High
Security advisory
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.
- Products
- Not specified
- Affected versions
- Not specified
- Fixed versions
- Not specified
- CVEs
- CVE-2021-21404
- Confirmed fixed CVEs
- Not specified
- Updated in this feed
- 31 Oct 2026, 11:32 UTC · Revision 1
Source evidence
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.
osv:CVE-2021-21404
Complete source record
{
"id": "3754664660ada3273a5b8db6d5455f6780eaab542965f16c2002260d7f92023d",
"url": "https://osv.dev/vulnerability/CVE-2021-21404",
"method": "structured",
"payload": {
"kind": "advisory",
"title": "CVE-2021-21404",
"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.",
"evidence": "osv:CVE-2021-21404",
"products": [],
"severity": "high",
"structured": {
"id": "CVE-2021-21404",
"aliases": [
"BIT-syncthing-2021-21404",
"CVE-2021-21404",
"GHSA-x462-89pf-6r5h",
"GO-2022-0888"
],
"metrics": [
{
"type": "CVSS_V3",
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
}
],
"affected": [
{
"ranges": [
{
"repo": "https://github.com/syncthing/syncthing",
"type": "GIT",
"events": [
{
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},
{
"fixed": "fb4fdaf4c0a79c22cad000c42ac1394e3ccb6a97"
}
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"database_specific": {
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{
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"range": [
{
"introduced": "0"
},
{
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}
],
"source": "CPE_RANGE"
},
{
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}
],
"source": "REFERENCES_COMMIT"
},
{
"range": [
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"source": "REFERENCES_TAG",
"original_tag": "v1.15.0"
}
]
}
}
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"database_specific": {
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}
}
],
"modified": "2026-10-20T10:54:28.576956036Z",
"provider": "osv",
"references": [
"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",
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],
"provider_metadata": {}
},
"related_cves": [
{
"value": "CVE-2021-21404",
"evidence": "/id"
}
],
"security_relevant": true,
"severity_evidence": "/severity/0/score"
},
"source_id": "core-osv",
"document_id": 385423,
"published_at": "2021-04-06T20:15:13.49+00:00"
}