{"id":"CVE-2021-39164","aliases":["GHSA-3x4c-pq33-4w3q","PYSEC-2021-425"],"title":"Improper authorisation of members discloses room membership to non-members","summary":"Improper authorisation of members discloses room membership to non-members","severity":"low","cvss":3.1,"cvssVector":"CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N","vendor":"matrix-synapse","product":"matrix-synapse","ecosystem":"pip","affected":["matrix-synapse < 1.41.1"],"patched":["matrix-synapse 1.41.1"],"published":"2021-09-01","updated":"2026-07-08","source":"OSV","sourceUrl":"https://osv.dev/vulnerability/GHSA-3x4c-pq33-4w3q","references":[{"url":"https://github.com/matrix-org/synapse/security/advisories/GHSA-3x4c-pq33-4w3q"},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2021-39164"},{"url":"https://github.com/matrix-org/synapse/commit/cb35df940a"},{"url":"https://github.com/matrix-org/synapse"},{"url":"https://github.com/matrix-org/synapse/releases/tag/v1.41.1"},{"url":"https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2021-425.yaml"},{"url":"https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/2VHDEPCZ22GJFMZCWA2XZAGPOEV72POF"},{"url":"https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/PXT7ID7DNBRN2TVTETU3SYQHJKEG6PXN"}],"tags":["osv","pip"],"epss":0.01457,"epssPercentile":0.72366,"ingestedAt":"2026-07-08T18:25:45.207Z","slug":"CVE-2021-39164","body":"## Overview\n\n### Impact\nUnauthorised users can access the membership (list of members, with their display names) of a room if they know the ID of the room. The vulnerability is limited to rooms with `shared` history visibility. Furthermore, the unauthorised user must be using an account on a vulnerable homeserver that is in the room.\n\n### Patches\nServer administrators should upgrade to 1.41.1 or later.\n\n### Workarounds\nAdministrators of servers that use a reverse proxy could, with potentially unacceptable loss of functionality, block the following endpoints:\n* `/_matrix/client/r0/rooms/{room_id}/members` with `at` query parameter\n* `/_matrix/client/unstable/rooms/{room_id}/members` with `at` query parameter\n\n### References\nn/a\n\n### For more information\nIf you have any questions or comments about this advisory, e-mail us at security@matrix.org.\n\n## Affected packages\n\n- `matrix-synapse < 1.41.1`\n\n## Remediation\n\nUpgrade to a patched release:\n\n- `matrix-synapse 1.41.1`","depth":"sunlit","depthScore":17,"depthScoreParts":{"impact":17.1,"likelihood":0.3,"exploitation":0,"ransomware":0},"changes":[]}