CVE-2021-20104
📋 TL;DR
Machform versions before 16 allow unauthenticated attackers to execute arbitrary code on the server by uploading malicious file attachments through forms. This affects all organizations using vulnerable Machform installations for form processing.
💻 Affected Systems
- Machform
📦 What is this software?
Machform by Machform
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise allowing attackers to install malware, steal data, pivot to internal networks, or establish persistent backdoors.
Likely Case
Webshell deployment leading to data theft, defacement, or use as part of a botnet for further attacks.
If Mitigated
Limited impact with proper network segmentation and file upload restrictions, potentially only affecting the web application.
🎯 Exploit Status
Exploitation requires sending specially crafted file uploads to upload.php without authentication.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Version 16
Vendor Advisory: https://www.machform.com/blog-machform-16-released/
Restart Required: No
Instructions:
1. Backup current installation and data. 2. Download Machform 16 from vendor. 3. Replace all files with new version. 4. Verify functionality of forms.
🔧 Temporary Workarounds
Restrict file upload types
allConfigure web server to block execution of uploaded files in upload directories
# Apache: Add to .htaccess in upload directory
<FilesMatch "\.(php|phtml|php3|php4|php5|phps|inc)">
Order Allow,Deny
Deny from all
</FilesMatch>
# Nginx: Add to server block
location ~* \.(php|phtml|php3|php4|php5|phps|inc)$ {
deny all;
}
Disable upload.php temporarily
linuxRename or remove upload.php file to prevent exploitation
mv upload.php upload.php.disabled
rm upload.php
🧯 If You Can't Patch
- Implement strict WAF rules to block malicious file upload patterns
- Isolate Machform server in DMZ with strict outbound traffic controls
🔍 How to Verify
Check if Vulnerable:
Check if upload.php exists and Machform version is below 16 in admin panel or version filesCheck Version:
Check /admin/ page or look for version in source filesVerify Fix Applied:
Confirm version shows 16 or higher in admin interface and test file upload functionality📡 Detection & Monitoring
Log Indicators:
- Unusual file uploads to upload.php with PHP extensions
- Multiple failed upload attempts with suspicious filenames
- POST requests to upload.php with executable content
Network Indicators:
- HTTP POST requests to /upload.php with file uploads
- Outbound connections from web server to unknown IPs after uploads
SIEM Query:
source="web_logs" AND uri="/upload.php" AND (method="POST" OR file_extension="php")