CVE-2026-91122: Discourse: Chat MessageBus delivers read-restricted messages to unauthorized users
Discourse is an open-source discussion platform. Prior to 2026.1.8, 2026.6.3, 2026.7.2, and 2026.8.0, the video placeholder component allowed crafted HTML to cause an attribute breakout and inject an attacker-controlled event handler. An authenticated user with default trust-level posting privileges could store the crafted placeholder in a post. When another user opened the post and clicked the video play overlay, the handler could execute arbitrary JavaScript in the viewer's session. Default Content Security Policy settings block inline event handlers, but instances with CSP disabled or relaxed could allow the script to read page content and make authenticated requests as the viewer. This issue is fixed in versions 2026.1.8, 2026.6.3, 2026.7.2, and 2026.8.0.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
Discourseto a version that resolves this vulnerability.Fixed in 2026.1.8 - Upgrade
Upgrade
Discourseto a version that resolves this vulnerability.Fixed in 2026.6.3 - Upgrade
Upgrade
Discourseto a version that resolves this vulnerability.Fixed in 2026.7.2 - Upgrade
Upgrade
Discourseto a version that resolves this vulnerability.Fixed in 2026.8.0
Event History
Frequently Asked Questions
Which deployments are realistically exposed to script execution?
Instances with Content Security Policy disabled or relaxed are exposed to arbitrary JavaScript execution in a viewer's session. Default CSP settings block inline event handlers, which prevents the described handler from running.
What does an attacker need to exploit this issue?
The attacker needs an authenticated Discourse account with default trust-level posting privileges. They must create a post containing a crafted video placeholder, and a victim must open that post and click its video play overlay.
What can be done if an upgrade cannot be applied immediately?
Ensure that Content Security Policy remains enabled and does not permit inline event handlers. This blocks the event-handler execution path described for the crafted placeholder.
Which versions contain the fix?
The issue is fixed in Discourse versions 2026.1.8, 2026.6.3, 2026.7.2, and 2026.8.0. Versions prior to those releases are affected.