What is CVE-2021-39163?

CVE-2021-39163 is a low-severity vulnerability in Matrix Synapse, classified under Information Disclosure. CVSS score: 3.1/10. Published 2021-08-31.

How severe is CVE-2021-39163?

Low severity. CVSS v3 base score is 3.1 out of 10.

Auth bypass in Matrix Synapse

CVE-2021-39163

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`.

Vulnerability class: Information Disclosure

EPSS: 0.009 (55.9th percentile) — read the EPSS interpretation.

CVSS v3 metric

CVSS v3 base score 3.1 (Low). Vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N.

Affected products

Matrix Synapse
Matrix-org Synapse — versions < 1.41.1
Fedoraproject Fedora — versions 34, 35

Weakness classification (CWE)

References

security-advisories@github.com (x_refsource_CONFIRM, Third Party Advisory, Mitigation)
security-advisories@github.com (Patch, Third Party Advisory, x_refsource_MISC)
security-advisories@github.com (Third Party Advisory, x_refsource_MISC, Release Notes)
security-advisories@github.com (x_refsource_FEDORA, vendor-advisory)
security-advisories@github.com (x_refsource_FEDORA, vendor-advisory)

Frequently asked questions

What is CVE-2021-39163?: CVE-2021-39163 is a low-severity vulnerability in Matrix Synapse, classified under Information Disclosure. CVSS score: 3.1/10. Published 2021-08-31.
How severe is CVE-2021-39163?: Low severity. CVSS v3 base score is 3.1 out of 10.