LocalSend is a free, open-source app that allows users to securely share files and messages with nearby devices over their local network without needing an internet connection. Prior to version 1.17.0, due to the missing sanitization of the path in the `POST /api/localsend/v2/prepare-upload` and the `POST /api/localsend/v2/upload` endpoint, a malicious file transfer request can write files to the arbitrary location on the system, resulting in the remote command execution. A malicious file transfer request sent by nearby devices can write files into an arbitrary directory. This usually allows command execution via the startup folder on Windows or Bash-related files on Linux. If the user enables the `Quick Save` feature, it will silently write files without explicit user interaction. Version 1.17.0 fixes this issue.
The product uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the product does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory.