CVE-2021-21404

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.

• CWE-20

• https://github.com/syncthing/syncthing/commit/fb4fdaf4c0a79c22cad000c42ac1394e3ccb6a97Patch, Third Party Advisory
• https://github.com/syncthing/syncthing/releases/tag/v1.15.0Release Notes, Third Party Advisory
• https://github.com/syncthing/syncthing/security/advisories/GHSA-x462-89pf-6r5hThird Party Advisory
• https://pkg.go.dev/github.com/syncthing/syncthingProduct, Third Party Advisory
• https://github.com/syncthing/syncthing/commit/fb4fdaf4c0a79c22cad000c42ac1394e3ccb6a97Patch, Third Party Advisory
• https://github.com/syncthing/syncthing/releases/tag/v1.15.0Release Notes, Third Party Advisory
• https://github.com/syncthing/syncthing/security/advisories/GHSA-x462-89pf-6r5hThird Party Advisory
• https://pkg.go.dev/github.com/syncthing/syncthingProduct, Third Party Advisory

Published

Apr 6, 2021

Last Modified

Jun 17, 2026

Status

Modified

• How Strix found a critical auth bypass in etcdStrix autonomously discovered a critical authentication bypass in etcd, later designated CVE-2026-33413.
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