Viruses: Contagious Pests
For all the publicity viruses have gotten, they remain a serious threat. While viruses can potentially destroy a computer's data, most of the widespread viruses have leaned more toward annoyance. The most famous are e-mail viruses that replicate and spread using e-mail addresses stored on a computer. They still cost computer users and their employers hundreds of millions of dollars annually.
The MS Blaster worm that caused havoc in the summer of 2003 exploited a vulnerability in the Remote Procedure Call (RPC) function of the Windows operating system. Anyone who did not install a patch issued by Microsoft was vulnerable, marking a new era in virus prevention for many Internet users. No longer was using care with e-mail attachments enough to keep you safe.
Trojans: RATS That Can Control Your Computer
Like the horse of old, a trojan carries with it an unexpected surprise. Trojans do not replicate like a virus, but they do leave behind a program that can be contacted by another computer. From there, they can do just about anything. While it's possible a trojan can be used to take control of a computer, the most common trojans are dialer programs. Dialers are used without your knowledge to make international or premium calls (900-type numbers) from your PC. That's more than an annoyance; it can get expensive.
Trojans are also known as RATS (remote access trojans) and they are most often hidden in games and other small software programs that unsuspecting users download then unknowingly execute on their PCs.
Two common trojans are known as Back Orifice and SubSeven. Back Orifice was originally developed as a remote administration tool. But it worked by exploiting holes in Microsoft software, which makes it a popular tool for nefarious applications. Both Back Orifice and SubSeven can be used to capture what is on a computer's screen and what is typed in using the keyboard; they can be use to remotely control devices, such as opening and closing the CD drive; or to set up FTP, HTTP or Telnet servers on an unsuspecting user's machine.
Basically, anything that can be done with a computer can be done remotely using a trojan.