Have exploitation attempts been observed for CVE-2021-20837?

Yes. Exploitation attempts have been reported for this vulnerability. Previdian lists CVE-2021-20837 as a known exploited vulnerability. Confidence is high.

What exploitation activity has Previdian observed?

Previdian has not yet recorded sensor-observed exploitation attempts for CVE-2021-20837. Our sensors continue to monitor for activity targeting this CVE.

No. CVE-2021-20837 is not currently listed in CISA KEV.

CVE-2021-20837 Exploitation Attempts Observed in the Wild — Movable Type

Q: Which versions are affected?

CVE-2021-20837 affects Six Apart Movable Type. Reported affected version: 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.

Decision summary

What security teams need to know first

Direct answers before the deeper technical record.

What it is

CVE-2021-20837 is an unauthenticated vulnerability affecting Six Apart Movable Type. Movable Type 7 r.5002 and earlier (Movable Type 7 Series), Movable Type 6.8.2 and earlier (Movable Type 6 Series), Movable Type...

Have exploitation attempts been observed?

Yes. Exploitation attempts have been reported for this vulnerability. Previdian lists it as a known exploited vulnerability. Also reported by third-party sources. Confidence is high.

Who is affected?

Six Apart / Movable Type 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 should we do?

Prioritize remediation. Validate affected assets and apply vendor fixes on an accelerated timeline.

Overview

Movable Type

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 allow remote attackers to execute arbitrary OS commands via unspecified vectors.

Note that all versions of Movable Type 4.0 or later including unsupported (End-of-Life, EOL) versions are also affected by this vulnerability.

Vendor Product Affected Status

Six Apart Ltd. Affected

Movable Type

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

Affected

Published: 26 Oct 2021
Exploitation Attempts Reported: 08 Jun 2026
Attack vector: Remote
Complexity: Low
Privileges: None
User interaction: None

CVE References

CVE Record CVE.org · CVE Record https://www.cve.org/CVERecord?id=CVE-2021-20837
movabletype.org/news/2021/10/mt-782-683-released.html movabletype.org · CVE Record https://movabletype.org/news/2021/10/mt-782-683-released.html
jvn.jp/en/jp/JVN41119755/index.html jvn.jp · CVE Record https://jvn.jp/en/jp/JVN41119755/index.html
packetstormsecurity.com/files/164705/Movable-Type-7-r.5002-XMLRPC-AP... packetstormsecurity.com · CVE Record http://packetstormsecurity.com/files/164705/Movable-Type-7-r.5002-XML...
packetstormsecurity.com/files/164818/Movable-Type-7-r.5002-XMLRPC-AP... packetstormsecurity.com · CVE Record http://packetstormsecurity.com/files/164818/Movable-Type-7-r.5002-XML...

Exploitation evidence

Evidence of exploitation attempts

Third-party attestation and first-party sensor observation are shown separately so teams can judge the evidence chain.

Exploitation attempts observed in the wild

The Shadowserver

Recorded 08 Jun 2026

A trusted third party reported exploitation attempts.

Proof of concept available

GitHub

Recorded 30 Oct 2021

Public scanner or PoC coverage increases practical exploitability.

Known exploited vulnerability sources

Per-source evidence links for KEV attestations are available through the Previdian Pro API.

Learn about Pro API access

Source	Added
The Shadowserver First	2025-06-13 00:00 UTC

Detection

Operational artifacts and observed signals

Make the evidence actionable in scanner, SOC, and edge-control workflows.

Scanner coverage

Nuclei template detected 25 Apr 2025.

View Nuclei template

Virtual patch status

No Previdian virtual patch is currently available. Future rules ship for ModSecurity, Cloudflare, and AWS WAF.

Learn about virtual patches →

Scanner Artifacts

Scanner and exploit-framework references linked to this CVE.

Scanner	Reference	Detected
Nuclei	https://github.com/projectdiscovery/nuclei-templates/blob/main/http/cves/2021/CVE-2021-20837.yaml	25 Apr 2025

Risk and context

Severity, weaknesses, and research context

CVSS v3.1

9.8 Critical

Potential damage if exploited. Separate from whether attackers are using it.

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

EPSS

88.1%

Estimated chance of exploitation in the next 30 days. Previdian's warning comes from evidence, not this score.

Weaknesses

Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection') CWE-78

All CVSS Scores

CVSS v3.1 9.8 Critical

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

CVSS v2.0 7.5 High

AV:N/AC:L/Au:N/C:P/I:P/A:P

Potential Proof of Concepts

These PoCs are unverified and could contain malware. Use at your own risk.

orangmuda/CVE-2021-20837

github · Created 2021-10-30 09:15:56 UTC · 21 stars · AI assessment 90%

XMLRPC - RCE in MovableTypePoC

MovableType - Remote Command Injection

nuclei · Created Unknown

Timeline

From disclosure to observed exploitation attempts

13 Jun 2025
00:00 UTC

Added to Previdian KEV Feed

High-confidence, third-party report of exploitation attempts
25 Apr 2025
00:00 UTC

Nuclei template available

Scanner coverage available
30 Oct 2021
09:15 UTC

Public PoC available

Public proof-of-concept code published
26 Oct 2021
05:15 UTC

CVE published

Vulnerability disclosed publicly
17 Dec 2020
00:00 UTC

CVE ID reserved

Identifier reserved by the CNA

Pro API

Automate this intelligence

Confidence, exploit status, sensor telemetry, PoCs, scanner integrations, mentions, and tags are available programmatically for VM, SOC, and CTI workflows.

Start 14-day Pro trial Read API docs

Evidence confidence and provenance
First-party sensor telemetry
PoC and scanner references
Affected versions and enrichment
Automation-ready JSON delivery

GET /api/v2/pro/kevs/CVE-2021-20837

Sample API response

{
  "cve_id": "CVE-2021-20837",
  "confidence": "High",
  "cvss_score": 9.8,
  "cvss_estimated": false,
  "epss_score": 0.88144,
  "exploit_status": {
    "exploited_in_the_wild": true,
    "active_exploitation_observed": false
  },
  "sensor_telemetry": { "attempts": 0, "sensors": 0 }
}

CVE-2021-20837

Movable Type

Tags

CVE References

Exploitation attempts observed in the wild

Proof of concept available

Known exploited vulnerability sources

Scanner coverage

Virtual patch status

Scanner Artifacts

Weaknesses

All CVSS Scores

Potential Proof of Concepts

Added to Previdian KEV Feed

Nuclei template available

Public PoC available

CVE published

CVE ID reserved

Automate this intelligence