A zombie (also known as a bot) is a computer that a remote attacker has accessed and set up to forward transmissions (including spam and viruses) to other computers on the Internet. The purpose is usually either financial gain or malice. Attackers typically exploit multiple computers to create a botnet, also known as a zombie army.
Typically, a zombie is a home-based PC whose owner is unaware that the computer is being exploited by an external party. The increasing prevalence of high speed connections makes home computers appealing targets for attack. Inadequate security measures make access relatively easy for an attacker. For example, if an Internet port has been left open, a small Trojan horse program can be left there for future activation.
There are a few other kinds of zombies:
In one form of denial of service attack, a zombie is an insecure Web server on which malicious people have placed code that, when triggered at the same time as other zombie servers, will launch an overwhelming number of requests toward an attacked Web site, which will soon be unable to service legitimate requests from its users. A pulsing zombie is one that launches requests intermittently rather than all at once.
The term originated in the West Indies, where a zombie is a will-less, automaton-like person who is said to have been revived from the dead and must now do the will of the living.