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Introduction:
In a recent bug bounty discovery, a seemingly innocuous feature allowing users to make projects public resulted in a significant information disclosure vulnerability. The root cause was identified in the backend API, where the endpoint responsible for serving the public project view was returning excessive data, including Personally Identifiable Information (PII) such as full names, email addresses, and team member roles from the internal workspace. This incident highlights the critical security principle of “least privilege” in data exposure and the ever-present risk of Insecure Direct Object References (IDOR) combined with mass data assignment.
Learning Objectives:
- Understand how to identify API endpoints that leak sensitive data during bug bounty hunting.
- Learn to analyze HTTP request/response pairs in Burp Suite to spot excessive data disclosure.
- Identify and mitigate common misconfigurations in access control that lead to PII leaks.
You Should Know:
- Identifying the Vulnerability: The Public vs. Private Data Boundary
The core issue stemmed from a feature that allowed a project to be shared publicly via a unique link. When the hunter, Omar Mokhtar, tested this feature, he accessed the public link. However, by monitoring the background network traffic, he noticed that the API endpoint (GET /api/project/public/{id}) returned a JSON payload that included a nested object containing the internal workspace configuration. This is a classic example of an application failing to create a separate “View” for public data versus internal data.
To replicate this analysis:
- Intercept the Traffic: Open Burp Suite and navigate to the “Proxy” -> “HTTP History” tab.
- Locate the Endpoint: Filter by the specific feature you are testing (e.g., the public project link).
- Analyze the Response: Send the request to Burp Repeater. Examine the raw JSON or XML response. Look for keys like
owner,workspace,members,email, orrole. - Compare Responses: If you have access to a private workspace version of the same page, compare the API response for the public link against the private link. If they are identical, or if the public link includes fields from the private context, you have found a leak.
2. Exploiting Excessive Data Exposure via API Manipulation
In this case, the hunter did not need to change the request method or parameters; the vulnerability was inherent in the response structure. However, a thorough test involves manipulating the request to see if you can force the server to return more data.
Step‑by‑step guide for API manipulation:
- Parameter Pollution: If the public request used
GET /api/project/123?visibility=public, try adding other parameters like `?fields=internal,email,workspace` or?debug=true. - HTTP Method Fuzzing: Change the request method. If the public view uses GET, try sending a POST or PUT request to the same endpoint with an empty JSON body (
{}). Sometimes, the server logic for different methods returns different data sets. - Versioning: Check for API versioning. If the current call is to
/api/v1/project/123, try changing it to `/api/v2/project/123` or `/api/internal/project/123` to see if older or internal versions leak more data.
Linux Command for Fuzzing (using cURL and FFUF):
To automate finding hidden parameters that might trigger verbose errors, you can use FFUF (Fuzz Faster U Fool) on a parameter. First, capture the base request:
Save the base request to a file (request.txt) Example content: GET /api/project/123?visibility=public HTTP/1.1 Host: target.com Use ffuf to fuzz for a 'debug' parameter ffuf -u https://target.com/api/project/123?visibility=public\&FUZZ=1 -w /usr/share/wordlists/param_mini.txt -H "Host: target.com" -fs 1234
Note: Adjust the `-fs` (filter size) to ignore the default response size, allowing you to see if adding a parameter like `&internal=true` changes the response content length.
3. Access Control Testing (IDOR)
While this was a data leak, it often goes hand-in-hand with broken access controls. To test if you can access other users’ public projects (or internal data) by guessing IDs:
1. Enumerate Identifiers: If your project ID is 123, change it to `124` or 122.
2. Tool Execution: Use Burp Intruder or a simple bash script with `seq` to iterate through IDs.
Windows PowerShell Command for Enumeration:
Loop through IDs 1 to 50 and check for leaks
for ($i=1; $i -le 50; $i++) {
$response = Invoke-WebRequest -Uri "https://target.com/api/project/public/$i" -Method Get
Check if the response contains an email address
if ($response.Content -match '\b[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+.[A-Z|a-z]{2,}\b') {
Write-Host "Potential leak found at ID: $i" -ForegroundColor Red
Write-Host $response.Content
}
}
4. Mitigation: Implementing DTOs and Strict Filtering
To prevent this, developers must use Data Transfer Objects (DTOs). Instead of serializing the entire database `Workspace` object to JSON for the public view, a specific `PublicProjectDTO` should be created that only contains the fields intended for public display (e.g., project_name, description, public_preview_image).
Conceptual Code Example (Python/Flask):
VULNERABLE CODE - Don't do this
@app.route('/api/project/public/<int:project_id>')
def public_project(project_id):
project = db.session.query(Project).get(project_id)
This returns the entire object, including relationships like workspace.members
return jsonify(project.serialize())
SECURE CODE
class PublicProjectSchema(Schema):
name = fields.Str()
description = fields.Str()
@app.route('/api/project/public/<int:project_id>')
def public_project_secure(project_id):
project = db.session.query(Project).get(project_id)
Explicitly filter out sensitive data
result = PublicProjectSchema().dump(project)
return jsonify(result)
5. Configuration Hardening in Web Servers/Cloud
If the API endpoint was inadvertently serving a directory listing or static files containing workspace metadata, cloud misconfigurations could be at play. For example, a misconfigured AWS S3 bucket or Azure Blob Storage serving a `config.json` file.
AWS CLI command to audit bucket permissions:
Check if a bucket allows public reads aws s3api get-bucket-acl --bucket target-company-assets List objects to see if sensitive files are exposed aws s3 ls s3://target-company-assets/ --no-sign-request
Note: Always ensure you have explicit permission from the target (via a bug bounty program) before running enumeration tools against their assets.
What Undercode Say:
- Key Takeaway 1: Never trust that a “public” feature means secure. Always inspect the underlying API response for fields that shouldn’t be there. The difference between what the UI shows and what the API returns is often where bugs are found.
- Key Takeaway 2: This bug underscores the “Mass Assignment” vulnerability category. Developers often reuse internal data structures for API responses without filtering, leading to silent data leaks. Strict output sanitization is as important as input sanitization.
In this specific case, the hunter’s success came from a simple act of observation—checking the Burp history after clicking a button. It wasn’t about complex exploitation chains, but about understanding the data flow. For organizations, the fix requires a shift in development practices: define clear contracts for every API endpoint, ensuring that responses for public access never include fields marked as `private` or `internal` in the database schema. This incident also serves as a reminder that during bug bounty hunting, focusing on new features can often yield high-impact vulnerabilities that standard scanners miss.
Prediction:
As applications become more API-driven, we will see a rise in “business logic” data leaks stemming from cloned endpoints. Automated scanners will continue to miss these vulnerabilities because they cannot differentiate between a public name and a private email address in a JSON payload. The future of bug bounty hunting will rely more heavily on manual source code review (when available) and deep business logic analysis to identify these nuanced exposure flaws, rather than relying solely on automated fuzzing for generic vulnerabilities like SQLi or XSS.
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IT/Security Reporter URL:
Reported By: Omar Mokhtar – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
