rh#1999844

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.

https://github.com/matrix-org/synapse/security/advisories/GHSA-3x4c-pq33-4w3q
https://github.com/matrix-org/synapse/releases/tag/v1.41.1
https://github.com/matrix-org/synapse/commit/cb35df940a

References:
https://bugzilla.redhat.com/show_bug.cgi?id=1999844
http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2021-39164
http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-39164
https://github.com/matrix-org/synapse/security/advisories/GHSA-3x4c-pq33-4w3q
https://github.com/matrix-org/synapse/releases/tag/v1.41.1
https://github.com/matrix-org/synapse/commit/cb35df940a