How severe is CVE-2021-36394?

CVE-2021-36394 has a CVSS score of 9.8/10, rated CRITICAL. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-36394?

CVE-2021-36394 affects the following packages: moodle/moodle (Packagist), moodle/moodle (Packagist), moodle/moodle (Packagist). Ecosystems affected: Packagist.

How do I fix CVE-2021-36394?

Update moodle/moodle to 3.11.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-36394 is resolved across your whole dependency graph.

How do I detect CVE-2021-36394 in my Packagist dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for moodle/moodle, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-36394 if there is no patch (or I can't update yet)?

Stop passing untrusted input into the interpreter or shell: call the affected binary with an argument array rather than a composed command string, reject anything outside a strict allowlist of expected values, and run the component under an account that cannot reach beyond the work it legitimately does.

Is CVE-2021-36394 actively exploited in the wild?

Yes. There are 4 known exploit references for CVE-2021-36394, including 2 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-2021-36394?

CVE-2021-36394 has an EPSS (Exploit Prediction Scoring System) score of 7.0%, placing it in the 94th 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. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-36394?

CVE-2021-36394 is classified as CWE-384 (CWE-384), Code Injection (CWE-94). These weakness types 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.

When was CVE-2021-36394 published, and has it been updated?

CVE-2021-36394 was published on March 6, 2023 and was last updated on October 24, 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

Not in CISA KEV

CRITICAL severity

CVE-2021-36394 — moodle

CRITICAL

CVE-2021-36394 is a critical-severity (CVSS 9.8) CWE-384 vulnerability in moodle/moodle. 4 public exploit or proof-of-concept references are indexed, so weaponization risk is real. A fix is available for moodle/moodle — see the affected versions and patch details below.

Moodle Session Fixation vulnerability

Also known asBIT-moodle-2021-36394GHSA-2563-fp9c-mgm8

Published

Mar 6, 2023

Updated

Oct 24, 2026

Affected

3 pkgs

Patched

3 / 3

Exploits

4 known

Exploitation data as of Oct 31, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

7.0%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs94th percentile — riskier than 94% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

3 pkgs affected

🐘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

In Moodle, a remote code execution risk was identified in the Shibboleth authentication plugin.

Affected Packages

3 total 3 fixed

Ecosystem	Package	Vulnerable range	Fix
🐘Packagist	`moodle/moodle`	≥ 3.11.0-beta&&< 3.11.1	3.11.1`composer require moodle/moodle:^3.11.1`
🐘Packagist	`moodle/moodle`	≥ 3.10.0-beta&&< 3.10.5	3.10.5`composer require moodle/moodle:^3.10.5`
🐘Packagist	`moodle/moodle`	all versions	3.9.8`composer require moodle/moodle:^3.9.8`

Affected Products

1 product · 3 configurations

Application

moodlemoodle

≥ 3.11.0 && < 3.11.1

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.

In Moodle, a remote code execution risk was identified in the Shibboleth…

github.comGithubAug 2021

Report on github.com

github.comGithubApr 2022

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, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update moodle/moodle to 3.11.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-36394 is resolved across your whole dependency graph.
Workarounds
Stop passing untrusted input into the interpreter or shell: call the affected binary with an argument array rather than a composed command string, reject anything outside a strict allowlist of expected values, and run the component under an account that cannot reach beyond the work it legitimately does.

Frequently Asked Questions

In Moodle, a remote code execution risk was identified in the Shibboleth authentication plugin.

O3 Security · Impact-Aware SCA

Is CVE-2021-36394 in your dependencies?

Find it across Packagist, including transitive dependencies.

Scan my dependencies How O3 SCA works

CVSS Score

9.8/10CVSS v3 base

Vector breakdown

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

None

Scope

Unchanged

Confidentiality

High

Integrity

High

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-384CWE-384 CWE-94Code Injection

Weaponization

No Nuclei template or Metasploit module yet. 4 public proof-of-concept references are listed on this page.

OpenSSF Scorecard

Package health score

moodle

3.4/10

Code-Review0/10

Maintained10/10

Branch-Protection0/10

Assessed Oct 26, 2026

Timeline

First exploit

Aug 28, 2021

Disclosed

Mar 6, 2023

Last updated

Oct 24, 2026

Identifiers

CVE-2021-36394

GHSA-2563-fp9c-mgm8

BIT-moodle-2021-36394

References

Advisorynvd.nist.gov · CVE 2021 36394 Fixmoodle.org · discuss Packagegithub.com · moodle

Related vulnerabilities

This CVE is critical severity — higher than any of the other 8 CVEs affecting moodle/moodle (highest there: high).

CVE-2026-26047CVE-2026-26045CVE-2025-67853CVE-2025-67850CVE-2025-67857CVE-2025-67856CVE-2025-67848CVE-2025-67849

Provenance

Sources

OSV.devNVDFIRST.org (EPSS)

Last verifiedOct 24, 2026

CVE-2021-36394 — moodle

EPSS Exploitation Probability

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

Description

Affected Packages

Affected Products

Detection & mitigation playbook

Detect

Fix

Workarounds

Frequently Asked Questions

Is CVE-2021-36394 in your dependencies?