Improper authorisation of members discloses room membership to non-members

Exploitation

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

CVSS

3.1 · CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N

EPSS

• CVE-2021-39164: 1.45% estimated exploitation activity in the next 30 days; percentile 73.41 (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 membership (list of members, with their display names) of a room if they know the ID of the room. The vulnerability is limited to rooms with shared history visibility. Furthermore, the unauthorised user must be using an account on a vulnerable homeserver that is in the room. Patches Server administrators should upgrade to 1.41.1 or later. Workarounds Administrators of servers that use a reverse proxy could, with potentially unacceptable loss of functionality, block the following endpoints: /matrix/client/r0/rooms/{roomid}/members with at query parameter /matrix/client/unstable/rooms/{roomid}/members with at query parameter References n/a For more information If you have any questions or comments about this advisory, e-mail us at security@matrix.org.

Published by the source

1 Sep 2021, 18:25 UTC

Last modified by the source

8 Jul 2026, 06:28 UTC

Source last checked

26 Oct 2026, 13:17 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-3x4c-pq33-4w3q

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-3x4c-pq33-4w3q

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# Patch Next advisory brief: CVE-2021-39164

Advisory: Improper authorisation of members discloses room membership to non-members
IDs: CVE-2021-39164, GHSA-3x4c-pq33-4w3q
Record: https://patchnext.com/advisories/cve-2021-39164 (revision 2, 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 3.1)
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-3x4c-pq33-4w3q (OSV record GHSA-3x4c-pq33-4w3q lists affected ranges: PyPI matrix-synapse >=0 <1.41.1 (fixed))
- Verified: https://github.com/advisories/GHSA-3x4c-pq33-4w3q (GitHub-reviewed advisory GHSA-3x4c-pq33-4w3q lists affected ranges: PyPI matrix-synapse >=0 <1.41.1)

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