CVE-2021-28931
📋 TL;DR
CVE-2021-28931 is an arbitrary file upload vulnerability in Fork CMS that allows attackers to upload malicious zip files to the Themes panel, enabling them to create or replace arbitrary files in the /themes directory. This affects Fork CMS 5.9.2 installations, potentially allowing attackers to execute arbitrary code, deface websites, or compromise the server.
💻 Affected Systems
- Fork CMS
📦 What is this software?
Fork Cms by Fork Cms
⚠️ Risk & Real-World Impact
Worst Case
Remote code execution leading to complete server compromise, data theft, or ransomware deployment.
Likely Case
Website defacement, file manipulation, or limited code execution within the web server context.
If Mitigated
No impact if proper file upload validation and access controls are implemented.
🎯 Exploit Status
Exploitation requires access to the Themes panel (admin privileges). Public proof-of-concept demonstrates the vulnerability.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 5.9.3 and later
Vendor Advisory: https://github.com/forkcms/forkcms/releases/tag/5.9.2
Restart Required: No
Instructions:
1. Backup your Fork CMS installation and database. 2. Download Fork CMS 5.9.3 or later from the official repository. 3. Replace the vulnerable files with the patched version. 4. Verify the update by checking the version in the admin panel.
🔧 Temporary Workarounds
Disable Themes Panel Access
allRestrict access to the Themes panel to prevent file uploads.
# Modify .htaccess or web server configuration to block /private/themes/upload
# Example for Apache: Deny from all in the themes directory
Implement File Upload Validation
allAdd server-side validation to reject zip files with malicious paths.
# Add validation in PHP to check zip file contents before extraction
# Example: Validate file paths don't contain directory traversal sequences
🧯 If You Can't Patch
- Restrict admin panel access to trusted IP addresses only.
- Monitor file uploads in the /themes directory for suspicious activity.
🔍 How to Verify
Check if Vulnerable:
Check if your Fork CMS version is 5.9.2 by logging into the admin panel and viewing the version in the footer or settings.Check Version:
Check the version in the admin panel interface or inspect the source code for version markers.Verify Fix Applied:
After updating, confirm the version is 5.9.3 or later in the admin panel and test that zip uploads in the Themes panel are properly validated.📡 Detection & Monitoring
Log Indicators:
- Unusual file uploads to /private/themes/upload
- Zip file extraction errors or warnings in web server logs
- Admin panel access from unexpected IP addresses
Network Indicators:
- HTTP POST requests to themes upload endpoints with zip files
- Unusual outbound connections from the web server after uploads
SIEM Query:
source="web_server_logs" AND (uri="/private/themes/upload" OR file_extension=".zip") AND status=200