What is CVE-2026-27166?

CVE-2026-27166 is a medium-severity vulnerability in Discourse, classified under Improper Neutralization of Script-Related HTML Tags in a Web Page (Basic XSS). CVSS score: 4.1/10. Published 2026-03-19.

How severe is CVE-2026-27166?

Medium severity. CVSS v3 base score is 4.1 out of 10.

XSS in Discourse

CVE-2026-27166

Discourse is an open source discussion platform. Prior to versions 2026.3.0-latest.1, 2026.2.1 and 2026.1.2, insufficient cleanup in the default Codepen allowed iframes value allows an attacker to trick a user into changing the URL of the main page. This issue has been fixed in versions 2026.3.0-latest.1, 2026.2.1 and 2026.1.2. To workaround this issue, remove Codepen from the list of allowed iframes.

Published 2026-03-19 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 4.1 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:N/I:L/A:N.

EPSS exploit prediction

EPSS: 0.003 (16.1th percentile), scored 2026-10-01.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 16th percentile — 16.1% 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: 0.003 · change: +0.001

Affected products

Discourse — versions < 2026.3.0-latest.1, >= 2026.1.0-latest, < 2026.1.2, >= 2026.2.0-latest, < 2026.2.1

Weakness classification (CWE)

CWE-80 — Improper Neutralization of Script-Related HTML Tags in a Web Page (Basic XSS)

References

github.com/discourse/discourse/security/advisories/GHSA-h653-cq78-vjj2 (Vendor Advisory)
github.com/discourse/discourse/commit/c4762a4fac2f61f52f325396b11d3aeb955ea925 (Patch)

Frequently asked questions

What is CVE-2026-27166?: CVE-2026-27166 is a medium-severity vulnerability in Discourse, classified under Improper Neutralization of Script-Related HTML Tags in a Web Page (Basic XSS). CVSS score: 4.1/10. Published 2026-03-19.
How severe is CVE-2026-27166?: Medium severity. CVSS v3 base score is 4.1 out of 10.