A computer worm is a self-replicating computer program, similar to a computer virus. The main difference between the two is that a virus attaches itself to, and becomes part of, another executable program, while a worm is self-contained; it does not need to be part of another program to propagate itself. In addition to replication, a worm may be designed to do any number of things, such as delete files on a host system, or send documents via email. It should be pointed out that worms are not always bad, and in fact can be occasionally useful, for instance they could be used to upgrade software on a very large privately run network. But even if worms do not have malicious intent if they reproduce quickly enough they can consume a lot of bandwidth and slow networks.

The name 'worm' was taken from a 1970s science fiction novel by John Brunner entitled The Shockwave Rider; researchers writing an early paper on experiments in distributed computing noted the similarities between their software and the program described by Brunner, and adopted that name.

The first worm to attract wide attention, known as the Morris Worm, was written by Robert Tappan Morris, Jr at the MIT Artificial intelligence Laboratory. It was released on November 2, 1988, and quickly infected a great many of the computers on the Internet. It propagated through a number of bugs in BSD Unix and its derivatives. Morris himself was convicted under the US Computer Crime and Abuse Act, received 3 years probation, community service and a fine in excess of $10,000.

More sophisticated worms such as the Klez worm are multi-headed and may carry other executables as a payload. This fact has sparked speculation that such worms could employ genetic algorithms.

Famous Worms

  • On March 26, 1999 the Melissa worm was released, causing a huge load on Microsoft Exchange and Outlook-based systems.
  • In the summer of 2001, the Code Red worm attacking Microsoft Internet Information Server was released. It was just another computer worm but it got much publicity because it targeted the White House website.
  • On January 24 2003, the SQL slammer worm attacking Microsoft SQL Server caused widespread problems on the Internet.
  • In late august 2003, two major worms named the Sobig worm and the Blaster worm began to attack several millions of Microsoft Windows computers very aggressively, resulting in the largest down-time and clean-up cost ever. The worms have also had political consequences as many companies in several countries have started to call for government action to prevent further damages from Windows worms.

The term 'worm' should not be confused with WORM (in capitals), Write Once, Read Many, a property of some computer storage media.