---
id: CVE-2021-39164
aliases:
  - GHSA-3x4c-pq33-4w3q
  - PYSEC-2021-425
title: Improper authorisation of members discloses room membership to non-members
summary: Improper authorisation of members discloses room membership to non-members
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-3x4c-pq33-4w3q'
references:
  - url: >-
      https://github.com/matrix-org/synapse/security/advisories/GHSA-3x4c-pq33-4w3q
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2021-39164'
  - 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-2021-425.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: 0.01457
epssPercentile: 0.72406
ingestedAt: '2026-07-08T18:25:45.207Z'
---

## Overview

### 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/{room_id}/members` with `at` query parameter
* `/_matrix/client/unstable/rooms/{room_id}/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.

## Affected packages

- `matrix-synapse < 1.41.1`

## Remediation

Upgrade to a patched release:

- `matrix-synapse 1.41.1`