exploiting-excessive-data-exposure-in-api

name	exploiting-excessive-data-exposure-in-api
description	Tests APIs for excessive data exposure where endpoints return more data than the client application needs, relying on the frontend to filter sensitive fields. The tester intercepts API responses and analyzes them for leaked PII, internal identifiers, debug information, or sensitive business data that the UI does not display but the API transmits. This maps to OWASP API3:2023 Broken Object Property Level Authorization. Activates for requests involving API data leakage testing, excessive data exposure, response filtering bypass, or API over-fetching.
domain	cybersecurity
subdomain	api-security
tags	["api-security","owasp","data-exposure","rest-security","pii-leakage"]
version	1.0.0
author	mahipal
license	Apache-2.0

Exploiting Excessive Data Exposure in API

When to Use

• Testing APIs where the frontend displays a subset of data but the API response includes additional fields
• Assessing mobile application APIs where responses are designed for multiple client types and may contain excess data
• Identifying PII leakage in API responses that include email addresses, phone numbers, SSNs, or payment data not shown in the UI
• Testing GraphQL APIs where clients can request arbitrary fields including sensitive attributes
• Evaluating APIs after microservice refactoring where internal service-to-service data leaks into public endpoints

Do not use without written authorization. Data exposure testing involves capturing and analyzing potentially sensitive personal data.

Prerequisites

• Written authorization specifying target API endpoints and scope
• Burp Suite Professional or mitmproxy configured as intercepting proxy
• Two test accounts at different privilege levels (regular user and admin)
• Browser developer tools or mobile proxy setup for traffic capture
• Python 3.10+ with requests and json libraries
• API documentation (OpenAPI spec) for comparison against actual responses

Workflow

Step 1: Response Schema Discovery

Compare documented API responses with actual responses:

import requests
import json

BASE_URL = "https://target-api.example.com/api/v1"
headers = {"Authorization": "Bearer <user_token>", "Content-Type": "application/json"}

# Fetch a resource and analyze all returned fields
endpoints_to_test = [
    ("GET", "/users/me", None),
    ("GET", "/users/me/orders", None),
    ("GET", "/products", None),
    ("GET", "/users/me/settings", None),
    ("GET", , ),
]

 method, path, body  endpoints_to_test:
    resp = requests.request(method, , headers=headers, json=body)
     resp.status_code == :
        data = resp.json()
        
         ():
            fields = []
             (obj, ):
                 k, v  obj.items():
                    full_key =   prefix  k
                    fields.append(full_key)
                    fields.extend(extract_fields(v, full_key))
             (obj, )  obj:
                fields.extend(extract_fields(obj[], ))
             fields

        all_fields = extract_fields(data)
        ()
         f  (all_fields):
            ()