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CVE Record

CVE-2021-39163: Adding a private/unlisted room to a community exposes room metadata in an unauthorised manner.

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 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. Server administrators should upgrade to 1.41.1 or higher to patch the vulnerability. There are two potential 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 endpoints `/_matrix/client/r0/groups/{group_id}/rooms` and `/_matrix/client/unstable/groups/{group_id}/rooms`.

LowCVSS 3.1Not KEV-listedUpdated
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Glexia's TakeAutomated analysislow

Security readout for executives and security teams

Plain-English summary

CVE-2021-39163 is a limited information disclosure in Matrix Synapse. On affected servers, a non-admin user who can create groups and knows a room ID may see metadata for a private or unlisted room, including name, avatar, topic, and member count. The sources do not indicate message disclosure.

Executive priority

Treat as low urgency unless private room metadata is highly sensitive or group creation was enabled for untrusted users. Patch during the normal maintenance cycle, with faster action for public or federated deployments using non-default group settings.

Technical view

Synapse before 1.41.1 can expose room metadata through group/community room listing behavior. Impact requires the vulnerable homeserver to be in the room and `enable_group_creation` set to `true`, allowing untrusted users to create groups. The issue is classified as CWE-200 with CVSS 3.1 score 3.1.

Likely exposure

Exposure is narrow: Matrix Synapse deployments below 1.41.1 with non-default `enable_group_creation=true`. Default configurations, where only administrators can create groups, are not meaningfully impacted because administrators can already access this metadata.

Exploitation context

No KEV listing or cited source indicates active exploitation. Abuse requires a low-privileged user, knowledge of the target room ID, and a server configuration that allows untrusted group creation. The disclosed data is metadata, not message contents, integrity, or availability.

Researcher notes

The issue is an unauthorised metadata exposure, not a full room access flaw. Sources identify affected product as matrix-org Synapse before 1.41.1. Evidence does not support broader affected products, message disclosure, active exploitation, or impact beyond confidentiality of room metadata.

Mitigation direction

  • Upgrade Matrix Synapse to version 1.41.1 or later.
  • Set `enable_group_creation` to `false` unless non-admin group creation is required.
  • If upgrading is delayed, review vendor workarounds for reverse proxy endpoint blocking.
  • Apply relevant downstream distribution updates where Synapse is installed from packages.

Validation and detection

  • Inventory Synapse servers and confirm all are running 1.41.1 or later.
  • Check homeserver configuration for `enable_group_creation=true`.
  • Confirm group creation is restricted to administrators unless explicitly required.
  • Review access logs for unusual requests to the documented group room endpoints.
Prepared
Confidence
high
Sources
7

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

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

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cwe · medium confidence lookup

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.

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description · low confidence lookup

Database behavior lookup

The CVE wording references database injection or access, so collection and exfiltration review may help. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.

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cve · low confidence lookup

CVE-2021-39163 mapping review

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Vulnerability profileCVE Program record
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

Official CVE source material

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.

1CVSS vectors
0Timeline events
0ADP providers
6Source links

CVSS vector scores

1 official score

We 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.

ScoreVersionSeverityVectorExploitImpactSource
3.1CVSS 3.1LowCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N1.61.4Primary CVE score

Vulnerability scoring details

Base CVSS 3.1 score

3.1Low
CVSS 3.1 vector shape for CVE-2021-39163Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

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

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Privileges Required
NoneLowHigh
User Interaction
NoneRequired
Scope
ChangedUnchanged
Confidentiality Impact
HighLowNone
Integrity Impact
HighLowNone
Availability Impact
HighLowNone

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
matrix-orgsynapse< 1.41.1Listed
Weakness

CWE details

CWE-200

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.

CWE-200 · source CWE mapping

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.

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