---
id: CVE-2021-39163
aliases:
  - GHSA-jj53-8fmw-f2w2
  - PYSEC-2021-424
title: >-
  Adding a private/unlisted room to a community exposes room metadata in an
  unauthorised manner.
summary: >-
  Adding a private/unlisted room to a community exposes room metadata in an
  unauthorised manner.
severity: low
cvss: 3.1
cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N'
vendor: matrix-synapse
product: matrix-synapse
ecosystem: pip
affected:
  - matrix-synapse < 1.41.1
patched:
  - matrix-synapse 1.41.1
published: '2021-09-01'
updated: '2026-07-08'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/GHSA-jj53-8fmw-f2w2'
references:
  - url: >-
      https://github.com/matrix-org/synapse/security/advisories/GHSA-jj53-8fmw-f2w2
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2021-39163'
  - url: 'https://github.com/matrix-org/synapse/commit/cb35df940a'
  - url: 'https://github.com/matrix-org/synapse'
  - url: 'https://github.com/matrix-org/synapse/releases/tag/v1.41.1'
  - url: >-
      https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2021-424.yaml
  - url: >-
      https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/2VHDEPCZ22GJFMZCWA2XZAGPOEV72POF
  - url: >-
      https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/PXT7ID7DNBRN2TVTETU3SYQHJKEG6PXN
tags:
  - osv
  - pip
epss: 0.00896
epssPercentile: 0.57868
ingestedAt: '2026-07-08T18:25:50.814Z'
---

## Overview

### 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.


## Affected packages

- `matrix-synapse < 1.41.1`

## Remediation

Upgrade to a patched release:

- `matrix-synapse 1.41.1`