Viruses

Computer Viruses.  You've certainly heard the term, but what are they, what do they do, what sorts of viruses are there, and how do you get them?   Here's a quick primer on viruses.

What is a virus?

Much like a biological virus, a computer virus is specifically defined as any computer program that spreads across computers or networks, creating copies of itself.   In a more real sense, a virus is software that is installed without your knowledge that may do any number of things to or with your computer.

What do viruses do?

Early viruses focused on destruction of data and rendering machines unusable, however, most modern viruses are less concerned with damage to your personal machine, and more concerned with the damage they can cause by using your machine for their ends.   Often, modern viruses spread silently and quickly infecting multitudes of machines, but doing absolutely nothing, until, at a specified time, the virus takes command of whatever parts of the computer it requires and executes some task, often sending large amounts of junk network traffic toward a pre-determined web site, in an effort to take the machine offline.   Another modern innovation in virus function is that of a spy, collecting e-mail addresses, credit card numbers, and any other important, personal data it can, and delivering that information back to some central location where the author can retrieve it.   Others are simply egotistical practical jokes created by the author to annoy or surprise infected users, often with an unsuspected pop-up window or sound playback at a certain predetermined time.   The possibilities are limited only by the author's imagination and code-writing abilities.

What sorts of viruses are there?

The two most common types of viruses seen now are called trojan horses and worms.

Trojan horses (trojans)

Trojan horses, commonly called trojans, are viruses that pretend to be a legitimate piece of software, like a critical system update, computer game, or other small application, but actually have a different purpose entirely.   The name is a reference to the famous wooden horse used by Odysseus and the Greek armies to invade the citadel of Troy in Virgil's epic poen The Aeneid. The horse was presented as a gift, but unbeknownst to the Trojans, it was hollow inside, and harbored a contingent of Greek warriors, led by Odysseus. The Trojans brought the horse inside the gates of Troy, and in the dark of night, the Greeks emerged and opened the gates of the city to the Greek army waiting outside. Much like that original Trojan horse, trojans will seem like a friendly gift, but will sack your computer in the end.

Trojans commonly do a few things. A major focus of trojan development has been in creating "zombie" computers.   Zombie computers are infected but unaware of the infection, much like the walking dead are unaware they are dead.   At a certain time the virus writer sends the appropriate signal and all computers infected with the virus will perform some task, most commonly attacking a web site that the virus author finds offensive, such as the Department of Defense or the website for The SCO Group.

Trojans may often be only one component of a larger virus, capturing and transmitting your keyboard input or distributing itself or a different virus via e-mail.

Worms

Unlike trojans, worms spread automatically.  They exploit security holes in network communications between computers to copy themselves to other, unprotected machines. The now-common flurry of Windows Security Updates are often to respond to security threats that may be easily exploited by worms. The name worm is meant to evoke the idea of a parasitic biological worm, writhing around the Internet into whatever systems it can enter.

They perform many of the same nefarious duties as trojans, creating zombie machines and capturing sensitive information to report.   The only major difference between a trojan and a worm is the method in which they propagate.

How do I get viruses?

In the general sense, a virus has to be run before it can infect your machine.   The problem is, there are many ways to execute a program that are outside your control as a user.   Trojans attach themselves to other software, such that when you run a seemingly benign application, you install and activate the virus.   Some are even disguised as other sorts of files like image files or screen savers.   When you try to open these files you install and activate the virus.   This is the most common type seen distributed via e-mail, often with the virus included as an attachment.   Worms exploit security flaws in an operating system that allow them to automatically access your machine.   These are also seen in e-mail, but are normally written to take advantage of security flaws in e-mail software.   Such flaws allow them to be executed as soon as the email message is viewed.   There are even viruses that infect Microsoft Office files.   These are often referred to as "Macro Viruses" and have prompted Microsoft to alert the user if a document contains macro information.