UNDERGROUND
Hand scanning was a pain, as ever, but if he was methodical and persistent, something should turn up. 111. 112. 113. 114. 115. On it went. Eventually he connected to something – a Sunos Unix system – which gave him a full IP address in its login message. Now that was handy. With the full IP address, he could connect to System X again through the Internet directly – avoiding the gateway if he chose to. It's always helpful in covering your tracks to have a few different routing options. Importantly, he could approach System X through more than just its front door.
Anthrax spiralled through the usual round of default user- names and passwords. Nothing. This system required a more strategic attack.
He backed out of the login screen, escaped from the gateway and went to another Internet site to have a good look at System X from a healthy distance. He 'fingered' the site, pulling up any bit of information System X would release to the rest of the Internet when asked. He probed and prodded, looking for openings. And then he found one. Sendmail.
The version of Sendmail run by System X had a security hole Anthrax could exploit by sending himself a tiny backdoor program. To do this, he used System X's mail-processing service to send a 'letter' which contained a tiny computer program. System X would never have allowed the program to run normally, but this program worked like a letter bomb. When System X opened the letter, the program jumped out and started running. It told System X that anyone could connect to port 2001 – to an interactive shell – of the computer without using a password.
A port is a door to the outside world. TCP/IP computers use a standard set of ports for certain services. Port 25 for mail. Port 79 for Finger. Port 21 for FTP. Port 23 for Telnet. Port 513 for Rlogin. Port 80 for the World Wide Web. A TCP/IP based computer system has $65\,535$ ports but most of them go unused. Indeed, the average Unix box uses only 35, leaving the remaining $65\,500$ ports sitting idle. Anthrax simply picked one