There are many kinds of software that are written to be troublesome and that can be dangerous to an unprotected system. These programs are referred to as malware; shorthand for malicious software.
The most common types of malware programs are listed below. Some types of software are written to mimic behavior of simple living organisms. That is, they reproduce themselves or live parasitically in other systems. It’s not that these programs are actually alive, but they can be just as annoying and hard to deal with as a living pest. The level of threat associated with malware corresponds to the intent and skill level of the programmer.
A Trojan horse program is named after the legendary Trojan horse used by the ancient Greeks to compromise the defenses of the city of Troy. A Trojan horse program appears to be a program that is useful or desirable, but in reality hides malicious software that can compromise a system. A Trojan horse program can do significant damage to a computer system, including deleting files or stealing private data such as passwords or credit card numbers. A Trojan horse program that hosts a server is referred to as a Remote Access Trojan. This type of Trojan is becoming increasingly popular.
A virus is a program fragment that uses other programs to run and reproduce itself. A typical virus is inserted into the code for an otherwise normal program. When the affected program runs, the virus code also runs, allowing the virus to operate. Usually the first thing a virus will do is try to insert copies of itself into other programs or more serious, the system code.
A worm is a type of program that uses the networking facilities of a computer to reproduce itself. E-mail is a common mechanism for worm reproduction. Even if a worm carries no hostile payload, it can easily duplicate itself to the point that network traffic involved in its reproduction consumes the bulk of resources available.
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