This indicates an attempt to exploit a directory traversal vulnerability in the Rhino Software Serv-U FTP Server.
The vulnerability is caused by an error that occurs when the vulnerable software handles a malformed RNTO command. It allows a remote attacker to write arbitrary files to any location on the vulnerable server.
RhinoSoft Serv-U 7.2 1
System Compromise: Remote attackers can gain control of vulnerable systems.
Update to version 18.104.22.168: