Which products and versions are affected by CVE-2021-21809?

Affected: Moodle versions 3.10.0, >= 3.10.0, >= Moodle 3.10; moodle/moodle (composer) versions >= 3.8.0. The full list is in the CVE brief.

Is CVE-2021-21809 being exploited in the wild?

It is not listed in any known-exploited vulnerabilities catalog.

Is CVE-2021-21809 in the CISA KEV catalog?

No, it is not currently listed in the CISA Known Exploited Vulnerabilities catalog.

Is there a public exploit for CVE-2021-21809?

Public exploit code is available.

Is there a fix for CVE-2021-21809?

Fixed versions are available, including Moodle 3.10.1.

How urgently should CVE-2021-21809 be patched?

Brandefense priority: patch immediately, because public exploit code exists for a high-severity flaw.

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Emerging threatsCritical

CVE-2021-21809Moodle OS Command Injection

Moodle command execution vulnerability exists in the default legacy spellchecker plugin

Published Oct 13, 2021 · Updated Sep 8, 2026 · Also known as BIT-moodle-2021-21809, GHSA-c7jj-vfmr-j9mj, UBUNTU-CVE-2021-21809

Key Facts

Patch immediately

CVE-2021-21809 is a critical-severity vulnerability (CVSS 9.1 v3.1) affecting Moodle, moodle/moodle (composer).
It is not listed in any known-exploited vulnerabilities catalog. Public exploit code is available.
EPSS estimates a 24% probability of exploitation in the next 30 days.
Fixed versions are available, including Moodle 3.10.1.
Brandefense priority: patch immediately, because public exploit code exists for a high-severity flaw.

Scores

CVSS v3.1

9.1

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

EPSS (30-day exploit likelihood)

24%

Higher than 97.8% of all scored CVEs

Exploit maturity

Weaponized

ATLAS score

60/100

Publication score, see How it works

Exploitation Verdict

KEV listed

Not listed

Federal due date

None

No federal due date

Ransomware use

Not reported

Attributed actors

None named

Exploit & Defence Guidance

Exploit maturity

Weaponized

Public exploit code

Proof-of-concept links

In-the-wild evidence

Reports of active use

Advisories And Exploit References

advisoryhttps://nvd.nist.gov/vuln/detail/CVE-2021-21809
advisoryhttps://ubuntu.com/security/CVE-2021-21809
advisoryhttps://www.cve.org/CVERecord?id=CVE-2021-21809
advisoryhttps://cxsecurity.com/issue/WLB-2021100060
advisoryhttps://cxsecurity.com/issue/WLB-2021100061
advisoryGHSA advisory

Description

A command execution vulnerability exists in the default legacy spellchecker plugin in Moodle 3.10. A specially crafted series of HTTP requests can lead to command execution. An attacker must have administrator privileges to exploit this vulnerabilities.

Affected Products

6 affected entries in total

Product	Affected versions	Fixed in
MoodleMoodle	3.10.0, >= 3.10.0, >= Moodle 3.10	3.10.1
moodle/moodlecomposer	>= 3.8.0	n/a

Patched Releases

Moodle

3.10.1

Timeline

Publicly disclosed
Oct 13, 2021
Last updated
Sep 8, 2026

Weakness Type

CWE-78 Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection')
CWE-732 Incorrect Permission Assignment for Critical Resource

References

8 more references in the full CVE brief

FAQ

Common Questions.

A command execution vulnerability exists in the default legacy spellchecker plugin in Moodle 3.10.
Affected: Moodle versions 3.10.0, >= 3.10.0, >= Moodle 3.10; moodle/moodle (composer) versions >= 3.8.0. The full list is in the CVE brief.
It is not listed in any known-exploited vulnerabilities catalog.
No, it is not currently listed in the CISA Known Exploited Vulnerabilities catalog.
Public exploit code is available.
Fixed versions are available, including Moodle 3.10.1.
Brandefense priority: patch immediately, because public exploit code exists for a high-severity flaw.

Is CVE-2021-21809 On Your Attack Surface?

Brandefense matches it against the products discovered on your perimeter and opens a finding the day a CVE like this is listed.