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A critical zero-day vulnerability has been discovered in Synapse, an open-source Matrix homeserver implementation. This flaw is actively being exploited in the wild and can lead to a denial-of-service condition.
What is Synapse and Matrix?
Synapse is an open-source Matrix homeserver implementation, developed and maintained by Element. Matrix is defined as the open standard for secure and interoperable real-time communications.
Vulnerability Details
- CVE-2025-30355 (CVSS 7.1): This vulnerability allows for a federation denial of service via malformed events.
Impact
A malicious server can craft specific events that, when received by a vulnerable Synapse server (up to version 1.127.0), prevent it from federating with other servers. This disrupts communication and can effectively isolate the affected server from the broader Matrix network. Importantly, this vulnerability is confirmed to be under active exploitation.
Affected Versions
Synapse versions up to 1.127.0 are vulnerable.
Patches
The vulnerability is fixed in Synapse version 1.127.1.
Workarounds
The following scenarios are not affected by this vulnerability:
- Closed federation environments consisting of trusted servers.
- Non-federating installations.
Recommendations
- Immediate Update: It is crucial for administrators of Synapse servers to upgrade to version 1.127.1 as soon as possible to mitigate the risk of exploitation.
- Monitoring: Closely monitor Synapse servers for any signs of unusual activity that might indicate an attempted exploit.
- Security Best Practices: Review and reinforce security best practices for server administration to minimize potential attack vectors.
