Authorization bypass through user-controlled key in Discourse - CVE-2026-31869
Published: July 1, 2026
Vulnerability identifier: #VU136138
CSH Severity: Low
CVSS v4.0: CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N/E:U/U:Clear
CVE-ID: CVE-2026-31869
CWE-ID: CWE-639
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Vendor: Civilized Discourse Construction Kit, Inc.
Affected software:
Discourse
Discourse
Detailed vulnerability description
The vulnerability allows a remote user to disclose sensitive information.
The vulnerability exists due to improper authorization in ComposerController#mentions when handling requests with user-supplied allowed_names values. A remote user can supply a crafted allowed_names parameter and probe usernames to disclose sensitive information.
The issue allows inference of hidden group membership based on whether user_reasons returns "private" for a given user.
How to mitigate CVE-2026-31869
Install security update from vendor's website.