API Testing Checklist

• MindAPI - API Testing Mindmap

Passive Reconnaissance

• Google Dorking to reveal specific information about the API

• Git Dorking: authorization: Bearer , filename: swagger.json

• Shodan Search: "content-type: application/json", "wp-json"

• Wayback Machine snapshots

Active Reconnaissance

• Amass: amass enum -active -d dns.com | grep api

  • amass enum -active -brute -w /usr/share/wordlists/API_superlist -d example.com -dir [directory name]

• Testing Directories: gobuster dir -u http://10.x.x.x -w /location/to/wordlist.txt

• Check for API calls in Devtools > network Tab or Web App Proxy

• Import curl request to Postman

Endpoint Analysis

• Search for API documentation, if not provided, tsee if it can be discovered

  • Example API doc Wordlist here

• Proxy all traffic to Postman/Burp and capture requests for history

• Perform the actions that can be performed within application; this can be filtered in Postman

OR

• Import api doc files as new collections

API Documentation Conventions

Convention	Example	Meaning
: or {}	/user/:id /user/{id} /user/1337 /account/:username /account/{username} /account/mart1an	The colon or curly brackets are used by some APIs to indicate a path variable. In other words, “:id” represents the variable for an ID number and “{username}” represents the account username you are trying to access.
[]	/api/v1/user?find=[name]	Square brackets indicate that the input is optional.
||	“blue” || “green” || “red”	Double bars represent different possible values that can be used.
< >	<find-function>	Angle Brackets represent a DomString, which is a 16-bit string

• Set parameters to variable in Postman

Testing

• To change versions: api/v3/login → api/v1/login

• Check other AuthN endpoints: /api/mobile/login → /api/v3/login /api/magic_link

• Verb Tampering: GET /api/trips/1 → POST /api/trips/1 POST /api/trips DELETE /api/trips/1

• Try Object IDs in HTTP headers and bodies, URLs tend to be less vulnerable.

• Try Numeric IDs when facing a GUID/UUID: GET /api/users/6b95d962-df38 → GET /api/users/1

• Wrap ID with an array: {"id":111} → {"id":[111]}

• Wrap ID with a JSON object: {"id":111} → {"id":{"id":111}}

• HTTP Parameter Pollution: /api/profile?user_id=legit&user_id=victim /api/profile?user_id=victim&user_id=legit

• JSON Parameter Pollution: {"user_id":legit,"user_id":victim} {"user_id":victim,"user_id":legit}

• Wildcard instead of ID: /api/users/1 → /api/users/* /api/users/% /api/users/_ /api/users/.

• Ruby application HTTP parameter containing a URL → Pipe as the first character and then a shell command.

• Developer APIs differs with mobile and web APIs. Test them separately.

• Change Content-Type to application/xml and see if the API parse it.

• Non-Production environments tend to be less secure (staging/qa/etc.) Leverage this fact to bypass AuthZ, AuthN, rate limiting & input validation.

• Export Injection if you see Convert to PDF feature.

• Expand your attack surface and test old versions of APKs IPAs.

Additional checks:

Mass Assignment Vulnerabilities

Mass Assignment with account registration for PrivEsc:

Admin Registration

POST /api/admin/create/user
Token: AdminAuthToken
-Redacted-
{
"username": "admin2",
"pass": "Iforgetit0ften",
"admin": true
}

POST /create/user
Token: StandardUserAuthToken
-Redacted-
{
"username": "tester",
"pass": "Test1234",
"admin": true
}

Blind Mass Assignment: If you suspect an API is vulnerable to Mass Assignment, there is a chance it may ignore the irrelevant variables and accept the variable that matches the expected name and format.

{
"username":"testern",
"email":"[e-mail]",
"admin": true,
"admin":1,
"isadmin": true,
"role":"admin",
"role":"administrator",
"user_priv": "admin",
"password":"Password1!"
}

Check different Content-Types

x-www-form-urlencoded --> user=test
application/json --> {"user": "test"}
application/xml --> <user>test</user>**

If it's regular POST data try sending arrays, dictionaries

username[]=John
username[$neq]=lalala**

If JSON is supported try to send unexpected data types

{"username": "John"}
{"username": true}
{"username": null}
{"username": 1}
{"username": [true]}
{"username": ["John", true]}
{"username": {"$neq": "lalala"}}**

If XML is supported, check for XXE