🛡️ CVE-2021-39163 — matrix-synapse

🟢 CVSS 2.0 — Low ✅ No Known Exploit CWE-200 OSV
2.0
CVSS Score
0 Low4 Medium7 High9 Critical10

Description

Adding a private/unlisted room to a community exposes room metadata in an unauthorised manner.

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.

How this vulnerability can be exploited

This issue can be reached over the network, attack complexity is high, an attacker needs low-level privileges on the target. No user interaction is required. Rated impact: confidentiality low, integrity none, availability none.

Weakness class

CVE-2021-39163 is classified as CWE-200: Exposure of Sensitive Information. Information that should stay internal is disclosed to someone who is not authorised to see it.

Affected software

CVE-2021-39163 is recorded against 1 package.

  • matrix-synapse (fixed in 1.41.1)

Timeline and source

Published on 1 September 2021 and last revised on 8 July 2026. No public exploit is currently recorded for this entry. A vendor advisory or fix has been published. Record sourced from OSV.

References

github.com (Web)
nvd.nist.gov (Advisory)
github.com (Web)
github.com (Package)
github.com (Web)
github.com (Web)
lists.fedoraproject.org (Web)
lists.fedoraproject.org (Web)

CVE-2021-39163 on other distributions

Each distribution ships its own build and its own fixed version. Pick the one you run:

Details

Severity LOW
CVSS Score 2.0
CVSS Vector CVSS:4.0/AV:N/AC:H/AT:N/PR:L/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N
CWE CWE-200
Public Exploit ✅ No
Source OSV
Published 2021-09-01
Updated 2026-08-12
Modified 2026-07-08

Affected Packages

Software From version Fixed in
matrix-synapse — 1.41.1

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