# CVE-2021-20837: Command injection in Movable Type

> Remote attackers can execute arbitrary OS commands in Movable Type CMS, requiring immediate upgrades to secure the web application.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 26 Oct 2021
- **Updated:** 3 Aug 2024
- **Also known as:** GHSA-fq4q-8q83-6gj7

## What is CVE-2021-20837?

CVE-2021-20837 is a critical command injection vulnerability in Six Apart Ltd. Movable Type, published on 26 Oct 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-78 (Improper Neutralization of Special Elements used in an OS Command)

## Which versions are affected by CVE-2021-20837?

It affects 1 product.


Products: Sixapart Movable Type.

## How do I fix CVE-2021-20837?

No fix has been published for CVE-2021-20837 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-20837 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2021-20837 exists.

Public exploit code:

- XMLRPC  - RCE in MovableTypePoC (github_poc): https://github.com/orangmuda/CVE-2021-20837
- PoC for the CVE-2021-20837 : RCE in MovableType (github_poc): https://github.com/ghost-nemesis/cve-2021-20837-poc
- Movable Type 7 r.5002  - XMLRPC API OS Command Injection (Metasploit) (exploitdb)
- MovableType XMLRPC  - RCE (github_poc): https://github.com/Cosemz/CVE-2021-20837
- MovableType - Remote Command Injection (nuclei): http/cves/2021/CVE-2021-20837.yaml

## Other vulnerabilities in Six Apart Ltd. Movable Type

- [CVE-2009-0752](https://insights.sbomify.com/CVE-2009-0752/): Other in n/a (critical 10.0)
- [CVE-2022-38078](https://insights.sbomify.com/CVE-2022-38078/): Command injection in Movable Type XMLRPC API (critical 9.8)
- [CVE-2016-5742](https://insights.sbomify.com/CVE-2016-5742/): SQL injection in n/a (critical 9.8)
- [CVE-2026-25776](https://insights.sbomify.com/CVE-2026-25776/): Code injection in Movable Type (critical 9.3)
- [CVE-2020-5576](https://insights.sbomify.com/CVE-2020-5576/): Cross-site request forgery in Movable Type (high 8.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-20837 and fix it if it is affected.

What it is: CVE-2021-20837 is a critical command injection vulnerability in Six Apart Ltd. Movable Type, published on 26 Oct 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2021-20837 exists.

Affected product: Six Apart Ltd. Movable Type. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2021-20837/ (Markdown: https://insights.sbomify.com/CVE-2021-20837/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-20837
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-20837
- OSV: https://osv.dev/vulnerability/CVE-2021-20837

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-20837/
Summaries are generated and may contain mistakes; the official description above is the record.