What is CVE-2021-21404?

CVE-2021-21404 is a high-severity vulnerability in Syncthing, classified under Improper Input Validation. CVSS score: 7.5/10. Published 2021-04-06.

How severe is CVE-2021-21404?

High severity. CVSS v3 base score is 7.5 out of 10.

Is CVE-2021-21404 known to be exploited?

1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.

Improper input validation in Syncthing

CVE-2021-21404

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.

Vulnerability class: Drupalgeddon 2 (CVE-2018-7600)

Published 2021-04-06 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 7.5 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.

EPSS exploit prediction

EPSS: 0.020 (79.8th percentile), scored 2026-10-07.

Low probability of exploitation in the next 30 days, but still tracked. 80th percentile — 79.8% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.020 · newest: 0.020 · change: 0.000

Affected products

Syncthing — versions < 1.15.0

Weakness classification (CWE)

CWE-20 — Improper Input Validation

Public proof-of-concept exploits

sustsoft/syncthing-broad

References

github.com/syncthing/syncthing/security/advisories/GHSA-x462-89pf-6r5h (Third Party Advisory)
pkg.go.dev/github.com/syncthing/syncthing (Product, Third Party Advisory)
github.com/syncthing/syncthing/commit/fb4fdaf4c0a79c22cad000c42ac1394e3ccb6a97 (Patch, Third Party Advisory)
github.com/syncthing/syncthing/releases/tag/v1.15.0 (Third Party Advisory, Release Notes)

Frequently asked questions

What is CVE-2021-21404?: CVE-2021-21404 is a high-severity vulnerability in Syncthing, classified under Improper Input Validation. CVSS score: 7.5/10. Published 2021-04-06.
How severe is CVE-2021-21404?: High severity. CVSS v3 base score is 7.5 out of 10.
Is CVE-2021-21404 known to be exploited?: 1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.