.json
curl -X POST https://target.com/graphql -d '{"query":"{__schema{types{name}}}"}'

If you find a swagger/openapi/graphql introspection — that IS the attack surface map. Use kiterunner to brute API routes if not.

2. OWASP API Top 10 — checklist per endpoint

API1: Broken Object Level Authorization (BOLA)

For every endpoint with an ID:

# Login as user A. Get user B's resource.
curl -H "Authorization: Bearer $A_TOKEN" https://api/users/$B_ID

Try: numeric→numeric swap, UUID enumeration via Wayback/JS, encoded ID decoding.

API2: Broken Authentication

• Missing / weak JWT verification
• Token reuse after logout
• Refresh-token abuse
• Hardcoded API keys in mobile bundles

API3: Broken Object Property Level (Mass Assignment + Excessive Data Exposure)

Mass assignment:

# Sign-up sends: {"email": "...", "password": "..."}
# Try: {"email": "...", "password": "...", "is_admin": true, "role": "admin"}

Excessive exposure: GET /users/me returns entire user object including hashed password / secret_question_answer / internal_notes — report it.

API4: Unrestricted Resource Consumption

Endpoints that allow ?limit=99999, deeply nested GraphQL queries, expensive operations without rate limit.

API5: Broken Function Level Authorization (BFLA)

Admin endpoints (POST /admin/users, DELETE /admin/posts/N) accessed as regular user.

API6: Unrestricted Access to Sensitive Business Flows

Anti-automation gaps: bulk-creating accounts, scraping product data, mass-redeeming gift cards.

API7: Server-Side Request Forgery (SSRF)

Any endpoint that takes a URL parameter (avatar URL, webhook URL, OAuth callback, image proxy) → SSRF target. See bounty-web SSRF section.

API8: Security Misconfiguration

• CORS: Access-Control-Allow-Origin: * with Allow-Credentials: true is a vuln
• Verbose error stack traces
• Default keys / debug endpoints
• HTTP methods like TRACE, OPTIONS revealing info

API9: Improper Inventory Management

Old API versions with weaker auth (/api/v1/) alongside new (/api/v2/). Test the old version.

API10: Unsafe Consumption of APIs

When the target API consumes a third-party API and trusts its response — SSRF chains, prototype pollution from upstream.

3. GraphQL specifics

# Introspection (if not disabled)
graphql-cop -t https://target/graphql
graphw00f https://target/graphql
clairvoyance https://target/graphql -w wordlist.txt    # if introspection is off

# Common issues
# 1. Introspection enabled in prod
# 2. No query depth limit → DoS
# 3. Batched queries bypassing rate limit
# 4. Fields exposed without auth
# 5. Mutations callable from queries (some impls)
# 6. Field-level authz missing — anonymous user fetches private fields

Useful tools: inql (Burp ext), gqlmap, BatchQL.

4. JWT

jwt_tool $TOKEN                      # decode + checks
jwt_tool $TOKEN -X a                 # alg=none
jwt_tool $TOKEN -X i                 # weak HMAC, brute
jwt_tool $TOKEN -X k -pk pubkey.pem  # key confusion (RS256→HS256)
jwt_tool $TOKEN -X kid               # kid injection

Also test: kid path traversal (kid: "../../../../dev/null" → empty key), JKU/X5U server-controlled injection.

5. Rate limit bypass

• Add header: X-Forwarded-For: 1.2.3.4, X-Real-IP, X-Originating-IP, X-Client-IP, X-Remote-IP
• Change User-Agent (some rate limiters key on UA+IP)
• Add trailing slash, change case, add path params (/login vs /login/)
• HTTP/2 — concurrent stream race
• Delay-based: many requesters bucket per second, sub-second burst slips through

6. Tool stack

kiterunner, arjun, nuclei, mitmproxy, BurpSuite (commercial), inql, graphql-cop, clairvoyance, jwt_tool, Postman/Hoppscotch for repro.

7. Reporting

Always include the exact request (method, path, headers, body). Mask tokens but show the structure. Demonstrate impact with two accounts (attacker + victim) where applicable.