CVE-2026-27166: Discourse vulnerable to HTML injection via prohibited iframe URLs

Severity: medium (CVSS 5.4 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-03-19 · updated 2026-06-17
Weakness: CWE-80

Description

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.

Affected products

discourse: before 2026.3.0; from 2026.1.0 before 2026.1.2; from 2026.2.0 before 2026.2.1; 2026.3.0 — fixed in 2026.1.2, 2026.2.1, 2026.3.0

How to fix

Upgrade to a fixed version: discourse 2026.1.2, 2026.2.1, 2026.3.0. Check the vendor advisory for mitigations.

References

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