Network Invaders The most common type of malicious software ("malware") is a virus, a bit of code that sneaks onto your machine, normally as an e-mail attachment or download. Traditional viruses self-replicate within a machine but need human intervention (such as sharing infected documents) to spread. Newer malware, including Trojan horses and worms, allows attacks of even greater dimensions.Worms, on the other hand, do replicate, but unlike traditional viruses, worms dont need any user assistance to move from machine to machine. The danger of a worm is that it can allow a variety of attacks to propagate over the Internet. For example, a well-crafted worm can look for vulnerable machines, embed itself in them, and wait to launch a synchronized denial-of-service (DoS) attack on a set target. By now most people are wary of e-mail attachments they didnt request. But even clicking on a link could allow ActiveX content embedded in a Web site to run programs on your PC, read your Clipboard, and even steal personal data. These days, you need to be vigilantnearly to the point of paranoiato stay safe.
Named for the Greek legend, Trojan horses, or Trojans, infiltrate your machine and wait for an opportune time to open the city gates. The Trojan listens on a designated network port (more on this below) and waits for a remote program to activate it, then takes control of the machine. Unlike viruses, Trojans dont replicate themselves.