A computer virus is one kind of threat to the security and integrity of computer systems. Like other threats, a computer virus can cause the loss or alteration of programs or data, and can compromise their confidentiality. Unlike many other threats, a computer virus can spread from program to program, and from system to system, without direct human intervention. The essential component of a virus is a set of instructions which, when executed, spreads itself to other, previously unaffected, programs or files. A typical computer virus performs two functions. First, it copies itself into previously-uninfected programs or files. Second, (perhaps after a specific number of executions, or on a specific date) it executes whatever other instructions the virus author included in it. Depending on the motives of the virus author, these instructions can do anything at all, including displaying a message, erasing files or subtly altering stored data. In some cases, a virus may contain no intentionally harmful or disruptive instructions at all. Instead, it may cause damage by replicating itself and taking up scarce resources, such as disk space, CPU time, or network connections. There are several problems similar to computer viruses. They too have colorful names: worms, bacteria, rabbits, and so on. Definitions of them are given in the glossary. Each shares the property of replicating itself within the computing system. This is the property on which we will focus, using viruses as an example. There are also a variety of security issues other than viruses. Here, we will deal only with viruses and related problems, since new measures are required to deal with them effectively.


A Worm is a small piece of software that uses computer networks and security holes to replicate itself. A copy of the worm scans the network for another machine that has a specific security hole. It copies itself to the new machine using the security hole, and then starts replicating from there, as well. The term worm was coined by John Brunner, a science fiction writer, in his 1975 novel Shockwave Rider. The hero, a talented programmer, created self-replicating computer programs that tunneled their way through a worldwide network.

A Trojan Horse is simply a malicious computer program that damages your computer system upon installation. The program claims to do one thing, such as pretending to be a game, but instead do something else when you run them. Some nasty Trojans will actually attempt to erase your hard drive. They were originally named after the historic Trojan horse used by the Greeks to conquer Troy because the first Trojan horse programs pretended to be innocent games or applications. Trojan horses have no way to replicate themselves automatically.

Spyware is any software that covertly gathers user information through the user’s Internet connection without his or her knowledge, usually for advertising purposes. Spyware applications are typically bundled as a hidden component of freeware or shareware programs that can be downloaded from the Internet. However, it should be noted that the majority of shareware and freeware applications do not come with spyware. Once installed, the spyware monitors user activity on the Internet and transmits that information in thebackground to someone else. Spyware can also gather information about E-mail addresses and even passwords and credit card numbers.

Spyware is similar to a Trojan horse in that users can unwittingly install the product when they install something else. A common way to become a victim of spyware is to download certain peer-to-peer file swapping products that are readily available on the internet. But more and more, users can be infected with spyware simply by surfing the internet. Many times spyware objects are invisibly and unethically embedded into web pages by the webmaster. And by simply visiting one of these websites, the user is unsuspectingly infected.Aside from the questions of ethics and privacy, spyware steals from the user by using the computer’s memory resources and also by eating bandwidth as it sends information back to the
Spyware’s home base via the user’s Internet connection. Because spyware is using memory and system resources, the applications running in the background can lead to system crashes or general system instability.

Because spyware exists as independent executable programs, they have the ability to monitor keystrokes, scan files on the hard drive, snoop other applications, such as chat programsor word processors, install other spyware programs, read cookies, change the default home page on the Web browser, consistently relaying this information back to the spyware author who will either use it for advertising, and/or marketing purposes, or sell the information to another party.


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