CVE-2021-39163: Adding a private/unlisted room to a community exposes room metadata in an unauthorised manner.

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.