What is a worm?

A computer worm is comparable to a computer virus, but unlike a virus a worm does not need to attach itself to an existing program. Instead, it is able to spread copies of itself from one computer to another without being activated by users.

Some worms have the ability to send themselves to everyone in your e-mail address book - and then to everyone in their e-mail address books, making the worms capable of spreading to millions of computers within hours. In the most extreme cases, worms have affected computer networks worldwide and drastically slowed down the Internet.
Worms typically exploit vulnerabilities in operating systems and spread through e-mail or other file transmission capabilities found on networked computers. They often hide in parts of operating systems that are automatic and usually invisible to you. It is common for worms to be noticed only when their uncontrolled replication consumes system resources, slowing or halting other tasks.

Many worms have been created only to spread and do not attempt to mess up computers they pass through. Other worms, however, carry a so-called "payload". A payload is code typically designed to do damage - it might delete files on your PC, encrypt files in an extortion attack, or attach itself to your outgoing e-mail. A very common payload for worms is to install a backdoor which can later be exploited by other malware, allowing the worm author to take control over infected computers.

The most famous and damaging worms - like MyDoom, Slammer, and Sasser - were all unleashed in 2003-2004 and since then major attacks have been rare. During 2007, the Storm Worm created a botnet containing millions of PCs. But in spite of its name, Storm Worm is in fact a Trojan horse with an executable file as an attachment.

Mobile devices have also experienced worm attacks. One example was Caribe which was able to send itself from one mobile phone to another across a network.

To protect yourself against worms, always keep your anti virus software up-to-date, your firewall turned on and your operating system and applications patched.