Welcome to Worm Library
A computer worm is a self-replicating malware computer program. It uses a computer network to send copies of itself to other nodes (computers on the network) and it may do so without any user intervention. This is due to security shortcomings on the target computer.
Unlike a virus, it does not need to attach itself to an existing program. Worms almost always cause at least some harm to the network, even if only by consuming bandwidth, as opposed to computer viruses which almost always corrupt or modify files on a targeted computer.
Many worms that have been created are only designed to spread, and don't attempt to alter the systems they pass through. However, as the Morris worm and Mydoom showed, the network traffic and other unintended effects can often cause major disruption.
Worms commonly 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, and the worm writers have been caught selling lists of IP addresses of infected machines. Others try to blackmail companies with threatened DoS attacks.
Backdoors can be exploited by other malware, including other worms. Examples include Doomjuice, which spreads better using the backdoor opened by Mydoom.