What’s Vulnerable › Vulnerabilities

CVE-2021-21404: Crash due to malformed relay protocol message

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 2.0% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-04-06 · updated 2026-06-17
• Weakness: CWE-20

Description

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: before 1.15.0 — fixed in 1.15.0

How to fix

Upgrade to a fixed version: syncthing 1.15.0. Check the vendor advisory for mitigations.

References

• https://github.com/syncthing/syncthing/commit/fb4fdaf4c0a79c22cad000c42ac1394e3ccb6a97
• https://github.com/syncthing/syncthing/releases/tag/v1.15.0
• https://github.com/syncthing/syncthing/security/advisories/GHSA-x462-89pf-6r5h
• https://pkg.go.dev/github.com/syncthing/syncthing
• https://github.com/syncthing/syncthing/commit/fb4fdaf4c0a79c22cad000c42ac1394e3ccb6a97
• https://github.com/syncthing/syncthing/releases/tag/v1.15.0
• https://github.com/syncthing/syncthing/security/advisories/GHSA-x462-89pf-6r5h
• https://pkg.go.dev/github.com/syncthing/syncthing

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