I ran across a very interesting technique this week called
Port Knocking and a related implementation called
Tumbler. This technique allows you to access a computer on the Internet via seemingly closed ports using a specific "knocking" sequence. There is a debate raging whether or not this is a security-through-obscurity play or a legitimate technique. I don't know enough yet to make a judgment. However, the first thing that came to my mind reading the introductory documentation is a hacker embedding one of these implementations into a Trojan or worm. Embedding one of these implementations in a Trojan or worm would be extremely dangerous and make the worm almost impossible to detect after it has planted itself on your computer. It would also open a door for an attacker to plant more code on your computer at his leisure to do whatever he wants. Not that this isn't possible today, but a small DLL running tumblerd wouldn't open any ports or communicate with a remote host until a connection is initiated by the remote host. A small DLL could be written that runs tumblerd, manipulates firewall settings, and sends a remote host any changes in IP settings, providing a universal gateway attack. This is seriously dangerous stuff in the wrong hands.
