CVE-2021-39163
Adding a private/unlisted room to a community exposes room metadata in an unauthorised manner.
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 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. Server administrators should upgrade to 1.41.1 or higher to patch the vulnerability. There are two potential 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 endpoints `/_matrix/client/r0/groups/{group_id}/rooms` and `/_matrix/client/unstable/groups/{group_id}/rooms`.
How to fix CVE-2021-39163
To remediate CVE-2021-39163, upgrade the affected package to a fixed version below.
- —upgrade to 1.41.1-1 or later
- —upgrade to 1.41.1 or later
- —upgrade to cb35df940a828bc40b96daed997b5ad4c7842fd3 or later
Is CVE-2021-39163 being exploited?
Low — EPSS is 0.2%, meaning exploitation activity has not been observed at scale.
Affected packages (3)
- from 0, < 1.41.1-1