How severe is CVE-2021-20185?

CVE-2021-20185 has a CVSS score of 5.3/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-20185?

CVE-2021-20185 affects the following packages: moodle/moodle (Packagist), moodle/moodle (Packagist), moodle/moodle (Packagist), moodle/moodle (Packagist). Ecosystems affected: Packagist.

How do I fix CVE-2021-20185?

Update moodle/moodle to 3.5.16 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-20185 is resolved across your whole dependency graph.

How do I detect CVE-2021-20185 in my Packagist dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for moodle/moodle. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.

How do I mitigate CVE-2021-20185 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

How does O3 Security protect against CVE-2021-20185?

O3 pinpoints whether CVE-2021-20185 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Is CVE-2021-20185 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-20185 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What type of vulnerability is CVE-2021-20185?

CVE-2021-20185 is classified as Uncontrolled Resource Consumption (CWE-400), CWE-770 (CWE-770). These weakness types describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-20185 published, and has it been updated?

CVE-2021-20185 was published on January 28, 2021 and was last updated on July 8, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐘 Packagist

CVE-2021-20185

MEDIUM

CVE-2021-20185 is a medium-severity (CVSS 5.3) Uncontrolled Resource Consumption vulnerability in moodle/moodle. O3 Security confirms whether CVE-2021-20185 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.

Moodle Client side denial of service via personal message

Also known asBIT-moodle-2021-20185GHSA-c3j6-33r4-89q3

Published

Jan 28, 2021

Updated

Jul 8, 2026

Affected

4 pkgs

Patched

4 / 4

Exploits

None indexed

Blast Radius

4 pkgs affected

🐘moodle/moodle🐘moodle/moodle🐘moodle/moodle🐘moodle/moodle

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Packagist packages — download data is not available via public APIs for these ecosystems.

Description

It was found in Moodle before version 3.10.1, 3.9.4, 3.8.7 and 3.5.16 that messaging did not impose a character limit when sending messages, which could result in client-side (browser) denial of service for users receiving very large messages.

Affected Packages

4 total 4 fixed

Ecosystem	Package	Vulnerable range	Fix
🐘Packagist	`moodle/moodle`	≥ 3.5&&< 3.5.16	3.5.16
🐘Packagist	`moodle/moodle`	≥ 3.8&&< 3.8.7	3.8.7
🐘Packagist	`moodle/moodle`	≥ 3.9&&< 3.9.4	3.9.4
🐘Packagist	`moodle/moodle`	≥ 3.10&&< 3.10.1	3.10.1

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for moodle/moodle. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update moodle/moodle to 3.5.16 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-20185 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2021-20185 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Tailored to CVE-2021-20185. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

O3 Security · Impact-Aware SCA

Is CVE-2021-20185 in your dependencies?

O3 detects CVE-2021-20185 across Packagist dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

Scan my dependencies How O3 SCA works