CVE-2021-42643
📋 TL;DR
CVE-2021-42643 is an arbitrary file write vulnerability in cmseasy CMS that allows attackers to write PHP script files to the web server. This can lead to remote code execution by accessing the uploaded malicious files. All users running vulnerable versions of cmseasy are affected.
💻 Affected Systems
- cmseasy CMS
📦 What is this software?
Cmseasy by Cmseasy
⚠️ Risk & Real-World Impact
Worst Case
Complete server compromise with attacker gaining full control over the web server, data theft, and lateral movement to other systems.
Likely Case
Website defacement, malware deployment, data exfiltration, and creation of persistent backdoors.
If Mitigated
Limited impact if proper file upload restrictions and web application firewalls are in place.
🎯 Exploit Status
Exploitation requires knowledge of vulnerable endpoints but is straightforward once identified.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Later versions after V7.7.5_20211012
Vendor Advisory: Not publicly documented by vendor
Restart Required: No
Instructions:
1. Upgrade to the latest version of cmseasy CMS. 2. Apply any available security patches. 3. Verify file upload functionality has proper validation.
🔧 Temporary Workarounds
Restrict File Uploads
allDisable or restrict file upload functionality in cmseasy configuration
Modify cmseasy configuration to disable file uploads or restrict to specific file types
Web Application Firewall Rules
allImplement WAF rules to block malicious file upload attempts
Add WAF rules to block requests containing PHP file extensions in upload parameters
🧯 If You Can't Patch
- Implement strict file upload validation in web server configuration
- Deploy network segmentation to isolate the vulnerable system
🔍 How to Verify
Check if Vulnerable:
Check if running cmseasy version V7.7.5_20211012 or earlier by examining version files or admin panelCheck Version:
Check cmseasy version in admin panel or examine version.txt file in installation directoryVerify Fix Applied:
Verify upgraded to version after V7.7.5_20211012 and test file upload functionality with malicious payloads📡 Detection & Monitoring
Log Indicators:
- Unusual file uploads to cmseasy directories
- PHP file creation in unexpected locations
- Multiple failed upload attempts
Network Indicators:
- HTTP POST requests to cmseasy upload endpoints with PHP file extensions
- Unusual outbound connections from web server
SIEM Query:
source="web_logs" AND (uri_path="/cmseasy/upload" OR uri_path CONTAINS "upload") AND (file_extension="php" OR file_extension="phtml")🔗 References
- https://jdr2021.github.io/2021/10/14/CmsEasy_7.7.5_20211012%E5%AD%98%E5%9C%A8%E4%BB%BB%E6%84%8F%E6%96%87%E4%BB%B6%E5%86%99%E5%85%A5%E5%92%8C%E4%BB%BB%E6%84%8F%E6%96%87%E4%BB%B6%E8%AF%BB%E5%8F%96%E6%BC%8F%E6%B4%9E/
- https://jdr2021.github.io/2021/10/14/CmsEasy_7.7.5_20211012%E5%AD%98%E5%9C%A8%E4%BB%BB%E6%84%8F%E6%96%87%E4%BB%B6%E5%86%99%E5%85%A5%E5%92%8C%E4%BB%BB%E6%84%8F%E6%96%87%E4%BB%B6%E8%AF%BB%E5%8F%96%E6%BC%8F%E6%B4%9E/
