CVE-2026-31869: Exposure of Sensitive Information to an Unauthorized Actor

Discourse is an open-source discussion platform. Prior to versions 2026.3.0-latest.1, 2026.2.1, and 2026.1.2, the ComposerControllermentions endpoint reveals hidden group membership to any authenticated user who can message the group. By supplying allowednames referencing a hidden-membership grou...