Malware: viruses, worms, trojans and spyware

By far the most common type of computer crime involves malware.

Malware (malicious software) is software created to damage or alter the computer data or its operations. Malware includes computer viruses, worms, trojan horses, most rootkits, spyware, dishonest adware, crimeware, and other malicious and unwanted software, including true viruses.

Viruses are self-replicating programs that spread by inserting copies of themselves into other executable code or documents. Thus, a computer virus behaves in a way similar to a biological

virus, which spreads by inserting itself into living cells. When the infected program is run, the virus propagates to other files or programs on the computer. Some viruses are designed to work at a particular time or on a specific date, e.g. on Friday 13th. An email virus spreads by sending a copy of

itself to everyone in an email address book.

A true virus can only spread from one computer to another (in some form of executable code) when its host is taken to the target computer; for instance because a user sent it over a network or the Internet, or carried it on a removable medium such as a floppy disk, CD, DVD, or USB drive.

Viruses can increase their chances of spreading to other computers by infecting files on a network file system or a file system that is accessed by another computer. Often, crackers create these programs just to see how far they will spread. Unfortunately, even a supposedly harmless virus can have a serious effect on processing and network operations.

The term "computer virus" is sometimes used as a catch-all phrase to include all types of malware. Viruses are sometimes confused with computer worms and Trojan horses, which are technically different.

Worms are self-copying programs that have the capacity to move from one computer to another without human help, by exploiting security flaws in computer networks. Worms are self-contained and don't need to be attached to a document or program the way viruses do. Like a virus, a worm is also a self-replicating program. The difference

between a virus and a worm is that a worm does not create multiple copies of itself on one system: it propagates through computer networks. A worm can exploit security vulnerabilities to spread itself to other computers without needing to be transferred as part of a host. After the comparison between computer viruses and biological viruses, the obvious comparison here is to a bacterium. Many people conflate the terms "virus" and "worm", using them both to describe any self-propagating program. It is possible for a program to have the blunt characteristics of both a worm and a virus.

Trojan horses are malicious programs disguised as innocent-looking files or embedded within legitimate software, it is a program that appears harmless but has a hidden agenda. Once they are activated, they may affect the computer in a variety of ways: some are just annoying, others are more ominous, creating a backdoor to the computer which can be used

to collect stored data. They don't copy themselves or reproduce by infecting other files.

A Trojan horse is a program designed as to seem to being or be doing one thing, such as legitimate software, but actually being or doing another. They are not necessarily malicious programs but can be. A Trojan horse can be used to set up a back door in a computer system so that the intruder can return later and gain access. Viruses that fool a user into downloading and/or executing them by pretending to be useful applications are also sometimes called Trojan horses. (The name refers to the horse from the Trojan War, with conceptually similar function of deceiving defenders into bringing an intruder inside.) Worms and Trojans, like viruses, may cause harm to either a computer system's hosted data, functional performance, or networking throughput, when they are executed. Some viruses and other malware have symptoms noticeable to the computer user, but many are surreptitious.

Spyware, software designed to collect information from computers for commercial or criminal purposes, is another example of malicious software. It usually comes hidden in fake freeware or shareware applications downloadable from the Internet. Any software that covertly gathers user information through the user's Internet connection without his or her knowledge is called spyware. That information may include surfing habits,

system details or, in its most dangerous form, passwords and login information for critical applications such as online banking. Many spyware programs are more annoying than dangerous, serving up pop-up ads or gathering e-mail addresses for use in spam campaigns. Even those programs, however, can cost you valuable time and computing resources. Often, spyware comes along with a free software application, such as a game or a supposed productivity booster.

Adware is similar to spyware and malware, in that it resides on a computer without the user's knowledge; adware specifically refers to programs that display pop-up advertisements. The subject matter of the ads is often based on surfing habits, but may also be tied to a specific advertiser.

While cookies aren't really malware, they can be used in similar ways. Cookies are small data files used by Web sites to store information on your computer. For example, a shopping site may want to identify items you've looked at, but not purchased, or store data on current purchases until you head for the checkout. A less scrupulous site, however, may decide to look through your cookies for personal information, such as recent sites you have visited.

Rootkits - a collection of programs that permits administrator-level control of a computer; not necessarily malware on its own, crackers use rootkits to control computers and evade detection. Backdoors - methods of circumventing the normal operating-system

procedures, allowing a cracker to access information on another computer.

Key loggers - programs that record keystrokes made by a user, allowing crackers to discover passwords, login codes and other confidential information.