Viruses

Viruses

Computer virus is that type of program which acts as an parasite to the computer system. They come unwantedly in the system and damage the system.

A virus is a computer program that is designed to replicate itself from file to file (or disk to disk) on a single computer. Viruses spread quickly to many files within a computer, but they do not spread between computers unless people exchange infected files over a network or share an infected floppy diskette.

Currently, there are three categories of malicious software threats: viruses, worms, and Trojan horses. All of these threats are built from the same basic instructions and computer logic that make up application programs on one's computer such as word processors, games, or spreadsheets. Like traditional application programs, malicious software is written by people and must be intentionally designed and programmed to self-replicate or cause damage.While almost all Trojan horses attempt to cause harm to the computer system, more than 70 percent of all computer viruses and worms are designed only to self-replicate. Those viruses, worms, and Trojan horses that do inflict intentional damage to computer systems are said to deliver a "payload." Common payloads include formatting a hard drive, deleting files, or gathering and sending passwords to an attacker. These threats typically have trigger criteria. They wait until the criteria are met before delivering the payload (for example, waiting until July 28 to reformat the hard drive).

The typical malicious software author is male between fourteen and twenty-five years of age (only a few female virus writers are known). These demographics are expected to change as organized crime, terrorist groups, and rogue organizations begin to target the Internet. In addition, many governments around the world are researching how to use malicious software for both offensive and defensive information warfare.

Virus Types:

Viruses are classified by the type of file or disk that the virus infects:

• Boot viruses attach themselves to floppy diskettes and hard drives. When a user boots from an infected floppy diskette or hard drive, the virus is activated and the computer becomes infected. The virus spreads to other floppy diskettes as they are used on the system.
• Application viruses spread from one application to another on the computer. Each time an infected application program is run, the virus takes control and spreads to other applications.
• Macro viruses spread through documents, spreadsheets, and other data files that contain computer macros. A macro is a small, self-contained program that is embedded directly within a document or spreadsheet file. Typically, macros are used to automate simple computer tasks such as summing a set of numbers in a spreadsheet. Modern macros are powerful enough to copy themselves between documents or spreadsheets.
• Script viruses infect other script files on the computer. Script viruses, which are written in high-level script languages such as Perl or Visual Basic, gain control when a user runs an infected script file.

A typical computer virus works as follows:

First, the user runs infected program A. Program A immediately executes its viral logic. The virus locates a new program, B, that it thinks it can infect. The virus checks to see if the program is already infected. If program B is already infected, the virus goes back to locate another program to infect. If it is not already infected, the virus appends a copy of its logic to the end of program B and changes program B such that it, too, will run the malicious logic. The virus then runs program A so the user does not suspect any malicious activities.

Viruses can be written in numerous computer programming languages including assembly language, scripting languages (such as Visual Basic or Perl), C, C, Java, and macro programming languages (such as Microsoft's VBA).