How severe is CVE-2021-21809?

CVE-2021-21809 has a CVSS 3.x base score of 9.1, rated critical severity. It is exploitable over network with low attack complexity, requires high privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.

Is CVE-2021-21809 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 24% (98th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-21809?

CVE-2021-21809 affects Moodle. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2021-21809?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.

When was CVE-2021-21809 published?

CVE-2021-21809 was published on 2021-06-23 and last updated on 2026-06-17.

CVE-2021-21809

CVE-2021-21809 is a critical-severity vulnerability in Moodle with a CVSS 3.x base score of 9.1. Its EPSS exploit-prediction score of 24% places it in the 98th percentile, indicating an elevated likelihood of exploitation. The underlying weakness is classified as CWE-78.

Key facts

Severity: Critical (CVSS 3.x base score 9.1)
CVSS v2: 9.0
EPSS exploit prediction: 24% (98th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-78
Affected product: Moodle
Published: 2021-06-23
Last modified: 2026-06-17

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.

Frequently asked questions

What is CVE-2021-21809?: 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.
How severe is CVE-2021-21809?: CVE-2021-21809 has a CVSS 3.x base score of 9.1, rated critical severity. It is exploitable over network with low attack complexity, requires high privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.
Is CVE-2021-21809 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 24% (98th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-21809?: CVE-2021-21809 affects Moodle. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2021-21809?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
When was CVE-2021-21809 published?: CVE-2021-21809 was published on 2021-06-23 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:moodle:moodle:3.10.0:-:*:*:*:*:*:*

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