What is CVE-2026-21631?

CVE-2026-21631 is a medium-severity vulnerability in Joomla Joomla\!, classified under Cross-site Scripting. CVSS score: 5.4/10. Published 2026-04-01.

How severe is CVE-2026-21631?

Medium severity. CVSS v3 base score is 5.4 out of 10.

XSS in Joomla Joomla\!

CVE-2026-21631

Lack of output escaping leads to a XSS vector in the multilingual associations component.

Vulnerability class: XSS (Cross-Site Scripting)

Published 2026-04-01 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 5.4 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N.

EPSS exploit prediction

EPSS: 0.002 (10.8th percentile), scored 2026-09-29.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 11th percentile — 10.8% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.002 · newest: 0.002 · change: 0.000

Affected products

Joomla Joomla\!
Joomla! Project Cms — versions 4.0.0-5.4.3, 6.0.0-6.0.3

Weakness classification (CWE)

CWE-79 — Cross-site Scripting

References

developer.joomla.org/security-centre/1029-20260303-core-xss-vector-in-com-assoc… (vendor-advisory, Vendor Advisory)
github.com/Shirshaw64p/security-advisories/tree/main/CVE-2026-21631 (Exploit, Third Party Advisory, third-party-advisory)

Frequently asked questions

What is CVE-2026-21631?: CVE-2026-21631 is a medium-severity vulnerability in Joomla Joomla\!, classified under Cross-site Scripting. CVSS score: 5.4/10. Published 2026-04-01.
How severe is CVE-2026-21631?: Medium severity. CVSS v3 base score is 5.4 out of 10.