Movable Type CMS RCE flaw actively exploited

15 December 2021 high vulnerability technology

An OS command injection vulnerability in Movable Type's XMLRPC API is being actively exploited in the wild, and initial patches proved incomplete.

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

Vulnerabilities CVE-2021-20837templated

Detection guidance

Movable Type XMLRPC API Command Injection

ATT&CK T1190

HTTP POST requests to Movable Type mt-xmlrpc.cgi endpoint with shell metacharacters or command substitution syntax in parameters, indicating exploitation of the OS command injection 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: Movable Type XMLRPC API Command Injection
description: Detects HTTP POST requests to Movable Type's vulnerable XMLRPC API endpoint
  (mt-xmlrpc.cgi) containing shell metacharacters or command substitution patterns
  indicative of OS command injection exploitation.
tags:
- attack.initial-access
- attack.t1190
logsource:
  category: proxy
  product: apache
detection:
  selection:
    http_method: POST
    uri|contains: mt-xmlrpc.cgi
  injection_patterns:
    http_request_body|contains:
    - '|'
    - '&'
    - $(
    - '`'
    - ;
    - \n
  condition: selection and injection_patterns
falsepositives:
- Legitimate XMLRPC API calls with special characters in XML payloads (rare; base64-encoding
  typical in legitimate calls)
level: high
id: 36d8d42a-9cbf-5d62-aebd-5be371f463fb
status: experimental
author: Vorant

Web Server Process Spawning Shell Interpreter from Movable Type

ATT&CK T1059.001

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

ATT&CK T1059.001

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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