CVE-2026-31869: medium-severity vulnerability in discourse
Discourse: Composer mentions endpoint leaks hidden group membership through PM `allowed_names` check
Published
E-mail alert when it lands in KEV, gets a public PoC or its score goes up.
13Vexday Risk Score
No sign of exploitation. No public exploitation artifact known so far.
ssvc Trackcvss 5.3epss 0.2%
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.
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
Affected products
discourse · discourseRelated CVEs — discourse
In the same product, most dangerous first.
CVE-2024-53991HIGHPotential Backup file leaked via Nginx in DiscourseEPSS 29.9%CVE-2021-41163CRITICALRCE via malicious SNS subscription payloadEPSS 19.8%CVE-2023-45131HIGHUnauthenticated access to new private chat messages in DiscourseEPSS 4.0%CVE-2026-28227LOWDiscourse Vulnerable to Unauthorized Topic Creation in Staff-Only Categories via Topic Timer publish_to_categoryEPSS 3.3%CVE-2022-36066CRITICALDiscourse vulnerable to RCE via admins uploading maliciously zipped fileEPSS 2.1%CVE-2024-47773HIGHAnonymous cache poisoning via XHR requests in DiscourseEPSS 1.8%