{"id":"CVE-2021-39164","aliases":["GHSA-3x4c-pq33-4w3q","PYSEC-2021-425"],"url":"https://o3.security/vulnerability/CVE-2021-39164","summary":"Improper authorisation of members discloses room membership to non-members","details":"### 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.","published":"2021-08-31T17:15:08.267Z","modified":"2026-07-08T06:27:08.987263429Z","cvss":{"score":3.1,"severity":"LOW","vector":"CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N"},"epss":null,"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"PyPI","name":"matrix-synapse","fixedVersion":"1.41.1"}],"fix":{"url":"https://github.com/matrix-org/synapse/commit/cb35df940a","label":"matrix-org/synapse@cb35df9"},"references":[{"type":"WEB","url":"https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/2VHDEPCZ22GJFMZCWA2XZAGPOEV72POF/"},{"type":"WEB","url":"https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/PXT7ID7DNBRN2TVTETU3SYQHJKEG6PXN/"},{"type":"ADVISORY","url":"https://github.com/matrix-org/synapse/releases/tag/v1.41.1"},{"type":"ADVISORY","url":"https://github.com/matrix-org/synapse/security/advisories/GHSA-3x4c-pq33-4w3q"},{"type":"FIX","url":"https://github.com/matrix-org/synapse/commit/cb35df940a"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2021-39164"},{"type":"PACKAGE","url":"https://github.com/matrix-org/synapse"},{"type":"WEB","url":"https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2021-425.yaml"},{"type":"WEB","url":"https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/2VHDEPCZ22GJFMZCWA2XZAGPOEV72POF"},{"type":"WEB","url":"https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/PXT7ID7DNBRN2TVTETU3SYQHJKEG6PXN"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-07-08T06:27:08.987263429Z"}}