CVE-2021-20279 is a medium-severity (CVSS 5.4) Cross-site Scripting (XSS) vulnerability in moodle/moodle. A fix is available for moodle/moodle — see the affected versions and patch details below.
Moodle contains Stored XSS via ID number user profile field
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2021-20279 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,156 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
moodle/moodle🐘moodle/moodle🐘moodle/moodle🐘moodle/moodleReal-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
The ID number user profile field required additional sanitizing to prevent a stored XSS risk in moodle before 3.10.2, 3.9.5, 3.8.8, 3.5.17.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐘Packagist | moodle/moodle | ≥ 3.10&&< 3.10.2 | 3.10.2composer require moodle/moodle:^3.10.2 |
| 🐘Packagist | moodle/moodle | ≥ 3.9&&< 3.9.5 | 3.9.5composer require moodle/moodle:^3.9.5 |
| 🐘Packagist | moodle/moodle | ≥ 3.8&&< 3.8.8 | 3.8.8composer require moodle/moodle:^3.8.8 |
| 🐘Packagist | moodle/moodle | ≥ 3.5&&< 3.5.17 | 3.5.17composer require moodle/moodle:^3.5.17 |
Detection & mitigation playbook
Open-source dependencyDetect
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 3.10.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-20279 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 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2021-20279 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2021-20279. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2021-20279 in your dependencies?
O3 Security finds CVE-2021-20279 across Packagist dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.