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

What is CVE-2021-39163?

CVE-2021-39163 is a low-severity vulnerability in Fedoraproject Fedora, published on 31 Aug 2021. Any signed-in user can exploit it over the network, with no action from the victim.

Official 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`.

Weakness: CWE-863, CWE-200

Which versions are affected by CVE-2021-39163?

3 open source packages ship an affected version, including matrix-synapse. It affects 2 products.

Products include Fedoraproject Fedora, Matrix Synapse.

How do I fix CVE-2021-39163?

Upgrade matrix-synapse to 1.41.1 or later.

Is CVE-2021-39163 being exploited?

There is no known exploitation of CVE-2021-39163 in the wild and no public exploit code.

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All Fedoraproject Fedora vulnerabilities

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as GHSA-jj53-8fmw-f2w2, PYSEC-2021-424, UBUNTU-CVE-2021-39163.