High
syncthing — 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
- syncthing
- Affected versions
- 0.14.36+ds1-1, 0.14.38+ds1-1, 0.14.43+ds1-5, 0.14.43+ds1-6, 1.1.4~ds1-4, 1.1.4~ds1-4ubuntu1, 1.1.4~ds1-4ubuntu1.1, 1.1.4~ds1-4ubuntu1.2, 1.12.1~ds1-4, 1.18.0~ds1-3, 1.18.0~ds1-3ubuntu0.1, 1.18.0~ds1-3ubuntu0.2, 1.18.0~ds1-3ubuntu0.3, 1.18.0~ds1-3ubuntu0.3+esm1, 1.18.0~ds1-3ubuntu0.3+esm2, 1.19.2~ds1-3, 1.27.2~ds4-1, 1.27.2~ds4-1ubuntu0.24.04.1, 1.27.2~ds4-1ubuntu0.24.04.2, 1.27.2~ds4-1ubuntu0.24.04.2+esm1, 1.27.2~ds4-1ubuntu0.24.04.3, 1.27.2~ds4-1ubuntu0.24.04.3+esm1, 1.27.2~ds4-1ubuntu0.24.04.3+esm2, 1.29.2~ds1-1, 1.29.5~ds1-2, 1.29.5~ds1-3, 1.29.5~ds1-3ubuntu0.26.04.1~esm1, 1.29.5~ds1-4
- Fixed versions
- Not specified
- CVEs
- CVE-2021-21404
- Confirmed fixed CVEs
- Not specified
- Updated in this feed
- 31 Oct 2026, 00:58 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:UBUNTU-CVE-2021-21404
syncthing
Fixed versions: Not specified
syncthing
Fixed versions: Not specified
syncthing
Fixed versions: Not specified
syncthing
Fixed versions: Not specified
syncthing
Fixed versions: Not specified
syncthing
Fixed versions: Not specified
syncthing
Fixed versions: Not specified
Complete source record
{
"id": "d873c5a04b034a6e56192a8c1ea94b12050ee8abe6995dd94aded93eb7609562",
"url": "https://osv.dev/vulnerability/UBUNTU-CVE-2021-21404",
"method": "structured",
"payload": {
"kind": "advisory",
"title": "UBUNTU-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:UBUNTU-CVE-2021-21404",
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{
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{
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{
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{
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"evidence": "/affected/6/package/name"
},
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"affected_versions": [
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},
{
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"evidence": "/affected/6/versions"
},
{
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],
"severity": "high",
"structured": {
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"aliases": [
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"metrics": [
{
"type": "CVSS_V3",
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
},
{
"type": "CVSS_V3",
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
},
{
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"affected": [
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],
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"ecosystem": "Ubuntu:18.04:LTS"
},
"versions": [
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"0.14.38+ds1-1",
"0.14.43+ds1-5",
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"database_specific": {
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],
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"1.18.0~ds1-3ubuntu0.2",
"1.18.0~ds1-3ubuntu0.3",
"1.18.0~ds1-3ubuntu0.3+esm1",
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],
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},
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{
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},
{
"binary_name": "syncthing",
"binary_version": "1.29.5~ds1-3ubuntu0.26.04.1~esm1"
},
{
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}
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],
"modified": "2026-10-17T12:56:19.654248313Z",
"provider": "osv",
"upstream": [
"CVE-2021-21404"
],
"references": [
"https://ubuntu.com/security/CVE-2021-21404",
"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://www.cve.org/CVERecord?id=CVE-2021-21404"
]
},
"related_cves": [
{
"value": "CVE-2021-21404",
"evidence": "/upstream"
}
],
"security_relevant": true,
"severity_evidence": "/severity/0/score"
},
"source_id": "core-osv",
"document_id": 355276,
"published_at": "2021-04-06T20:15:00+00:00"
}