:: Viruses, Trojans and Worms

In the mid-1980's two brothers in Pakistan discovered that people were pirating their software. They responded by writing the first computer virus, a program that would put a copy of itself and a copyright message on any floppy disk copies their customers made. Today new viruses sweep the planet in minutes and can corrupt data, slow networks or harm your reputation.

A virus (or worm) is a computer program that can spread across computers and networks by making copies of itself, usually without the users knowledge. Viruses can have harmful effects. These range from displaying irritating messages to stealing data or giving other users control of a computer.

How does a virus infect computers? A virus program has to be run before it can infect your computer. Viruses have ways of making sure that this happens. They can attach themselves to other programs or hide in code that is run automatically when you open certain types of file. Sometimes they can exploit security flaws in your computers operating system to run and spread themselves automatically.
You might receive an infected file in an email attachment, in a download from the internet, or on a disk. As soon as the file is launched, the virus code runs. Then the virus can copy itself to other files or disks and make changes on your computer.

Trojan Horses are programs that pretend to be legitimate software, but actually carry out hidden, harmful functions.
For example, DLoader-L arrives in an email attachment and claims to be an urgent update from Microsoft for Windows XP. If you run it, it downloads a program that uses your computer to connect to certain websites, in an attempt to overload them (this is called a denial of service attack).
Trojans cannot spread as fast as viruses because they do not make copies of themselves. However they now often work hand-in-hand with viruses. A virus may download Trojans which record keystrokes or steal information. On the other hand, some Trojans are used as a means of infecting a computer with a virus.

Worms are similar to viruses but do not need a carrier program or document.
Worms simply create exact copies of themselves and use communications between computers to spread.
Many viruses, such as MyDoom or Bagle, behave like worms and use email to forward themselves.

Viruses used to play pranks or stop your computer working, but now they compromise security in more insidious ways. Here are the things viruses can do.

Slow down email - Viruses that spread by email, such as Sobig, can generate so much email traffic that servers slow down or crash. Even if this doesn't happen, companies may react to the risk by shutting down servers anyway.

Steal confidential data - The Bugbear-D worm records the users keystrokes, including passwords, and gives the virus writer access to them.

Use your computer to attack websites - MyDoom used infected computers to flood the SCO software company's website with data, making the site unusable (a denial of service attack).

Let other users hijack your computer - Some viruses place 'backdoor Trojans' on the computer, allowing the virus writer to connect to your computer and use it for their own purposes.

Corrupt data - The compatible virus makes changes to data in Excel spreadsheets.

Delete data - The Sircam worm may attempt to delete or overwrite the hard disk on a certain day.

Disable hardware - CIH, also known as Chernobyl, attempts to overwrite the BIOS chip on April 26th, making the computer unusable.

Play pranks - the Netsky-D worm made computers beep sporadically for several hours one morning.

Display messages - Cone-F displays a political message if the month is May.

Damage your credibility - If a virus forwards itself from your computer to your customers and business partners, they may refuse to do business with you, or demand compensation.

Cause you embarrassment - For example, PolyPost places your documents and your name on sex-related newsgroups.

You can read more about viruses by selecting the first option from the table below.

:: See Also ::

Virus Guide Continued...    Spam Guide...    Hoaxes Guide...    Safety Guide...