Discourse versions before 2026.1.2, 2026.2.1, and 2026.3.0-latest.1 contain an authorization bypass in PostsController#display_post. The controller calls post.revert_to(params[:version]) directly whenever a version query parameter is present, without checking whether the corresponding PostRevision is hidden or whether the caller has permission to view edit history. By requesting a post at its publicly known version number via GET /posts/:id.json?version=<public_version>, the next PostRevision's stored modifications are applied unconditionally. If staff have hidden that revision, its pre-edit content is returned to an unauthenticated caller. On patched installs the same request is rejected with 403 because guardian.ensure_can_see!(post_revision) is evaluated first.
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