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

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

2.3 · 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 · score source

EPSS

• CVE-2021-39163: 0.90% estimated exploitation activity in the next 30 days; percentile 59.61 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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 enablegroupcreation has been set to true are impacted. Patches Server administrators should upgrade to 1.41.1 or higher. Workarounds Server administrators can set enablegroupcreation 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/{groupid}/rooms…

Published by the source

1 Sep 2021, 18:25 UTC

Last modified by the source

8 Jul 2026, 06:29 UTC

Source last checked

26 Oct 2026, 13:17 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-jj53-8fmw-f2w2

2. Revision 2 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-jj53-8fmw-f2w2

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# Patch Next advisory brief: CVE-2021-39163

Advisory: Adding a private/unlisted room to a community exposes room metadata in an unauthorised manner.
IDs: CVE-2021-39163, GHSA-jj53-8fmw-f2w2
Record: https://patchnext.com/advisories/cve-2021-39163 (revision 3, source checked 26 Oct 2026, 13:17 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Low (CVSS 2.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: matrix-synapse 1.41.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI matrix-synapse: affected < 1.41.1; fixed in 1.41.1

## Suggested inspection (read-only)
- `python -m pip show matrix-synapse`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on matrix-synapse.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-jj53-8fmw-f2w2 (OSV record GHSA-jj53-8fmw-f2w2 lists affected ranges: PyPI matrix-synapse >=0 <1.41.1 (fixed))
- Verified: https://github.com/advisories/GHSA-jj53-8fmw-f2w2 (GitHub-reviewed advisory GHSA-jj53-8fmw-f2w2 lists affected ranges: PyPI matrix-synapse >=0 <1.41.1)

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