Vulnerability · Published Mar 20, 2026 · Assigned by GitHub_M
CVE-2026-31869Discourse: Composer mentions endpoint leaks hidden group membership through PM `allowed_names` check
Medium 5.3
In plain English
A medium-severity information exposure in discourse from discourse. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 9% of all known vulnerabilities).
Official description
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.
Affected products (as reported)
Weakness: CWE-200, CWE-285, CWE-639 · 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
References
- https://github.com/discourse/discourse/security/advisories/GHSA-5f9h-vp7v-7vq5 x_refsource_CONFIRM