Zombie (computer science)

In computer science, a zombie is a computer connected to the Internet that has been compromised by a hacker, computer virus or trojan horse program and can be used to perform malicious tasks of one sort or another under remote direction. Botnets of zombie computers are often used to spread e-mail spam and launch denial-of-service attacks (DOS attacks). Most owners of "zombie" computers are unaware that their system is being used in this way. Because the owner tends to be unaware, these computers are metaphorically compared to fictional zombies. A coordinated DDoS attack by multiple botnet machines also resembles a "zombie horde attack", as depicted in fictional zombie films.

Zombie computers have been used extensively to send e-mail spam; as of 2005, an estimated 50–80% of all spam worldwide was sent by zombie computers. This allows spammers to avoid detection and presumably reduces their bandwidth costs, since the owners of zombies pay for their own bandwidth. This spam also greatly furthers the spread of Trojan horses, as Trojans are not self-replicating. They rely on the movement of e-mails or spam to grow, whereas worms can spread by other means. For similar reasons zombies are also used to commit click fraud against sites displaying pay-per-click advertising. Others can host phishing or money mule recruiting websites.

Zombies can be used to conduct distributed denial-of-service (DDOS) attacks, a term which refers to the orchestrated flooding of target websites by large numbers of computers at once. The large number of Internet users making simultaneous requests of a website's server is intended to result in crashing and the prevention of legitimate users from accessing the site. A variant of this type of flooding is known as distributed degradation-of-service. Committed by "pulsing" zombies, distributed degradation-of-service is the moderated and periodical flooding of websites, done with the intent of slowing down rather than crashing a victim site. The effectiveness of this tactic springs from the fact that intense flooding can be quickly detected and remedied, but pulsing zombie attacks and the resulting slow-down in website access can go unnoticed for months and even years. Notable incidents of distributed denial- and degradation-of-service attacks in past include the attack upon the SPEWS service in 2003, and the one against Blue Frog service in 2006. In 2000, several prominent Web sites (Yahoo, eBay, etc.) were clogged to a standstill by a distributed denial of service attack mounted by ‘MafiaBoy’, a Canadian teenager. An attack on grc.com is discussed at length, and the perpetrator, a 13-year-old probably from Kenosha, Wisconsin, was identified on the Gibson Research Web site. Steve Gibson disassembled a 'bot' which was a zombie used in the attack, and traced it to its distributor. In his account about his research, he describes the operation of a 'bot'-controlling IRC channel.

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