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What is a Virus?

In computer security technology, a virus is a self-replicating program that spreads by inserting copies of itself into other executable programs or documents.

A computer virus behaves in a way similar to a biological virus , which spreads by inserting itself into living cells. Extending the analogy, the insertion of the virus into a program is termed infection , and the infected file (or executable program that is not part of a file) is called a host . Viruses are one of the several types of malware or malicious software. In common parlance, the term virus is often extended to refer to computer worms and other sorts of malware. This can confuse computer users, since viruses in the narrow sense of the word are less common than they used to be, compared to other forms of malware such as worms. This confusion can have serious consequences, because it may lead to a focus on preventing one genre of malware over another, potentially leaving computers vulnerable to future damage. However, a basic rule is that computer viruses cannot directly damage hardware, only software is damaged directly. The software in the hardware however may be damaged.

While viruses can be intentionally destructive (for example, by destroying data), many other viruses are fairly benign or merely annoying. Some viruses have a delayed payload, which is sometimes called a bomb . For example, a virus might display a message on a specific day or wait until it has infected a certain number of hosts. A time bomb occurs during a particular date or time, and a logic bomb occurs when the user of a computer takes an action that triggers the bomb. However, the predominant negative effect of viruses is their uncontrolled self-reproduction, which wastes or overwhelms computer resources.

Today (as of 2005 ), viruses are somewhat less common than network-borne worms, due to the popularity of the Internet . Anti-virus software , originally designed to protect computers from viruses, has in turn expanded to cover worms and other threats such as spyware .

What is a Worm?

A computer worm is a self-replicating computer program , similar to a computer virus . A virus attaches itself to, and becomes part of, another executable program; however, a worm is self-contained and does not need to be part of another program to propagate itself. They are often designed to exploit the file transmission capabilities found on many computers.

The name 'worm' was taken from The Shockwave Rider , a 1970s science fiction novel by John Brunner . Researchers writing an early paper on experiments in distributed computing noted the similarities between their software and the program described by Brunner and adopted the name.

The first implementation of a worm was by two researchers at Xerox PARC in 1978.

The first worm to attract wide attention, the Morris worm , was written by Robert Tappan Morris , who at the time was a graduate student at Cornell University . It was released on November 2 , 1988 , and quickly infected a great number of computers on the Internet at the time. It propagated through a number of bugs in BSD Unix and its derivatives. Morris himself was convicted under the US Computer Crime and Abuse Act and received three years probation, community service and a fine in excess of $10,000.

In addition to replication, a worm may be designed to do any number of things, such as delete files on a host system or send documents via email . More recent worms may be multi-headed and carry other executables as a payload . However, even in the absence of such a payload, a worm can wreak havoc just with the network traffic generated by its reproduction. Mydoom , for example, caused a noticeable worldwide Internet slowdown at the peak of its spread.

A common payload is for a worm to install a backdoor in the infected computer, as was done by Sobig and Mydoom . These zombie computers are used by spam senders for sending junk email or to cloak their website's address. Spammers are thought to pay for the creation of such worms, and worm writers have been caught selling lists of IP addresses of infected machines. Others try to blackmail companies with threatened DoS attacks. The backdoors can also be exploited by other worms, such as Doomjuice , which spreads using the backdoor opened by Mydoom .

Whether worms can be useful is a common theoretical question in computer science and artificial intelligence . The Nachi family of worms, for example, tried to download then install patches from Microsoft's website to fix various vulnerabilities in the host system — the same vulnerabilities that they exploited. This eventually made the systems affected more secure, but generated considerable network traffic (often more than the worms they were protecting against), rebooted the machine in the course of patching it, and, maybe most importantly, did its work without the explicit consent of the computer's owner or user. As such, most security experts deprecate worms, whatever their payload.

What is a Trojan?

In the context of computer software , a Trojan horse is a malicious program that is disguised as legitimate software. The term is derived from the classical myth of the Trojan horse . In the siege of Troy , the Greeks left a large wooden horse outside the city. The Trojans were convinced that it was a gift, and moved the horse to a place within the city walls. It turned out that the horse was hollow, containing Greek soldiers who opened the city gates of Troy at night, making it possible for the Greek army to pillage the city. Trojan horse programs work in a similar way: they may look useful or interesting (or at the very least harmless) to an unsuspecting user, but are actually harmful when executed.

Often the term is shortened to simply Trojan , even though this turns the adjective into a noun, reversing the myth (Greeks were gaining malicious access, not Trojans).

Trojan horse programs cannot replicate themselves, in contrast to some other types of malware , like viruses or worms . A Trojan horse can be deliberately attached to otherwise useful software by a cracker , or it can be spread by tricking users into believing that it is a useful program.

What is Spyware?

Spyware is a broad category of malicious software intended to intercept or take partial control of a computer 's operation without the user's informed consent . While the term taken literally suggests software that surreptitiously monitors the user, it has come to refer more broadly to software that subverts the computer's operation for the benefit of a third party.

Spyware differs from viruses and worms in that it does not usually self-replicate. Like many recent viruses , spyware is designed to exploit infected computers for commercial gain. Typical tactics furthering this goal include delivery of unsolicited pop-up advertisements ; theft of personal information (including financial information such as credit card numbers ); monitoring of Web-browsing activity for marketing purposes; or routing of HTTP requests to advertising sites.

As of 2005 , spyware has only been developed for computers running Microsoft Windows operating systems. There have been no reports of spyware attacking Mac OS X , Linux , or other platforms.

What is Malware?

Malware (a portmanteau of "malicious software ") is software program designed to fulfil any purpose contrary to the interests of the person running it. Examples of malware include viruses and trojan horses .

Malware can be classified based on how it is executed, how it spreads, and/or what it does. The classification is not perfect, however, in the sense that the groups often overlap and the difference is not always obvious, giving rise to frequent flame wars .




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