cve/2021/CVE-2021-21404.md

### [CVE-2021-21404](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-21404)
![](https://img.shields.io/static/v1?label=Product&message=syncthing&color=blue)
![](https://img.shields.io/static/v1?label=Version&message=n%2Fa&color=blue)
![](https://img.shields.io/static/v1?label=Vulnerability&message=CWE-20%20Improper%20Input%20Validation&color=brighgreen)

### 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.

### POC

#### Reference
No PoCs from references.

#### Github
- https://github.com/sustsoft/syncthing-broad