{"cve":"CVE-2021-21404","nvd":{"published":"2021-04-06T20:15:13.490","modified":"2026-06-17T03:35:30.003","status":"Modified","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.","cvss_v4":null,"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","score":7.5},"cwes":["CWE-20"],"references":["https://github.com/syncthing/syncthing/commit/fb4fdaf4c0a79c22cad000c42ac1394e3ccb6a97","https://github.com/syncthing/syncthing/releases/tag/v1.15.0","https://github.com/syncthing/syncthing/security/advisories/GHSA-x462-89pf-6r5h","https://pkg.go.dev/github.com/syncthing/syncthing","https://github.com/syncthing/syncthing/commit/fb4fdaf4c0a79c22cad000c42ac1394e3ccb6a97"],"products":["syncthing:syncthing"],"source":"nvd","synced_at":"2026-10-19T21:17:01.373894544+00:00"},"epss":{"cve":"CVE-2021-21404","score":0.0197,"percentile":0.80411,"date":"2026-10-31"},"kev":null,"osv":[{"id":"GHSA-x462-89pf-6r5h","url":"https://osv.dev/vulnerability/GHSA-x462-89pf-6r5h","summary":"Crash due to malformed relay protocol message","modified":"2026-07-08T06:30:05.046463672Z","packages":[{"ecosystem":"Go","name":"github.com/syncthing/syncthing","purl":"pkg:golang/github.com/syncthing/syncthing","ranges":["<1.15.0"],"fixed":["1.15.0"]}]},{"id":"GO-2022-0888","url":"https://osv.dev/vulnerability/GO-2022-0888","summary":"Crash due to malformed relay protocol message in github.com/syncthing/syncthing","modified":"2026-03-03T04:52:24.853224Z","packages":[{"ecosystem":"Go","name":"github.com/syncthing/syncthing","purl":"pkg:golang/github.com/syncthing/syncthing","ranges":["<1.15.0"],"fixed":["1.15.0"]}]}],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}