CVE-2021-21404: Syncthing

High severity, CVSS 7.5. EPSS: 2% chance of exploitation in the next 30 days.

Syncthing is a continuous file synchronization program. In Syncthing before version 1.15.0, the relay server `strelaysrv` can be caused to crash and exit by sending a relay message with a negative length field. Similarly, Syncthing itself can crash for the same reason if given a malformed message from a malicious relay server when attempting to join the relay. Relay joins are essentially random (from a subset of low latency relays) and Syncthing will by default restart when crashing, at which point it's likely to pick another non-malicious relay. This flaw is fixed in version 1.15.0.

Affected products

• Syncthing Syncthing: before 1.15.0 (fixed in 1.15.0)

Published 2021-04-06. Last modified 2026-06-17.