LowCVE-2021-39164

    Improper authorisation of /members discloses room membership to non-members (CVE-2021-39164)

    Published Sources: CVE.org, OSV

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    What is affected, what to do

    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.

    Update Matrix-org Synapse to version 1.41.1.

    Affected
    • Matrix-org Synapse lt *..1.41.1
    Fixed in
    • Matrix-org Synapse 1.41.1

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    3.1

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: NVD, CVSS v3

    3.1

    CVSS v3

    Severity Low on the scale from 0 to 10.

    Source: CVE.org, OSV

    1.5 %

    EPSS score

    Moderate — model estimates 1-10% exploitation likelihood.

    Source: FIRST EPSS, percentile 73.4

    Show all metrics
    Severity
    Low
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    58.58 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

    Read in glossary
    no public PoC known
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

    Read in glossary
    3.1 Methodology
    CVSS v3

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    3.1
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

    Read in glossary
    1.5 %
    Moderate — model estimates 1-10% exploitation likelihood.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

    Read in glossary
    73.4
    High — this CVE ranks above the median of all CVEs scored today (rank ≥ 36%).
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    —
    Published
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-200

    Detection rules

    No NEOSEC detection rule is available for this vulnerability yet.

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

    Read in glossary
    Handle the vulnerability in regular vulnerability handling under Art. 21(2)(e) NIS2.
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    Description

    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.

    Source: NVD (NIST), OSV (Open Source Vulnerabilities), cvelistv5

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    matrix-orgsynapse
    • < 1.41.1
    • 1.41.1
    matrixsynapse
    • < 1.41.1
    • 1.41.1
    pypimatrix-synapse
    • 1.36.0
    • 1.26.0rc2
    • 1.10.0rc2
    • 1.9.0.dev2
    • 1.31.0rc1
    • 1.12.0
    • 1.20.1
    • 0.99.5.2
    • 1.18.0rc2
    • 0.99.4rc1
    • 1.20.0rc5
    • 1.40.0rc3
    • 1.5.0rc1
    • 1.28.0
    • 1.10.0
    • 1.41.0
    • 1.21.0rc1
    • 1.22.0
    • 1.0.0
    • 1.40.0rc2
    • 1.12.4rc1
    • 1.7.1
    • 0.99.0
    • 1.19.2
    • 1.13.0
    • 1.3.0
    • 1.17.0rc1
    • 1.35.0rc3
    • 1.16.0rc2
    • 0.99.3
    • 1.39.0rc1
    • 1.22.1
    • 1.10.0rc5
    • 0.99.5.1
    • 0.34.1.1
    • 0.33.7rc1
    • 1.18.0
    • 0.99.2rc1
    • 1.4.1rc1
    • 1.21.0rc3
    • 1.21.2
    • 1.20.0rc1
    • 0.33.9
    • 0.33.7rc2
    • 1.7.0
    • 1.34.0
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    Vendors and products

    Affected operating systems

    Affected operating systems
    Otherfedoraprojectfedora34
    Otherfedoraprojectfedora35

    Weakness classes (CWE)

    • CWE-200Exposure of Sensitive Information to an Unauthorized Actor

      Class

      The product exposes sensitive information to an actor that is not explicitly authorized to have access to that information.

      Entry at MITRE CWE-200

    References & sources

    • github.commatrix-org/synapse/security/advisories/GHSA-3x4c-pq33-4w3qweb, x_refsource_CONFIRM, Third-party advisory
    • NVD (NIST)vuln/detail/CVE-2021-39164advisory
    • github.commatrix-org/synapse/commit/cb35df940aweb, x_refsource_MISC, Patch, Third-party advisory
    • github.commatrix-org/synapsepackage
    • github.commatrix-org/synapse/releases/tag/v1.41.1web, x_refsource_MISC, Release notes, Third-party advisory
    • github.compypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-202…web
    • lists.fedoraproject.orgarchives/list/package-announce@lists.fedoraproject.org/message/…web
    • lists.fedoraproject.orgarchives/list/package-announce@lists.fedoraproject.org/message/…web
    • lists.fedoraproject.orgFEDORA-2021-2e8ed15b14Vendor advisory, x_refsource_FEDORA
    • lists.fedoraproject.orgFEDORA-2021-f12fdca1bfVendor advisory, x_refsource_FEDORA
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