Discourse is an open-source discussion platform. From versions 2026.1.0-latest to before 2026.1.3, 2026.2.0-latest to before 2026.2.2, and 2026.3.0-latest to before 2026.3.0, an attacker with the ability to create shared AI conversations could inject arbitrary HTML and JavaScript via crafted conversation titles. This payload would execute in the browser of any user viewing the onebox preview, potentially allowing session hijacking or unauthorized actions on behalf of the victim. This issue has been patched in versions 2026.1.3, 2026.2.2, and 2026.3.0.