A zombie computer (abbreviated zombie) is a computer attached to the Internet that has been compromised by a Hacker, a computer virus, or a trojan horse (computing), and performs malicious tasks of one sort of another, under the direction of the hacker. Many owners of zombie computers are unaware that their systems are zombies or that any hacker attack ever occurred.

Zombies have been used extensively to send e-mail spam; between 50% to 80% of all spam worldwide is now sent by zombie computers. [http://www.pcworld.com/news/article/0,aid,121381,00.asp] This allows spammers to avoid detection of the source of spam, and presumably reduces their bandwidth costs, since the owners of zombies pay for their computers use of bandwidth.

For similar reasons, zombies are also used to commit click fraud against sites displaying pay per click advertising.

Zombies have also conducted DDoS attacks, such as the attack upon the SPEWS service in 2003.

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