CVE-2021-21404
HIGH7.5EPSS 0.25%Crash due to malformed relay protocol message
Published: 5/21/2021Modified: 4/28/2026
Also known as:DEBIAN-CVE-2021-21404
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 packages (4)
- Bitnami/syncthingfrom 0, < 1.15.0
- Debian/syncthingfrom 0, < 1.12.1~ds1-3
- Go/github.com/syncthing/syncthingfrom 0, < 1.15.0
- Go/github.com/syncthing/syncthingfrom 0, < 1.15.0
CVSS scores
| Source | Version | Severity | Vector |
|---|---|---|---|
| osv | CVSS 3.1 | HIGH7.5 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H |
References (6)
- ADVISORYhttps://nvd.nist.gov/vuln/detail/CVE-2021-21404
- ADVISORYhttps://security-tracker.debian.org/tracker/CVE-2021-21404
- WEBhttps://github.com/syncthing/syncthing/commit/fb4fdaf4c0a79c22cad000c42ac1394e3ccb6a97
- WEBhttps://github.com/syncthing/syncthing/releases/tag/v1.15.0
- WEBhttps://github.com/syncthing/syncthing/security/advisories/GHSA-x462-89pf-6r5h
- WEBhttps://pkg.go.dev/github.com/syncthing/syncthing