CVE-2021-21392
Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse).
Does this matter?
Lower severity and a low EPSS score (0.94%). Track it; it rarely justifies an emergency change on its own.
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.
- CVSS 3.1
- 6.3 MEDIUMCVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:H/A:N
- EPSS
- 0.94% probability · 61th percentile
- Public exploits
- None in Exploit-DB
- CISA KEV
- Not listed
- Weakness
- CWE-601
- Affected
- matrix/synapse · fedoraproject/fedora
- Source
- security-advisories@github.com
EPSS 0.9% since 26 October 2026; no change recorded yet. Points are recorded when the score first appears and whenever it moves by a percentage point or more.
References
- https://github.com/matrix-org/synapse/pull/9240Patch, Third Party Advisory
- https://github.com/matrix-org/synapse/security/advisories/GHSA-5wrh-4jwv-5w78Patch, Third Party Advisory
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/
- https://pypi.org/project/matrix-synapse/Product, Third Party Advisory
- https://github.com/matrix-org/synapse/pull/9240Patch, Third Party Advisory
- https://github.com/matrix-org/synapse/security/advisories/GHSA-5wrh-4jwv-5w78Patch, Third Party Advisory
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/
- https://pypi.org/project/matrix-synapse/Product, Third Party Advisory
Source: NVD record, EPSS from FIRST.org, KEV from CISA, exploits from Exploit-DB. Refreshed daily. Download this record as JSON.