Impact

Unauthorised users can access the name, avatar, topic and number of members of a room if they know the ID of the room. This vulnerability is limited to homeservers where:

• the vulnerable homeserver is in the room; and
• untrusted users are permitted to create groups (communities).

By default, only homeserver administrators can create groups. However, homeserver administrators can already access this information in the database or using the admin API. As a result, only homeservers where the configuration setting enable_group_creation has been set to true are impacted.

Patches

Server administrators should upgrade to 1.41.1 or higher.

Workarounds

Server administrators can set enable_group_creation to false in their homeserver configuration (this is the default value) to prevent creation of groups by non-administrators.

Administrators that are using a reverse proxy could, with partial loss of group functionality, block the following endpoints:

• /_matrix/client/r0/groups/{group_id}/rooms
• /_matrix/client/unstable/groups/{group_id}/rooms

References

n/a

For more information

If you have any questions or comments about this advisory, e-mail us at security@matrix.org.