CVE-2026-27454 PoC — Discourse has check revision visibility on posts endpoint

Source

https://github.com/projectdiscovery/nuclei-templates/blob/main/http/cves/2026/CVE-2026-27454.yaml

Associated Vulnerability

Title: Discourse has check revision visibility on posts endpoint (CVE-2026-27454)
Description:Discourse is an open-source discussion platform. Prior to versions 2026.3.0-latest.1, 2026.2.1, and 2026.1.2, requesting /posts/:id.json?version=X bypassed authorization checks on post revisions. The display_post method called post.revert_to directly without verifying whether the revision was hidden or if the user had permission to view edit history. This meant hidden revisions (intentionally concealed by staff) could be read by any user by simply enumerating version numbers. Starting in versions 2026.3.0-latest.1, 2026.2.1, and 2026.1.2, Discourse looks up the PostRevision and call guardian.ensure_can_see! before reverting, consistent with how the /posts/:id/revisions/:revision endpoint already authorizes access. No known workarounds are available.

Description

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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