CVE-2026-31869

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

State: Published
Severity: 5.3 Medium
Published: Mar 20, 2026
Updated: Mar 20, 2026
Reserved: Mar 9, 2026

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.

Scores

CVSS	Score	Vector	Source
4.0	5.3 Medium	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	GitHub_M

Weaknesses

CWE-200 Exposure of Sensitive Information to an Unauthorized Actor
CWE-285 Improper Authorization
CWE-639 Authorization Bypass Through User-Controlled Key

Affected Products

Product	Versions
discoursediscourse	>= 2026.1.0-latest, < 2026.1.2Affected >= 2026.2.0-latest, < 2026.2.1Affected = 2026.3.0-latestAffected

References

https://github.com/discourse/discourse/security/advisories/GHSA-5f9h-vp7v-7vq5

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