Malicious Software: Spyware Worms and Viruses Fact Sheet

What is malicious software?

"Computer programs" or "computer software" are recipes of specially coded instructions that tell your computer to do something. You are probably familar with many common, helpful types of software, such as your computer's operating system (for example, Windows, MacOS, Linux) or popular software applications like Microsoft Word, the Windows calculator, iPhoto, iTunes, etc.

When someone writes a computer program with the intent to steal information, cause annoyance and mayhem, or conceal what they are doing, we call that program "malicious software". Viruses, worms, trojans, spyware and rootkits are all forms of malicious software. In general, malicious software may:

• attempt to reproduce itself automatically and secretly
• try to conceal itself from routine forms of detection (for example, using random file names) and elimination (for example, turning off your antivirus software)
• spread itself to other computers via the network, such as by e-mail, unsecured file shares, password guessing or exploiting security problems on other computers
• modify the operating system or other legitimate software
• make copies of itself to floppy disks, USB storage devices, CD-RW discs, or other writable media
• send personal information gleaned from your computer back to the maker of the malicious software, or his or her criminal associates, for purposes of identity theft or to collect market data
• display unwanted advertising banners on web sites or in pop-up windows
• allow malicious individuals to monitor your computer remotely over the network
• delete, damage or modify your documents and data files

When malicious software carries some specific traits, computer professionals may give it a special name, such as:

• Spyware: sends information about the computer, your personal information or your Internet browsing activities to a third party.
• Adware: displays unwanted advertisements.
• Viruses: copy themselves into other legitimate programs, turning the innocent software into carriers of the virus.
• Worms: take advantage of operating system bugs and other security problems to copy themselves - usually silently and automatically - to other computers on a network.
• Trojans, or Trojan Horses: masquerade as legitimate software but engage in various kits of malicious activity.
• Rootkits: modify the operating system to hide themselves, then use worm-like methods to propgate to other computers.

In practice, you will often find a specific name like virus applied to a broad range of malicious software. The important thing to remember is that malicious software may combine any of the traits specified above.