GHSA-mggc-4xg6-vcxf

## Summary

Default SCP remote-path handling places caller-supplied paths into the command that runs scp on the server. On a shell-based server that command is interpreted by a shell, so an attacker-influenced path that is not quoted to suit that shell can execute as a command as the authenticated SSH user. SSH.NET provides `ScpClient.RemotePathTransformation` to control escaping behaviour (defaulting to `RemotePathTransformation.DoubleQuote`) but cannot guarantee safety for arbitrary remote command interpreters. This is inherent to running scp over a remote shell (cf. CVE-2020-15778).

## Impact

Command execution on the SSH server as the authenticated SSH user, when an application passes an attacker-influenced remote path to ScpClient against a shell-based server.

Exploitation depends on conditions beyond the attacker's control: the remote server must be shell-based, and the attacker-influenced path must be crafted to defeat the quoting applied by the transformation in effect — that is, to use metacharacters that it does not neutralise (for example $(...) or backticks, which survive the default double-quoting on a POSIX shell). A path that does not meet these escaping rules, or a non-shell-based server, does not result in command execution.

## Remediation

The fixed release obsoletes the constructors that silently defaulted the path transformation and adds constructors that require an explicit `IRemotePathTransformation`, so callers are required to choose one that suits their server and trust environment. For POSIX shells, the library provides `RemotePathTransformation.ShellQuote`. The legacy default (`RemotePathTransformation.DoubleQuote`) is unchanged for backwards compatibility. SCP is considered legacy and SFTP, which does not involve a remote shell, is recommended where possible (via `SftpClient`).