{"id":"CVE-2021-21404","aliases":["BIT-syncthing-2021-21404","GHSA-x462-89pf-6r5h","GO-2022-0888"],"url":"https://o3.security/vulnerability/CVE-2021-21404","summary":"Crash due to malformed relay protocol message","details":"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.","published":"2021-04-06T20:15:13.490Z","modified":"2026-07-09T10:21:17.295396Z","cvss":{"score":7.5,"severity":"HIGH","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"},"epss":null,"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"Go","name":"github.com/syncthing/syncthing","fixedVersion":"1.15.0"}],"fix":{"url":"https://github.com/syncthing/syncthing/commit/fb4fdaf4c0a79c22cad000c42ac1394e3ccb6a97","label":"syncthing/syncthing@fb4fdaf"},"references":[{"type":"ADVISORY","url":"https://github.com/syncthing/syncthing/releases/tag/v1.15.0"},{"type":"ADVISORY","url":"https://github.com/syncthing/syncthing/security/advisories/GHSA-x462-89pf-6r5h"},{"type":"ADVISORY","url":"https://pkg.go.dev/github.com/syncthing/syncthing"},{"type":"FIX","url":"https://github.com/syncthing/syncthing/commit/fb4fdaf4c0a79c22cad000c42ac1394e3ccb6a97"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-07-09T10:21:17.295396Z"}}