CVE-2021-21404

medium

Published 2021-05-21 · Modified 2026-03-03

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CVSS v3

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CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS v4 NEW

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not yet in upstream

VIR risk

5.5

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.

Predictions

Exploit likelihood

30%

Patch ETA

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Heuristic predictions, AS-IS, for prioritization only.

Mitigations

No mitigations published for this CVE yet.

The vendor-content worker queues fetches as references arrive (check back in a few minutes). Or — if you've already worked around this in production — publish your fix to the community-verified tier.

✚ Propose a mitigation on Community → Mitigations published via the community go through AI scoring + 2 human reviewers + 7-day silent objection window before landing here with source_tier=community-verified.

OS impact

OS	Version	Status	Fixed in
arch		fixed	`1.15.1-1`
debian	bookworm	fixed	`1.12.1~ds1-3`
debian	bullseye	fixed	`1.12.1~ds1-3`
debian	forky	fixed	`1.12.1~ds1-3`
debian	sid	fixed	`1.12.1~ds1-3`
debian	trixie	fixed	`1.12.1~ds1-3`

Package impact

Ecosystem	Package	Vulnerable	Fixed
Go	github.com/syncthing/syncthing	`<1.15.0`	`1.15.0`

References

Community-verified mitigations for this CVE will appear above when contributors publish them.

Verify integrity in audit chain (admin only). AS-IS.