Description
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.
References6
- github.com/discourse/discourse/commit/0a8015e6c7a50799818437214…
- github.com/discourse/discourse/commit/1304b0c04f87534053b0d574a…
- github.com/discourse/discourse/commit/bac7dd1201911851462483271…
- github.com/discourse/discourse/commit/fed3a58c48412ce7207b72a46…
- github.com/discourse/discourse/pull/42882
- github.com/discourse/discourse/security/advisories/GHSA-54vw-ch…
Metrics
- CVSS 3.1
-
5.4
MEDIUM
Score source: CNA
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N - EPSS — exploit probability
- 0% percentile: 12 · as of 9 November 2026
- CWE
- CWE-20
Could this be exploited in your environment?
A patch closes the known hole. A penetration test shows what an attacker can still reach — with proof.