CVE-2021-21404

Name: CVE-2021-21404
Creator: NVD / NIST
Published: 2021-04-06T20:15:13.49+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

HIGHCVSS 7.5/10EPSS 1.97%

Last modified Jun 17, 2026

CVE-2021-21404 is a high-severity vulnerability rated 7.5/10 on the CVSS scale. 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. EPSS estimates a 1.97% chance of exploitation in the next 30 days.

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.

Metrics

CVSS 3.1

7.5/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS Probability

1.97%

77.9th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-20

Affected Software

Vendor	Product	Versions
Syncthing	Syncthing	< 1.15.0

References

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

Timeline

Published: Apr 6, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-21404?

How severe is CVE-2021-21404?

CVE-2021-21404 has a CVSS score of 7.5/10 (HIGH severity). The EPSS model estimates a 1.97% probability of exploitation in the next 30 days.

How do I fix CVE-2021-21404?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2021-21404?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST