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TI-2021-04502 Aliases CVE-2021-21404GHSA-x462-89pf-6r5h

Crash due to malformed relay protocol message

Priority LOW 37CWE-20Published · Updated

Key facts

Attack vector
Network
Privileges required
None
User interaction
None
Attack complexity
Low
Impact type
—
Confidentiality · integrity · availability
None · None · High
Fix status
Fixed

Attack profile from CVSS 3.1 · CNA. Fix status from CNA and GitHub.

Summary

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.

Why it matters now

EPSS 0.0197 80th percentileScored 2026-11-07
Fix Fixed syncthing (syncthing): 1.15.0github.com/syncthing/syncthing (Go): 1.15.0

SSVC decision points

Not available: CISA has not published SSVC decision points for this record.

Affected products

VendorProductAffected versionsFixed inStatusSource
syncthingsyncthingbefore 1.15.01.15.0Patch availableCNA
—github.com/syncthing/syncthing (Go)before 1.15.01.15.0Patch availableGHSA-x462-89pf-6r5h
  • syncthing Vendorsyncthing Affected before 1.15.0 Fixed in 1.15.0 Patch available CNA
  • github.com/syncthing/syncthing (Go) Vendor— Affected before 1.15.0 Fixed in 1.15.0 Patch available GHSA-x462-89pf-6r5h

CPE configurations NVD · 1

CPEVersionsRoleVendor
cpe:2.3:a:syncthing:syncthing:*:*:*:*:*:*:*:*before 1.15.0Vulnerablesyncthing
  • cpe:2.3:a:syncthing:syncthing:*:*:*:*:*:*:*:* Versions before 1.15.0 Vulnerable ·syncthing

Remediation

  • Fix · CNA syncthing (syncthing) Fixed in 1.15.0
  • Fix · GHSA-x462-89pf-6r5h github.com/syncthing/syncthing (Go) Fixed in 1.15.0 Advisory →

Vendor assessments

Red Hat

Red Hat's page for this CVE →

Not affected: 1 product
  • Red Hat

Timeline

  1. 2026-10-02 Threatist record created from CVE-2021-21404
  2. 2026-10-03 Alias added: GHSA-x462-89pf-6r5h

References

Patch 1

Advisory 4

  • TI-2022-22809 Cross-site Scripting (XSS) in Web GUI in syncthing CVSS 3.1MEDIUM 5.4
  • TI-2017-14639 Syncthing version 0.14.33 and older is vulnerable to symlink traversal resulting in arbitrary file overwrite CVSS 3.0HIGH 7.5

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences