Zombie

To successfully attack someone using a DDoS attack, you need hundreds or thousands of PCs all simultaneously generating attack traffic. However, the attacker is usually unwilling to go to the nearest hardware store and buy those PCs himself. Therefore, he usually "borrows" the services of your home PC.

For an attacker to borrow your PC, your PC must get infected by a piece of software that allows someone else to remotely control it. Using that software, an attacker can then use your PC to generate spam email, attack someone else, or infect other PCs. All this happens in the background without your knowing it. Your PC is now a zombie.

NOTE Why are remotely controlled PCs called zombies?

A zombie is a computer, which after being compromised, is being used to perform malicious tasks under remote control by someone on the Internet. This can be compared to the stories of Voodoo magic where a sorcerer used spells to animate dead people in order to perform evil tasks. The computer (and the computer's owner) are usually totally unaware of what is going on, and the computer can therefore be compared to a zombie.

Almost all computer viruses and Trojan horses written today contain a backdoor, which allows someone else to control your PC. An infected PC announces itself to an external server, telling the virus writer that it's now available. The virus writer can use those PCs to start his own attacks or, more commonly, charge money to give someone else access to those PCs. (Currently [mid-2007], the going rate for 1000 zombies is about $100.) Clearly, virus writers now have good reason to attempt to infect and gain control of your PC; they can make serious money.

NOTE Zombies are a relatively recent phenomenon. It was not until always-on Internet connections, such as digital subscriber line (DSL), became popular that the number of zombies on the Internet dramatically surged. The reason for this is if the infected PC uses a dialup connection to connect to the Internet, it cannot be controlled unless it's online. Today's high-speed, always-on connections make it possible to remotely control the zombie computer as long as it is on.

Continue reading here: Botnet

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