A worm is a self-replicating computer program, which sends copies of itself to other computers on the same network without any user intervention, taking advantage of security shortcomings on the target computer.Unlike a virus, a worm 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, whereas viruses almost always corrupt or modify files on the target computer.
Worms are often carried inside another file, usually a Word document or an Excel spreadsheet. A worm will usually release a document that already has the worm macro inside it. This document travels from system to system, so the entire document could be regarded as a worm. W32.Mydoom.AX@mm is an example of a worm. It was designed to allow spammers to send spam e-mail from infected computers.
In January 2012 it was reported that a worm named Ramnit had stolen the names and passwords of more than 45,000 Facebook users. The attackers responsible used the stolen credentials to log-in to victims’ Facebook accounts and send malicious links to their friends, allowing the worm to spread even further. They also took advantage of the fact that users often use the same password for different sites to obtain access to banks and corporate networks. In total, around 800,000 machines are believed to have been infected.
Different worms exploit different weaknesses in the programs that allow computers to communicate with each other. A common purpose of worms is to install a backdoor into a computer. These are programs that allow others to gain access to your computer. You can find out more about the differences between viruses, Trojans and worms by visiting the following web site: Viruses, Trojans and Worms.