CVE-2021-20837Known exploitationPublic exploitNetwork attack vector

Unauthenticated command execution via XMLRPC backtick evaluation

Published Oct 26, 2021 · Updated Aug 3, 2024

OS command injection in affected Movable Type releases through 7 r.5004 allows remote attackers to execute arbitrary commands via XMLRPC. The mt.handler_to_coderef method base64-decodes an attacker-controlled value and evaluates backtick-delimited content during Perl package resolution. The endpoint requires no authentication when mt-xmlrpc.cgi is exposed, and observed attacks placed unauthorized files in public web directories.

CVSS severity9.8
Critical
EPSS probability88.14%
Next 30 days · Sep 16, 2026
Known exploitationReported
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

OS command injection in affected Movable Type releases through 7 r.5004 allows remote attackers to execute arbitrary commands via XMLRPC. The mt.handler_to_coderef method base64-decodes an attacker-controlled value and evaluates backtick-delimited content during Perl package resolution. The endpoint requires no authentication when mt-xmlrpc.cgi is exposed, and observed attacks placed unauthorized files in public web directories.

The record

CVE
CVE-2021-20837
Published
Oct 26, 2021
Updated
Aug 3, 2024
Vendor
Six Apart Ltd.
Product
Movable Type
Classifications
CWE-78, T1059, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Oct 26, 2021CVE publishedPublication date reported by the CVE source.
  2. Nov 18, 2021Exploitation reportedAttacks placing suspicious files through vulnerable XMLRPC APIs
  3. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=Movable Type 7 r.5002 and earlier (Movable Type 7 Series), Movable Type 6.8.2 and earlier (Movable Type 6 Series), Movable Type Advanced 7 r.5002 and earlier (Movable Type Advanced 7 Series), Movable Type Advanced 6.8.2 and earlier (Movable Type Advanced 6 Series), Movable Type Premium 1.46 and earlier, and Movable Type Premium Advanced 1.46 and earlier

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Six Apart Ltd. · Movable Typeversion=Movable Type 7 r.5002 and earlier (Movable Type 7 Series), Movable Type 6.8.2 and earlier (Movable Type 6 Series), Movable Type Advanced 7 r.5002 and earlier (Movable Type Advanced 7 Series), Movable Type Advanced 6.8.2 and earlier (Movable Type Advanced 6 Series), Movable Type Premium 1.46 and earlier, and Movable Type Premium Advanced 1.46 and earlier

Published CVSS scores

9.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
7.5NIST NVDAV:N/AC:L/Au:N/C:P/I:P/A:P
7.5JPCERT/CCAV:N/AC:L/Au:N/C:P/I:P/A:P
9.8JPCERT/CCCVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationReported
Public exploitPublished

Weakness, pattern, technique

CWE-78Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection')
T1059Command and Scripting Interpreter
T1190Exploit Public-Facing Application

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Reported exploitation

Technologies

Your stack

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Your stack

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