Movable Type CMS RCE flaw actively exploited
An OS command injection vulnerability in Movable Type's XMLRPC API is being actively exploited in the wild, and initial patches proved incomplete.
Sourceipa.go.jp — open the original
What to check
- Applies to
- The 1 CVE named below. Whether you run an affected version is not assessed: check the versions the source lists.
- Exploitation
- A public exploit is recorded for CVE-2021-20837 (templated); no exploitation in the wild is recorded.
IPA (Japan's information security agency) issued an alert regarding a critical OS command injection vulnerability in the XMLRPC API of Six Apart's Movable Type CMS, affecting all versions from 4.0 onward, including end-of-life releases. The flaw allows a remote, unauthenticated attacker to execute arbitrary OS commands on the underlying server, and the derivative product PowerCMS is also affected.
As of November 5, 2021, IPA confirmed active exploitation of this vulnerability in the wild, prompting urgent calls for organizations to apply vendor patches or mitigations immediately. On December 16, 2021, Six Apart disclosed that the fix released on October 20, 2021 was insufficient, requiring further remediation. IPA recommends applying updated patches from the vendor or, where patching is not immediately possible, applying a workaround to the mt-config.cgi configuration file to reduce risk.
Given the CVSS v3 base score of 9.8 (critical) and confirmed real-world exploitation against a widely used CMS platform, affected organizations should prioritize patching or mitigation without delay, particularly given the initial patch's incomplete remediation.
Mentioned in this report
Detection guidance
Web Server Process Spawning Shell Interpreter from Movable Type
A behavioural hunt for the technique in general, not a signature for the CVEs this report names.
Web server process (httpd, apache, nginx) spawning cmd.exe, bash, sh, or perl following a request to mt-xmlrpc.cgi or mt-config.cgi, indicating command execution via the CMS vulnerability. Auto-generated starting point — validate and tune in your environment before deploying. IOC matches can false-positive on shared infrastructure and decay as adversary infrastructure rotates.
title: Web Server Process Spawning Shell Interpreter from Movable Type
description: Detects web server processes (httpd, apache, nginx) spawning shell interpreters
or script engines immediately after handling requests to Movable Type CGI endpoints,
consistent with exploitation of the XMLRPC command injection flaw.
tags:
- attack.execution
- attack.t1059.001
logsource:
category: process_creation
product: windows
detection:
selection:
ParentImage|endswith:
- \httpd.exe
- \apache.exe
- \nginx.exe
- \apache2.exe
Image|endswith:
- \cmd.exe
- \powershell.exe
- \perl.exe
filter_legitimate:
CommandLine|contains:
- cgi-bin
- ScriptAlias
condition: selection and not filter_legitimate
falsepositives:
- Web server CGI scripts legitimately executing shell commands for site functionality
level: medium
id: 85d0c8dc-d330-5c1c-bd83-c6425b8d22ff
status: experimental
author: Vorant
Suspicious Perl or Python Execution from mt-config.cgi Access
A behavioural hunt for the technique in general, not a signature for the CVEs this report names.
Perl or Python interpreter spawned by web server process with command-line arguments suggesting OS command execution, correlated with recent HTTP requests to Movable Type CGI files. Auto-generated starting point — validate and tune in your environment before deploying. IOC matches can false-positive on shared infrastructure and decay as adversary infrastructure rotates.
title: Suspicious Perl or Python Execution from mt-config.cgi Access
description: Detects Perl or Python interpreters spawned by web server processes with
command-line patterns (backticks, system(), exec()) indicative of OS command injection
exploitation via Movable Type CGI endpoints.
tags:
- attack.execution
- attack.t1059.001
logsource:
category: process_creation
product: linux
detection:
selection:
ParentImage|contains:
- httpd
- apache2
- nginx
Image|endswith:
- /perl
- /python
- /python3
command_execution:
CommandLine|contains:
- system
- exec
- '`'
- subprocess
condition: selection and command_execution
falsepositives:
- Web application scripts legitimately executing system commands as part of normal
operation
level: medium
id: 3d750052-0557-5b87-bfe9-a0a37e7be4d1
status: experimental
author: Vorant
Behavioural rules are generated from public reporting — validate in your environment before deploying.
Source reporting: https://www.ipa.go.jp/archive/security/security-alert/2021/20211020-jvn.html
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