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Vulnerability · Published Sep 24, 2026 · Assigned by GitHub_M

CVE-2026-91134Discourse: Block post iframes whose encoded userinfo bypasses the allowed_iframes allowlist

Medium 5.4

In plain English

A medium-severity improper input validation in discourse from discourse. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 12% of all known vulnerabilities).

Severity (CVSS 3.1)5.4Medium
Exploit odds (30 days)0.2%top 87.9% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known

Official 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.

Affected products (as reported)

VendorProductVersions
discourse discourse < 2026.8.0>= 2026.7.0-latest, < 2026.7.2>= 2026.6.0-latest, < 2026.6.3>= 2026.1.0-latest, < 2026.1.8

Weakness: CWE-20 · Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N

References