CVE-2021-39608
📋 TL;DR
This vulnerability allows remote attackers to execute arbitrary PHP code on FlatCore-CMS 2.0.7 systems via the upload addon plugin. Attackers can achieve full system compromise by uploading malicious files. All systems running the vulnerable version with the upload addon enabled are affected.
💻 Affected Systems
- FlatCore-CMS
📦 What is this software?
Flatcore Cms by Flatcore
⚠️ Risk & Real-World Impact
Worst Case
Complete system takeover with attacker gaining full administrative control, data theft, website defacement, and potential lateral movement to other systems.
Likely Case
Website defacement, malware deployment, credential theft, and backdoor installation for persistent access.
If Mitigated
Limited impact with proper file upload restrictions and web application firewalls blocking malicious uploads.
🎯 Exploit Status
Public exploit code available on Packet Storm Security. Attack requires no authentication.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 2.0.8 or later
Vendor Advisory: https://github.com/flatCore/flatCore-CMS/issues/52
Restart Required: No
Instructions:
1. Backup your current installation. 2. Download latest version from official repository. 3. Replace vulnerable files with patched version. 4. Verify upload functionality still works correctly.
🔧 Temporary Workarounds
Disable Upload Addon
linuxTemporarily disable the vulnerable upload addon plugin
mv /path/to/flatcore/addons/upload /path/to/flatcore/addons/upload.disabled
Restrict File Upload Types
allConfigure web server to block PHP file uploads
Add to .htaccess: <FilesMatch "\.(php|php5|php7|phtml)$">
Order Allow,Deny
Deny from all
</FilesMatch>
🧯 If You Can't Patch
- Implement strict file upload validation (allow only specific extensions like .jpg, .png)
- Deploy web application firewall with RCE protection rules
🔍 How to Verify
Check if Vulnerable:
Check if running FlatCore-CMS version 2.0.7 with upload addon enabledCheck Version:
Check /core/version.php or admin panel version displayVerify Fix Applied:
Verify version is 2.0.8 or higher and test upload functionality with malicious files📡 Detection & Monitoring
Log Indicators:
- Unusual file uploads to upload directory
- PHP file uploads with suspicious names
- Multiple failed upload attempts
Network Indicators:
- HTTP POST requests to upload endpoints with PHP files
- Unusual outbound connections from web server
SIEM Query:
source="web_logs" AND (uri_path="/addons/upload/" OR uri_path="/upload/") AND (file_extension="php" OR file_extension="php5" OR file_extension="phtml")