{"id":"BIT-discourse-2026-31869","summary":"Discourse: Composer mentions endpoint leaks hidden group membership through PM `allowed_names` check","details":"Discourse is an open-source discussion platform. Prior to versions 2026.3.0, 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, 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.","aliases":["CVE-2026-31869","GHSA-5f9h-vp7v-7vq5"],"modified":"2026-09-08T08:46:31.642409539Z","published":"2026-03-27T07:10:33.910Z","database_specific":{"cpes":["cpe:2.3:a:discourse:discourse:*:*:*:*:*:*:*:*"],"severity":"Medium"},"references":[{"type":"ADVISORY","url":"https://github.com/discourse/discourse/security/advisories/GHSA-5f9h-vp7v-7vq5"},{"type":"WEB","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-31869"}],"affected":[{"package":{"name":"discourse","ecosystem":"Bitnami","purl":"pkg:bitnami/discourse"},"ranges":[{"type":"SEMVER","events":[{"introduced":"2026.1.0"},{"fixed":"2026.1.2"},{"introduced":"2026.2.0"},{"fixed":"2026.2.1"},{"introduced":"2026.3.0"},{"fixed":"2026.3.0"}]}],"database_specific":{"source":"https://github.com/bitnami/vulndb/tree/main/data/discourse/BIT-discourse-2026-31869.json"},"severity":[{"type":"CVSS_V4","score":"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:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X"}]}],"schema_version":"1.9.0"}