viruses defination and types, Exams of Computer Science

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COMPUTER VIRUS a c o m p l e t e s t u d y.... June,

What is a Virus? A virus is just a computer program. Like any other program, it contains instructions that tell your computer what to do. But unlike an application, a virus usually tells your computer to do something you don't want it to do, and it can usually spread itself to other files on your computer -- and other people's

General virus types

While there are thousands of

variations of viruses, most fall

into one of the following general

categories, each of which works

slightly differently.

Boot Sector Virus :

Replaces or implants itself in

the boot sector. This kind of

virus can prevent you from being able to boot your hard disk.

Multipartite Virus

Infects both files and the boot

sector-- a double whammy that

can reinfect your system dozens of

times before it's caught.

Polymorphic Virus :

Changes code whenever it

passes to another machine.

E-mail viruses:

An e-mail virus moves around

in e-mail messages, and

usually replicates itself by

automatically mailing itself to

dozens of people in the victim's

e-mail address book.

Worms : A worm is a computer program that has the ability to copy itself from machine to machine. Worms normally move around and infect other machines through computer networks. Worms eat up storage space and slows down the computer. But worms don't alter or delete files.

When loaded onto your machine, a Trojan horse can capture information from your system -- such as user names and passwords or could allow a malicious hacker to remotely control your computer. Trojan horses have no way to replicate automatically.

Origins of Viruses :

People create viruses. A person has to write the code, test it to make sure it spreads properly and then release the virus. A person also designs the virus's attack phase, whether it's a silly message or destruction of a hard disk. In most of the cases people create viruses just for the thrill or fun.

The virus loads itself into memory and looks around to see if it can find any other programs on the disk. If it can find one, it modifies it to add the virus's code to the unsuspecting program. Then the virus launches the "real program." The user really has no way to know that the virus ever ran. Unfortunately, the virus has now reproduced itself, so two programs are infected. The next time either of those programs gets executed, they infect other programs, and the cycle continues.

If one of the infected programs is given to another person on a floppy disk, or if it is uploaded to internet, then other programs get infected. This is how the virus spreads.

SOME TRICKS THE VIRUSES PLAY : One important trick is the ability to load viruses into memory so that they can keep running in the background as long as the computer remains on. This gives viruses a much more effective way to replicate themselves.

Another trick is the ability to infect the boot sector on floppy disks and hard disks. The boot sector is a small program that is the first part of the operating system that the computer loads and tells the computer how to load the rest of the operating system. By putting its code in the boot sector, a virus can guarantee that it gets executed. It can load itself into memory immediately, and it is able to run whenever the computer is on. Boot sector viruses can infect the boot sector of any floppy disk inserted in the machine, and on campuses where lots of people share machines they spread like wildfire.