Test with SSH-MITM
A proof of concept exploit is integrated in ssh-mitm.
This exploit also integrates CVE-2019-6111 to inject additional files.
To inject files during a file transfer from the server to the client you have to start ssh-mitm with following arguments:
$ ssh-mitm --scp-interface inject_file --scp-inject /path/to/additional/file
The file transfer for the injected file will be hidden because of the described vulnerability and CVE-2019-6109.