CVE-2021-39164

medium

Published 2021-09-01 · Modified 2026-02-26

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CVSS v3

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CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N

CVSS v4 NEW

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not yet in upstream

VIR risk

5.5

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.

Predictions

Exploit likelihood

30%

Patch ETA

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Heuristic predictions, AS-IS, for prioritization only.

Mitigations

No mitigations published for this CVE yet.

The vendor-content worker queues fetches as references arrive (check back in a few minutes). Or — if you've already worked around this in production — publish your fix to the community-verified tier.

✚ Propose a mitigation on Community → Mitigations published via the community go through AI scoring + 2 human reviewers + 7-day silent objection window before landing here with source_tier=community-verified.

OS impact

Arch Fixed 1 release

Version	Status	Fixed in
—	Fixed	`1.41.1-1`

Debian Fixed 2 releases

Version	Status	Fixed in
sid	Fixed	`1.41.1-1`
forky	Fixed	`1.41.1-1`

Package impact

Ecosystem	Package	Vulnerable	Fixed
PyPI	matrix-synapse	`<1.41.1`	`1.41.1`
PyPI	matrix-synapse	`<cb35df940a828bc40b96daed997b5ad4c7842fd3\|\|<1.41.1`	`cb35df940a828bc40b96daed997b5ad4c7842fd3`

References

Community-verified mitigations for this CVE will appear above when contributors publish them.

Verify integrity in audit chain (admin only). AS-IS.