CVE-2020-21481
📋 TL;DR
CVE-2020-21481 is an arbitrary file upload vulnerability in RGCMS v1.06 that allows attackers to upload malicious .txt files that can later be renamed to PHP files, leading to remote code execution. This affects all systems running the vulnerable version of RGCMS content management software.
💻 Affected Systems
- RGCMS
📦 What is this software?
Rgcms by Rgcms Project
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise through remote code execution, allowing attackers to install backdoors, steal data, or pivot to other systems.
Likely Case
Webshell deployment leading to data theft, defacement, or use as part of a botnet.
If Mitigated
File uploads blocked or properly validated, preventing malicious file execution.
🎯 Exploit Status
Simple file upload bypass technique with publicly available proof-of-concept.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Unknown
Vendor Advisory: No official vendor advisory found
Restart Required: No
Instructions:
No official patch available. Consider upgrading to a newer version if available or implementing workarounds.
🔧 Temporary Workarounds
File Upload Restriction
allImplement strict file upload validation to block .txt files with PHP code or restrict uploads to trusted extensions only.
# Configure web server to block execution of uploaded files
# In .htaccess for Apache:
<FilesMatch "\.(txt|php)$">
Order Allow,Deny
Deny from all
</FilesMatch>
Web Application Firewall Rules
allDeploy WAF rules to detect and block file upload attempts with suspicious content.
# Example ModSecurity rule:
SecRule FILES_TMPNAMES "@rx \.txt$" \
"id:1001,phase:2,deny,msg:'Suspicious file upload attempt'"
🧯 If You Can't Patch
- Disable file upload functionality entirely in RGCMS
- Implement network segmentation to isolate the vulnerable system
🔍 How to Verify
Check if Vulnerable:
Check if running RGCMS v1.06 and test file upload functionality with crafted .txt files containing PHP code.Check Version:
Check RGCMS configuration files or admin panel for version information.Verify Fix Applied:
Attempt to upload a test .txt file with PHP code and verify it cannot be executed or renamed to .php.📡 Detection & Monitoring
Log Indicators:
- Unusual file uploads with .txt extension
- Files being renamed from .txt to .php
- Webshell creation in upload directories
Network Indicators:
- POST requests to upload endpoints with suspicious file content
- Unexpected outbound connections from web server
SIEM Query:
source="web_logs" AND (uri_path="*upload*" AND file_extension="txt") AND (response_code=200 OR response_code=201)