What’s Vulnerable › Vulnerabilities

CVE-2021-39164: Improper authorisation of /members discloses room membership to non-members

• Severity: low (CVSS 3.1 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.5% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-08-31 · updated 2026-06-17
• Weakness: CWE-200

Description

Matrix is an ecosystem for open federated Instant Messaging and Voice over IP. In versions 1.41.0 and prior, unauthorised 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. Server administrators should upgrade to 1.41.1 or later in order to receive the patch. One workaround is available. Administrators of servers that use a reverse proxy could, with potentially unacceptable loss of functionality, block the endpoints: `/_matrix/client/r0/rooms/{room_id}/members` with `at` query parameter, and `/_matrix/client/unstable/rooms/{room_id}/members` with `at` query parameter.

Affected products

• Fedoraproject Fedora: 34, 35
• matrix-org synapse: before 1.41.1 — fixed in 1.41.1

How to fix

Upgrade to a fixed version: matrix-org synapse 1.41.1. Check the vendor advisory for mitigations.

References

• https://github.com/matrix-org/synapse/commit/cb35df940a
• https://github.com/matrix-org/synapse/releases/tag/v1.41.1
• https://github.com/matrix-org/synapse/security/advisories/GHSA-3x4c-pq33-4w3q
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/2VHDEPCZ22GJFMZCWA2XZAGPOEV72POF/
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/PXT7ID7DNBRN2TVTETU3SYQHJKEG6PXN/
• https://github.com/matrix-org/synapse/commit/cb35df940a
• https://github.com/matrix-org/synapse/releases/tag/v1.41.1
• https://github.com/matrix-org/synapse/security/advisories/GHSA-3x4c-pq33-4w3q
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/2VHDEPCZ22GJFMZCWA2XZAGPOEV72POF/
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/PXT7ID7DNBRN2TVTETU3SYQHJKEG6PXN/

Get alerted when CVE-2021-39164 or the next vulnerability affects your stack