Bypass File Attachment Restrictions In Google Groups Via Email Posting

This article discusses a critical security flaw in Google Groups where file attachment restrictions could be bypassed via email posting. The finding emphasizes that front-end enforcement alone is insufficient—server-side validation must be implemented across all input vectors, including emails, APIs, and mobile clients.

🔗 Reference: Writeup

You Should Know:

1. Understanding the Vulnerability

The bug allowed users to bypass file attachment restrictions by sending emails directly to the group instead of using the web interface. This highlights the importance of:
– Input Validation: Ensure all input methods (email, API, UI) follow the same security checks.
– Server-Side Controls: Never rely solely on client-side restrictions.

2. Testing for Similar Vulnerabilities

Here’s how you can test for similar issues:

Using cURL to Simulate Email Posting

curl -X POST \
-H "Content-Type: multipart/form-data" \
-F "[email protected]" \
"https://groups.google.com/group/YOUR_GROUP_ID/post"

Check if the file bypasses front-end restrictions.

Automating with Python

import requests

url = "https://groups.google.com/group/TEST_GROUP/post"
files = {'file': open('test.pdf', 'rb')}
response = requests.post(url, files=files)

if response.status_code == 200:
print("File uploaded successfully!")
else:
print("Failed to bypass restrictions.")

3. Mitigation Steps for Developers

Validate all input sources uniformly.

Use regex or MIME-type checks for file uploads:

Linux command to check file type
file --mime-type upload.txt

Implement rate-limiting for email-based submissions.

4. Bug Bounty Hunting Tips

Fuzz email-to-post endpoints with tools like Burp Suite or OWASP ZAP.
Monitor HTTP headers for discrepancies between UI and email submissions.

What Undercode Say

This finding underscores the necessity of holistic security validation. Many platforms enforce restrictions only on the front end, leaving back-end APIs or email handlers exposed.

Relevant Linux & Windows Commands for Security Testing
– Check file permissions (Linux):

ls -la /var/www/html/uploads

– Monitor HTTP traffic (Windows):

netsh trace start capture=yes tracefile=C:\traces.etl

– Extract metadata from files (Linux):

exiftool suspicious.docx

– Scan for open ports (Cross-Platform):

nmap -sV target.com

Always verify server-side logic—client-side checks are easily bypassed.

Expected Output:

A detailed analysis of the Google Groups file restriction bypass, including PoC code snippets, mitigation strategies, and related security commands for testers and developers.