Improper authorisation of /members discloses room membership to non-members
Key facts
- Attack vector
- Network
- Privileges required
- Low
- User interaction
- None
- Attack complexity
- High
- Impact type
- Information disclosure, Security bypass
- Confidentiality · integrity · availability
- Low · None · None
- Fix status
- Fixed
Attack profile from CVSS 3.1 · CNA. Fix status from CNA and GitHub. Impact type from CWE-200 and CWE-863.
Summary
Matrix is an ecosystem for open federated Instant Messaging and Voice over IP. In versions 1.41.0 and prior, 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. Server administrators should upgrade to 1.41.1 or later in order to receive the patch. One workaround is available. Administrators of servers that use a reverse proxy could, with potentially unacceptable loss of functionality, block the endpoints: `/_matrix/client/r0/rooms/{room_id}/members` with `at` query parameter, and `/_matrix/client/unstable/rooms/{room_id}/members` with `at` query parameter.
Why it matters now
SSVC decision points
Not available: CISA has not published SSVC decision points for this record.
Affected products
| Vendor | Product | Affected versions | Fixed in | Status | Source |
|---|---|---|---|---|---|
| matrix-org | synapse | before 1.41.1 | 1.41.1 | Patch available | CNA |
| — | matrix-synapse (PyPI) | before 1.41.1 | 1.41.1 | Patch available | GHSA-3x4c-pq33-4w3q |
- synapse Vendormatrix-org Affected before 1.41.1 Fixed in 1.41.1 Patch available CNA
- matrix-synapse (PyPI) Vendor— Affected before 1.41.1 Fixed in 1.41.1 Patch available GHSA-3x4c-pq33-4w3q
CPE configurations NVD · 3
| CPE | Versions | Role | Vendor |
|---|---|---|---|
| cpe:2.3:a:matrix:synapse:*:*:*:*:*:*:*:* | before 1.41.1 | Vulnerable | matrix-org |
| cpe:2.3:o:fedoraproject:fedora:34:*:*:*:*:*:*:* | 34 | Vulnerable | Fedoraproject |
| cpe:2.3:o:fedoraproject:fedora:35:*:*:*:*:*:*:* | 35 | Vulnerable | Fedoraproject |
- cpe:2.3:a:matrix:synapse:*:*:*:*:*:*:*:* Versions before 1.41.1 Vulnerable ·matrix-org
- cpe:2.3:o:fedoraproject:fedora:34:*:*:*:*:*:*:* Versions 34 Vulnerable ·Fedoraproject
- cpe:2.3:o:fedoraproject:fedora:35:*:*:*:*:*:*:* Versions 35 Vulnerable ·Fedoraproject
Remediation
- Fix · CNA synapse (matrix-org) Fixed in 1.41.1
- Fix · GHSA-3x4c-pq33-4w3q matrix-synapse (PyPI) Fixed in 1.41.1 Advisory →
Fixed in the same release
Records fixed by the same vendor advisory or release, as the CNA lists them.
- Advisory ·
matrix-org synapse — FEDORA-2021-f12fdca1bf
2 records Highest priority Priority LOW 2 LOW
Other records, highest priority first
- TI-2021-16549 Adding a private/unlisted room to a community exposes room metadata in an unauthorised manner.Priority LOW 17
- Advisory ·
matrix-org synapse — FEDORA-2021-2e8ed15b14
2 records Highest priority Priority LOW 2 LOW
Other records, highest priority first
- TI-2021-16549 Adding a private/unlisted room to a community exposes room metadata in an unauthorised manner.Priority LOW 17
Vendor assessments
Timeline
- 2026-10-02 Threatist record created from CVE-2021-39164
- 2026-10-03 Alias added: GHSA-3x4c-pq33-4w3q
References
Advisory 5
Other 4
- https://github.com/matrix-org/synapse
- https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2021-425.yaml
- https://lists.fedoraproject.org/archives/list/[email protected]/message/2VHDEPCZ22GJFMZCWA2XZAGPOEV72POF
- https://lists.fedoraproject.org/archives/list/[email protected]/message/PXT7ID7DNBRN2TVTETU3SYQHJKEG6PXN
More matrix-org Synapse vulnerabilities
- TI-2023-15617 Synapse Outgoing federation to specific hosts can be disabled by sending malicious invites CVSS 3.1MEDIUM 5.0
- TI-2024-35327 Synapse allows a a malformed invite to break the invitee's `/sync` CVSS 4.0HIGH 8.7
- TI-2019-13571 Matrix Synapse before 1.5.0 mishandles signature checking on some federation APIs. CVSS 3.1CRITICAL 9.8
- TI-2024-36056 Synapse can be forced to thumbnail unexpected file formats, invoking external, potentially untrustworthy decoders CVSS 4.0HIGH 8.2
- TI-2024-35318 Synapse allows unsupported content types to lead to memory exhaustion CVSS 4.0HIGH 8.2
- TI-2018-11846 Matrix Synapse before 0.33.3.1 allows remote attackers to spoof events and possibly have… CVSS 3.0HIGH 8.8
Sources
- CVE-2021-39164 · CVE Program
- CVE-2021-39164 · NVD
- FIRST EPSS
- GHSA-3x4c-pq33-4w3q · GitHub Advisory Database · CC BY 4.0
- CVE-2021-39164 · Red Hat security data · CC BY 4.0
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