Improper input validation in Discourse
CVE-2026-91134
Discourse is an open-source discussion platform. Prior to 2026.1.8, 2026.6.3, 2026.7.2, and 2026.8.0, the Discourse post sanitizer allowed a stored cross-origin iframe to bypass the allowed_iframes prefix policy when the iframe src contained encoded userinfo. The sanitizer validated a decoded form differently from the stored iframe src, allowing the browser to interpret an attacker-controlled host while the allowlist check accepted the encoded URL as an allowed prefix. An authenticated user with posting privileges could persist the iframe in a post and cause attacker-controlled cross-origin content to be rendered. This issue is fixed in versions 2026.1.8, 2026.6.3, 2026.7.2, and 2026.8.0.
Vulnerability class: Drupalgeddon 2 (CVE-2018-7600)
Published · last modified .
CVSS v3 metric
CVSS v3 base score 5.4 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N.
EPSS exploit prediction
No EPSS score from FIRST.org yet. New CVEs usually get one within a few days; no score means no signal yet, not low risk. How EPSS works.
Affected products
- Discourse — versions < 2026.8.0, >= 2026.1.0-latest, < 2026.1.8, >= 2026.6.0-latest, < 2026.6.3
Weakness classification (CWE)
References
- github.com/discourse/discourse/security/advisories/GHSA-54vw-chv3-wjpv
- github.com/discourse/discourse/pull/42882
- github.com/discourse/discourse/commit/0a8015e6c7a5079981843721494d30c9a91f5415
- github.com/discourse/discourse/commit/1304b0c04f87534053b0d574ae5a9b9ba13dd590
- github.com/discourse/discourse/commit/bac7dd1201911851462483271b4567246fa1309d
- github.com/discourse/discourse/commit/fed3a58c48412ce7207b72a4629ddbc44482cd64
Frequently asked questions
- What is CVE-2026-91134?
- CVE-2026-91134 is a medium-severity vulnerability in Discourse, classified under Improper Input Validation. CVSS score: 5.4/10. Published 2026-09-24.
- How severe is CVE-2026-91134?
- Medium severity. CVSS v3 base score is 5.4 out of 10.