Security readout for executives and security teams
Plain-English summary
This is a privacy leak in Matrix Synapse. A non-member who knows a room ID could see that room's member list and display names, but only under narrow conditions involving shared history visibility and a vulnerable homeserver already in the room.
Executive priority
Treat as low priority unless Matrix rooms contain sensitive membership information or the organization operates public or federated Synapse infrastructure. Patch during the next normal maintenance window, sooner for sensitive deployments.
Technical view
Synapse versions before 1.41.1 improperly authorized /members lookups with an at query parameter. The issue is CWE-200 information exposure with CVSS 3.1 score 3.1. Impact is limited to confidentiality; sources do not indicate integrity or availability impact.
Likely exposure
Exposure is limited to Matrix Synapse homeservers older than 1.41.1, especially those participating in rooms using shared history visibility. The attacker also needs a valid account on a vulnerable homeserver in the room and the room ID.
Exploitation context
The source bundle does not show active exploitation, and KEV is false. Abuse requires prior knowledge and constrained access, so business urgency is mainly privacy and trust risk for sensitive Matrix rooms.
Researcher notes
Focus validation on Synapse version, room history visibility, and homeserver participation. The vulnerability is an authorization flaw causing member-list disclosure, not message disclosure according to the supplied sources.
Mitigation direction
- Upgrade Matrix Synapse to 1.41.1 or later.
- Use fixed Fedora packages where Synapse is installed from Fedora repositories.
- If upgrade is delayed, review the vendor's reverse proxy workaround.
- Block affected /members endpoints with at query parameter only after assessing functionality loss.
- Review sensitive rooms that use shared history visibility.
Validation and detection
- Inventory all Matrix Synapse homeservers and record their versions.
- Confirm each exposed Synapse instance is running 1.41.1 or later.
- Identify rooms using shared history visibility on vulnerable homeservers.
- Check whether vulnerable homeservers participate in sensitive rooms.
- Verify any reverse proxy workaround matches vendor-described endpoint conditions.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CWE-200: Information exposure and cloud metadata lookup
Information exposure and SSRF weaknesses can make discovery, cloud metadata, and credential material review relevant. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupCVE-2021-39164 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- Low
- CVSS
- 3.1 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N1.61.4Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
3.1LowVector: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N
Source materials
- CVE List V5 sourceCVE List V5
- https://github.com/matrix-org/synapse/commit/cb35df940aCVE reference · x_refsource_MISC
- https://github.com/matrix-org/synapse/releases/tag/v1.41.1CVE reference · x_refsource_MISC
- https://github.com/matrix-org/synapse/security/advisories/GHSA-3x4c-pq33-4w3qCVE reference · x_refsource_CONFIRM
- FEDORA-2021-2e8ed15b14CVE reference · vendor-advisory, x_refsource_FEDORA
- FEDORA-2021-f12fdca1bfCVE reference · vendor-advisory, x_refsource_FEDORA
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Exposure of Sensitive Information to an Unauthorized Actor
Exposure of Sensitive Information to an Unauthorized Actor represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.