What’s Vulnerable › Vulnerabilities

CVE-2021-21392: Open redirect via transitional IPv6 addresses on dual-stack networks

• Severity: medium (CVSS 6.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.9% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-04-12 · updated 2026-06-17
• Weakness: CWE-601

Description

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.

Affected products

• Fedoraproject Fedora: 34
• matrix-org synapse: before 1.28.0 — fixed in 1.28.0

How to fix

Upgrade to a fixed version: matrix-org synapse 1.28.0. Check the vendor advisory for mitigations.

References

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

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