A computer worm is a standalone malware computer program that replicates itself in order to spread to other computers. Often, it uses a computer network to spread itself, relying on security failures on the target computer to access it. Unlike a computer virus, it does not need to attach itself to an existing program.〔(【引用サイトリンク】url=http://scienceline.ucsb.edu/getkey.php?key=52 )〕 Worms almost always cause at least some harm to the network, even if only by consuming bandwidth, whereas viruses almost always corrupt or modify files on a targeted computer.
Many worms that have been created are designed only to spread, and do not attempt to change the systems they pass through. However, as the Morris worm and Mydoom showed, even these "payload free" worms can cause major disruption by increasing network traffic and other unintended effects. A "payload" is code in the worm designed to do more than spread the worm—it might delete files on a host system (e.g., the ExploreZip worm), encrypt files in a ransomware attack, or send documents via e-mail. A very common payload for worms is to install a backdoor in the infected computer to allow the creation of a "zombie" computer under control of the worm author. Networks of such machines are often referred to as botnets and are very commonly used by spam senders for sending junk email or to cloak their website's address. Spammers are therefore thought to be a source of funding for the creation of such worms,〔(【引用サイトリンク】title=Unavailable )〕 and the worm writers have been caught selling lists of IP addresses of infected machines. Others try to blackmail companies with threatened DoS attacks.
Users can minimize the threat posed by worms by keeping their computers' operating system and other software up-to-date, avoiding opening unrecognized or unexpected emails, and running firewall and antivirus software.〔(【引用サイトリンク】title=Computer Worm Information and Removal Steps )〕
Backdoors can be exploited by other malware, including worms. Examples include Doomjuice, which can spread using the backdoor opened by Mydoom, and at least one instance of malware taking advantage of the rootkit and backdoor installed by the Sony/BMG DRM software utilized by millions of music CDs prior to late 2005.
The actual term "worm" was first used in John Brunner's 1975 novel, ''The Shockwave Rider''. In that novel, Nichlas Haflinger designs and sets off a data-gathering worm in an act of revenge against the powerful men who run a national electronic information web that induces mass conformity. "You have the biggest-ever worm loose in the net, and it automatically sabotages any attempt to monitor it... There's never been a worm with that tough a head or that long a tail!"
On November 2, 1988, Robert Tappan Morris, a Cornell University computer science graduate student, unleashed what became known as the Morris worm, disrupting a large number of computers then on the Internet, guessed at the time to be one tenth of all those connected During the Morris appeal process, the U.S. Court of Appeals estimated the cost of removing the virus from each installation was in the range of $200–53,000, and prompting the formation of the CERT Coordination Center〔(【引用サイトリンク】title=Security of the Internet )〕 and Phage mailing list.〔(【引用サイトリンク】title=Phage mailing list )〕 Morris himself became the first person tried and convicted under the 1986 Computer Fraud and Abuse Act.
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