CVE-2025-30355: Synapse vulnerable to federation denial of service via malformed events

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 1.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2025-03-27 · updated 2026-06-17
Weakness: CWE-20

Description

Synapse is an open source Matrix homeserver implementation. A malicious server can craft events which, when received, prevent Synapse version up to 1.127.0 from federating with other servers. The vulnerability has been exploited in the wild and has been fixed in Synapse v1.127.1. No known workarounds are available.

Affected products

matrix-org synapse: before 1.127.1 — fixed in 1.127.1

How to fix

Upgrade to a fixed version: matrix-org synapse 1.127.1. Check the vendor advisory for mitigations.

References

https://github.com/element-hq/synapse/commit/2277df2a1eb685f85040ef98fa21d41aa4cdd389
https://github.com/element-hq/synapse/releases/tag/v1.127.1
https://github.com/element-hq/synapse/security/advisories/GHSA-v56r-hwv5-mxg6

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