CVE-2021-21392

    Dashboard / Vulnerabilities / CVE-2021-21392

    CVE-2021-21392

    Published: 12 Apr 2021Last Modified: 8 Jul 2026

    Summary:

    Details: Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is an ecosystem for open federated Instant Messaging and VoIP. In Synapse before version 1.28.0 requests to user provided domains were not restricted to external IP addresses when transitional IPv6 addresses were used. Outbound requests to federation, identity servers, when calculating the key validity for third-party invite events, sending push notifications, and generating URL previews are affected. This could cause Synapse to make requests to internal infrastructure on dual-stack networks. See referenced GitHub security advisory for details and workarounds.

    References: https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/, https://github.com/matrix-org/synapse/pull/9240, https://github.com/matrix-org/synapse/security/advisories/GHSA-5wrh-4jwv-5w78, https://pypi.org/project/matrix-synapse/

    Affected packages

    Package

    Name:

    Purl:

    Affected ranges

    Type: GIT

    Events:

    Introduced- 0
    Fixed -2756517f7a6e17d2403de44981569dc18329315b

    Affected versions

    v1.28.0rc1

    Common Vulnerability Scoring System

    Attack Vector
    Network
    Adjacent
    Local
    Physical
    Privileges Required
    None
    Low
    High
    User Interaction
    None
    Required
    Scope
    Unchanged
    Changed
    Confidentiality
    None
    Low
    High
    Integrity
    None
    Low
    High
    Availability
    None
    Low
    High
    CVE-2021-21392 | CVE-DB