How severe is CVE-2021-20837?

CVE-2021-20837 has a CVSS score of 9.8/10, rated CRITICAL. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-20837?

Affected package information for CVE-2021-20837 is not yet fully mapped. Check the official advisory links for details.

How do I fix CVE-2021-20837?

No fixed version has been published for CVE-2021-20837 yet. Monitor the advisory for updates, apply mitigations recommended by the vendor, and use O3 Security to track when a fix becomes available in your stack.

Is CVE-2021-20837 actively exploited in the wild?

Yes. There are 15 known exploit references for CVE-2021-20837, including 1 documented proof-of-concept exploit on Exploit-DB and 12 in-the-wild exploitations observed on GitHub and other sources. Treat this as actively exploitable and prioritize patching immediately. All exploit code should only be run in an isolated sandbox environment for research or authorized testing — never against production systems without explicit written authorization.

What is the EPSS score for CVE-2021-20837?

CVE-2021-20837 has an EPSS (Exploit Prediction Scoring System) score of 94.2%, placing it in the 100th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. A score above 50% indicates significantly elevated exploitation risk — immediate patching is strongly recommended.

What type of vulnerability is CVE-2021-20837?

CVE-2021-20837 is classified as OS Command Injection (CWE-78). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

Which products and versions are affected by CVE-2021-20837?

CVE-2021-20837 affects sixapart movable type. Check the affected-products list on this page against your deployed version to confirm exposure.

Is there exploit tooling available for CVE-2021-20837?

A Nuclei detection template exists for CVE-2021-20837, meaning automated scanners can fingerprint it. Public exploit tooling significantly increases real-world risk — prioritize patching and ensure detection coverage. Exploit tooling should only ever be used in isolated, authorized testing environments.

When was CVE-2021-20837 published, and has it been updated?

CVE-2021-20837 was published on October 26, 2021 and was last updated on November 21, 2024. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

CVE-2021-20837

CRITICAL

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…

Published

Oct 26, 2021

Updated

Nov 21, 2024

Affected

0 pkgs

Patched

None yet

Exploits

15 known

EPSS Exploitation Probability

via FIRST.org ↗

94.2%probability of exploitation in next 30 days

Very High Risk100th percentile0.00%

EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.

Description

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.

Affected Products

1 product · 7 configurations

Application

movable typesixapart

≥ 7.0 && ≤ 7.8.1

range

Exploits & PoCs

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

EDB-50464webappscgi✓ Verified

Movable Type 7 r.5002 - XMLRPC API OS Command Injection (Metasploit)

by Charl-Alexandre Le Brun · Oct 29, 2021

Exploit-DB Source

Frequently Asked Questions

O3 Security · Impact-Aware SCA

Is CVE-2021-20837 in your stack?

O3 detects CVE-2021-20837 across dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

Scan my stack How O3 SCA works

CVSS Score

9.8/10

CVSS v3 base score

Vector breakdown

AV:Attack Vector

Network

AC:Attack Complexity

Low

PR:Privileges Required

None

UI:User Interaction

None

S:Scope

Unchanged

C:Confidentiality

High

I:Integrity

High

A:Availability

High

Raw vector

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

CWE Classification

CWE-78OS Command Injection

Weaponization

Timeline

Disclosed

Oct 26, 2021

First exploit

Oct 29, 2021

3 days after disclosure — rapid exploitation

Last updated

Nov 21, 2024

Identifiers

CVE-2021-20837

References

Advisorypacketstormsecurity.com Advisoryjvn.jp Advisorymovabletype.org Advisorypacketstormsecurity.com Advisoryjvn.jp Advisorymovabletype.org Exploit / PoCpacketstormsecurity.com Exploit / PoCpacketstormsecurity.com

Data from OSV.dev, enriched by O3 Security. Refreshed hourly.