CVE-2026-31869CWE-200CWE-285CWE-639

Discourse: Composer mentions endpoint leaks hidden group membership through PM `allowed_names` check

Medium · published March 20, 2026

CVSS v4.0
5.3
EPSS
0%
Percentile
22.6
In the wild
Unconfirmed
What it is

Discourse is an open-source discussion platform. Prior to versions 2026.3.0-latest.1, 2026.2.1, and 2026.1.2, the ComposerController#mentions endpoint reveals hidden group membership to any authenticated user who can message the group. By supplying allowed_names referencing a hidden-membership group and probing arbitrary usernames, an attacker can infer membership based on whether user_reasons returns "private" for a given user. This bypasses group member-visibility controls. Versions 2026.3.0-latest.1, 2026.2.1, and 2026.1.2 contain a patch. To work around this issue, restrict the messageable policy of any hidden-membership group to staff or group members only, so untrusted users cannot reach the vulnerable code path.

The record
Technical detail
CVSS v4.0
5.3 · MEDIUM
Vector
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
EPSS
0.00310 · 22.6th percentile
Weaknesses
CWE-200 · Exposure of Sensitive Information to an Unauthorized Actor; CWE-285 · Improper Authorization; CWE-639 · Authorization Bypass Through User-Controlled Key
Published
2026-03-20T03:10Z
EPSS history
Timeline
  • 20 MAR 03:10Z
    Discourse: Composer mentions endpoint leaks hidden group membership through PM `allowed_names` check
    cvelistv5