CVE-2021-21404
Crash due to malformed relay protocol message
Reported by github_m · Published April 6, 2021
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.
Fixed versions
Upgrade to a fixed version or later. Language registries are listed first, then Linux distributions, then vendor products.
| Package | Ecosystem | Fixed in |
|---|---|---|
| syncthing | Arch Linux | 1.15.1-1 |
Risk scores
Affected products
| Vendor | Product | Versions |
|---|---|---|
| syncthing | syncthing | < 1.15.0 |
| syncthing | syncthing | 0 |
Timeline
- Apr 6, 2021 CVE Published
- Apr 14, 2021 EPSS Score (30 in total, the first dated here)
- May 20, 2021 CVE Updated
- Mar 26, 2026 Security Advisory (3 in total, the first dated here)
- Aug 26, 2026 Coalition ESS Score
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