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A computer virus is a computer program that can copy itself and infect a computer without permission or knowledge of the user. The original may modify the copies or the copies may modify themselves, as occurs in a metamorphic virus . A virus can only spread from one computer to another when its host is taken to the uninfected computer, for instance by a user sending it over a network or carrying it on a removable medium such as a floppy disk , CD , USB drive or by the Internet. Additionally, viruses can spread to other computers by infecting files on a network file system or a file system that is accessed by another computer. Viruses are sometimes confused with computer worms and Trojan horses . A worm, however, can spread itself to other computers without needing to be transferred as part of a host. INTRODUCTION
In the context of computer software , a Trojan horse is a program that installs malicious software while under the guise of doing something else. A Trojan horse differs from a virus in that a Trojan horse does not insert its code into other computer files and appears harmless until executed. The term is derived from the classical myth of the Trojan Horse . Trojan horses may appear to be useful or interesting programs (or at the very least harmless) to an unsuspecting user, but are actually harmful when executed. (See Social engineering .) Often the term is shortened to simply Trojan . There are two common types of Trojan horses. One is ordinary software that has been corrupted by a hacker . A hacker inserts malicious code into the program that executes while the program is used. Examples include various implementations of weather alerting programs, computer clock setting software, and peer-to-peer file sharing utilities. The other type of Trojan is a standalone program that masquerades as something else, like a game or image file, in order to trick the user into running the program. A Trojan horse is a file that appears harmless until executed. In contrast to viruses, Trojan horses do not insert their code into other computer files. Many personal computers are now connected to the Internet and to local-area networks, facilitating their spread.
Trojan horse programs cannot operate autonomously, in contrast to some other types of malware , like viruses or worms . Just as the Greeks needed the Trojans to bring the horse inside for their plan to work, Trojan horse programs depend on actions by the intended victims. As such, if Trojans replicate and distribute themselves, each new victim must run the Trojan. Therefore their virulence is of a different nature, depending on successful implementation of social engineering concepts rather than flaws in a computer system's security design or configuration. In the field of computer architecture, 'Trojan Horse' can also refer to security loopholes that allow Kernel Code to access anything for which it is not authorized.
Some viruses are programmed to damage the computer by damaging programs, deleting files, or reformatting the hard disk. Others are not designed to do any damage, but simply replicate themselves and perhaps make their presence known by presenting text, video, or audio messages. Even these benign viruses can create problems for the computer user . They typically take up computer memory used by legitimate programs. As a result, they often cause erratic behavior and can result in system crashes. In addition, many viruses are bug -ridden, and these bugs may lead to system crashes and data loss . There are many viruses operating in the general Internet today, and new ones are created and discovered every day.
Today's viruses may also take advantage of network services such as the World Wide Web , e-mail , and file sharing systems to spread, blurring the line between viruses and worms. Furthermore, some sources use an alternative terminology in which a virus is any form of self-replicating malware .
The role of software development Because software is often designed with security features to prevent unauthorized use of system resources, many viruses must exploit software bugs in a system or application to spread. Software development strategies that produce large numbers of bugs will generally also produce potential exploits.
Anti-virus software and other preventive countermeasures There are two common methods that an anti-virus software application uses to detect viruses. The first, and by far the most common method of virus detection is using a list of virus signature definitions. The disadvantage of this detection method is that users are only protected from viruses that pre-date their last virus definition update. The second method is to use a heuristic algorithm to find viruses based on common behaviors. This method has the ability to detect viruses that anti-virus security firms’ have yet to create a signature for. Many users install anti-virus software that can detect and eliminate known viruses after the computer downloads or runs the executable. They work by examining the content heuristics of the computer's memory (its RAM , and boot sectors ) and the files stored on fixed or removable drives (hard drives, floppy drives), and comparing those files against a database of known virus "signatures". Some anti-virus programs are able to scan opened files in addition to sent and received emails 'on the fly' in a similar manner. This practice is known as "on-access scanning." Anti-virus software does not change the underlying capability of host software to transmit viruses. Users must update their software regularly to patch security holes. Anti-virus software also needs to be regularly updated in order to gain knowledge about the latest threats.
Recovery methods Once a computer has been compromised by a virus, it is usually unsafe to continue using the same computer without completely reinstalling the operating system. However, there are a number of recovery options that exist after a computer has a virus. These actions depend on severity of the type of virus. Operating system reinstallation As a last ditch effort, if a virus is on your system and anti-viral software can't clean it, then reinstalling the operating system may be required. To do this properly, the hard drive is completely erased (partition deleted and formatted) and the operating system is installed from media known not to be infected. Important files should first be backed up, if possible, and separately scanned for infection before erasing the original hard drive and reinstalling the operating system.
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