A network administrator discovers several unknown files in the root directory of his Linux FTP server. One of the files is a tarball, two are shell script files, and one is a binary file named nc (netcat). The logs show the user logged in anonymously, uploaded the files, extracted the contents, and ran the script using a function provided by the ftp servers software. The ps command shows that the nc file is running as process and the netstat command shows the nc process is listening on a network port. What kind of vulnerability had to have existed to make this remote attack possible?