The vulnerability in SSH.NET's ScpClient.Download method arises because it writes files and directories using names returned by the remote SCP server during recursive downloads without validating that the resulting local path remains inside the requested directory. A malicious, compromised, or man-in-the-middle SCP server can exploit this by returning filenames containing '../' sequences or absolute paths, causing the client to write or overwrite files anywhere the client process has write access. This can lead to overwriting sensitive files such as ~/.ssh/authorized_keys, shell rc files, cron entries, or application binaries/configuration, enabling persistence, privilege escalation, or remote code execution on the client host. The issue is similar to CVE-2019-6111 in OpenSSH but includes directory traversal capability. The vendor has released a fix that ensures remote filenames are validated and throws an exception when invalid names are detected.

An attacker controlling or intercepting the SCP server can cause the client to write arbitrary files outside the intended download directory. This can overwrite critical files leading to persistence mechanisms, privilege escalation, or remote code execution on the client system. The attack requires the victim to initiate a recursive directory download from the malicious SCP server.

A fixed release of SSH.NET is available that validates all remotely supplied file and directory names to ensure the constructed local paths remain within the specified local directory. The client throws an ScpException when invalid names are detected, preventing directory traversal. Users should upgrade to the fixed version to mitigate this vulnerability.