CVE-2021-21404

Crash due to malformed relay protocol message

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.

Category

7.5
CVSS
Severity: High
CVSS 3.1 •
CVSS 2.0 •
EPSS 1.16% Top 25%
Third-Party Advisory github.com Third-Party Advisory github.com Third-Party Advisory github.com Third-Party Advisory go.dev
Affected: syncthing syncthing
Published at:
Updated at:

References

Link Tags
https://github.com/syncthing/syncthing/security/advisories/GHSA-x462-89pf-6r5h third party advisory
https://pkg.go.dev/github.com/syncthing/syncthing product third party advisory
https://github.com/syncthing/syncthing/commit/fb4fdaf4c0a79c22cad000c42ac1394e3ccb6a97 third party advisory patch
https://github.com/syncthing/syncthing/releases/tag/v1.15.0 third party advisory release notes

Frequently Asked Questions

What is the severity of CVE-2021-21404?
CVE-2021-21404 has been scored as a high severity vulnerability.
How to fix CVE-2021-21404?
To fix CVE-2021-21404, make sure you are using an up-to-date version of the affected component(s) by checking the vendor release notes. As for now, there are no other specific guidelines available.
Is CVE-2021-21404 being actively exploited in the wild?
It is possible that CVE-2021-21404 is being exploited or will be exploited in a near future based on public information. According to its EPSS score, there is a ~1% probability that this vulnerability will be exploited by malicious actors in the next 30 days.
What software or system is affected by CVE-2021-21404?
CVE-2021-21404 affects syncthing syncthing.
This platform uses data from the NIST NVD, MITRE CVE, MITRE CWE, First.org and CISA KEV but is not endorsed or certified by these entities. CVE is a registred trademark of the MITRE Corporation and the authoritative source of CVE content is MITRE's CVE web site. CWE is a registred trademark of the MITRE Corporation and the authoritative source of CWE content is MITRE's CWE web site.
© 2025 Under My Watch. All Rights Reserved.