CVE-2021-20837
📋 TL;DR
CVE-2021-20837 is a critical remote command injection vulnerability in Movable Type's XMLRPC API that allows unauthenticated attackers to execute arbitrary operating system commands on affected servers. This affects all versions of Movable Type 4.0 and later, including unsupported end-of-life versions. The vulnerability has a CVSS score of 9.8, indicating critical severity.
💻 Affected Systems
- Movable Type 7 Series
- Movable Type 6 Series
- Movable Type Advanced 7 Series
- Movable Type Advanced 6 Series
- Movable Type Premium
- Movable Type Premium Advanced
📦 What is this software?
Movable Type by Sixapart
Movable Type by Sixapart
Movable Type by Sixapart
Movable Type by Sixapart
Movable Type by Sixapart
Movable Type by Sixapart
Movable Type by Sixapart
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise allowing attackers to execute arbitrary commands with the web server's privileges, potentially leading to data theft, ransomware deployment, or creation of persistent backdoors.
Likely Case
Remote code execution leading to website defacement, data exfiltration, or use of the server as part of a botnet.
If Mitigated
Limited impact if proper network segmentation, web application firewalls, and least-privilege configurations are in place.
🎯 Exploit Status
Multiple public exploit proofs-of-concept exist, and the vulnerability is actively exploited in the wild. Attack requires network access to the XMLRPC endpoint.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Movable Type 7.8.2, Movable Type 6.8.3, and corresponding versions for Advanced/Premium editions
Vendor Advisory: https://movabletype.org/news/2021/10/mt-782-683-released.html
Restart Required: Yes
Instructions:
1. Backup your Movable Type installation and database. 2. Download the patched version from the official Movable Type website. 3. Replace the affected files with the patched version. 4. Restart the web server. 5. Verify the patch is applied by checking the version.
🔧 Temporary Workarounds
Disable XMLRPC API
allTemporarily disable the vulnerable XMLRPC API endpoint if immediate patching is not possible.
# Edit your web server configuration to block access to XMLRPC
# For Apache: Add to .htaccess: <Files "xmlrpc.cgi"> Order deny,allow Deny from all </Files>
# For Nginx: Add to server block: location ~* /xmlrpc\.cgi$ { deny all; }
Network Access Control
linuxRestrict access to the XMLRPC endpoint using firewall rules.
# Example iptables rule to block external access to XMLRPC
iptables -A INPUT -p tcp --dport 80 -m string --string "POST /xmlrpc.cgi" --algo bm -j DROP
iptables -A INPUT -p tcp --dport 443 -m string --string "POST /xmlrpc.cgi" --algo bm -j DROP
🧯 If You Can't Patch
- Implement a web application firewall (WAF) with rules to block XMLRPC command injection attempts.
- Isolate the Movable Type server in a segmented network zone with strict egress filtering.
🔍 How to Verify
Check if Vulnerable:
Check if your Movable Type version is within the affected range by examining the version number in the admin interface or checking the mt-config.cgi file.Check Version:
Check the Movable Type admin dashboard or examine the mt-config.cgi file for version information.Verify Fix Applied:
Verify the version has been updated to 7.8.2 or 6.8.3 (or corresponding patched versions for Advanced/Premium) in the admin dashboard.📡 Detection & Monitoring
Log Indicators:
- Unusual POST requests to /xmlrpc.cgi with suspicious parameters
- Commands like 'system', 'exec', 'shell_exec' in web server logs
- Unexpected process execution from the web server user
Network Indicators:
- Unusual outbound connections from the web server to external IPs
- Spike in traffic to the XMLRPC endpoint
SIEM Query:
source="web_server_logs" AND (uri_path="/xmlrpc.cgi" AND (method="POST" AND (param="*system*" OR param="*exec*" OR param="*shell*")))🔗 References
- http://packetstormsecurity.com/files/164705/Movable-Type-7-r.5002-XMLRPC-API-Remote-Command-Injection.html
- http://packetstormsecurity.com/files/164818/Movable-Type-7-r.5002-XMLRPC-API-Remote-Command-Injection.html
- https://jvn.jp/en/jp/JVN41119755/index.html
- https://movabletype.org/news/2021/10/mt-782-683-released.html
- http://packetstormsecurity.com/files/164705/Movable-Type-7-r.5002-XMLRPC-API-Remote-Command-Injection.html
- http://packetstormsecurity.com/files/164818/Movable-Type-7-r.5002-XMLRPC-API-Remote-Command-Injection.html
- https://jvn.jp/en/jp/JVN41119755/index.html
- https://movabletype.org/news/2021/10/mt-782-683-released.html
