Improper Input Validation The advisory has been revoked - it doesn't affect any version of package syncthing (opens in a new tab)
Threat Intelligence
The probability is the direct output of the EPSS model, and conveys an overall sense of the threat of exploitation in the wild. The percentile measures the EPSS probability relative to all known EPSS scores. Note: This data is updated daily, relying on the latest available EPSS model version. Check out the EPSS documentation for more details.
Do your applications use this vulnerable package?
In a few clicks we can analyze your entire application and see what components are vulnerable in your application, and suggest you quick fixes.
Test your applications- Snyk IDSNYK-UBUNTU2510-SYNCTHING-15025283
- published17 Jan 2026
- disclosed6 Apr 2021
Introduced: 6 Apr 2021
CVE-2021-21404 (opens in a new tab)Amendment
The Ubuntu security team deemed this advisory irrelevant for Ubuntu:25.10.
NVD Description
Note: Versions mentioned in the description apply only to the upstream syncthing package and not the syncthing package as distributed by Ubuntu.
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.
References
- http://people.ubuntu.com/~ubuntu-security/cve/CVE-2021-21404
- https://github.com/syncthing/syncthing/security/advisories/GHSA-x462-89pf-6r5h
- https://github.com/syncthing/syncthing/commit/fb4fdaf4c0a79c22cad000c42ac1394e3ccb6a97
- https://github.com/syncthing/syncthing/releases/tag/v1.15.0
- https://pkg.go.dev/github.com/syncthing/syncthing