Computer worm:-

A computer worm is a self-replicating computer program. It uses a network to send copies of itself to other nodes (computers on the network) and it may do so without any user intervention. 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, if only by consuming bandwidth, whereas viruses almost always corrupt or devour 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 My doom showed, the network traffic and other unintended effects can often cause major disruption. A "payload" is code designed to do more than spread the worm - it might delete files on a host system (e.g., the Explore Zip worm), encrypt files in a crypto viral extortion 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 - So big and My doom are examples which created zombies. 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.

Beginning with the very first research into worms at Xerox PARC there have been attempts to create useful worms. The Nachi family of worms, for example, tried to download and install patches from Microsoft's website to fix vulnerabilities in the host system – by exploiting those same vulnerabilities. In practice, although this may have made these systems more secure, it generated considerable network traffic, rebooted the machine in the course of patching it, and did its work without the consent of the computer's owner or user.

Some worms, such as XSS worms, have been written for research to determine the factors of how worms spread, such as social activity and change in user behavior, while other worms are little more than a prank, such as one that sends the popular image macro of an owl with the phrase "O RLY?" to a print queue in the infected computer.

Worms spread by exploiting vulnerabilities in operating systems. All vendors supply regular security updates, and if these are installed to a machine then the majority of worms are unable to spread to it. If a vendor acknowledges vulnerability, but has yet to release a security update to patch it, a zero day exploit is possible. However, these are relatively rare.