Computer virus

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Not to be confused with Malware.

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A computer virus is a computer program that can copy itself^[1] and infect a computer. The term "virus" is also commonly but erroneously used to refer to other types of malware, including but not limited to adware and spyware programs that do not have the reproductive ability. A true virus can 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.^[2]^[3]

As stated above, the term "computer virus" is sometimes used as a catch-all phrase to include all types of malware, even those that do not have the reproductive ability. Malware includes computer viruses, computer worms, Trojan horses, most rootkits, spyware, dishonest adware and other malicious and unwanted software, including true viruses. Viruses are sometimes confused with worms and Trojan horses, which are technically different. A worm can exploit security vulnerabilities to spread itself automatically to other computers through networks, while a Trojan horse is a program that appears harmless but hides malicious functions. Worms and Trojan horses, like viruses, may harm a computer system's data or performance. Some viruses and other malware have symptoms noticeable to the computer user, but many are surreptitious or simply do nothing to call attention to themselves. Some viruses do nothing beyond reproducing themselves.

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History

Academic work

The first academic work on the theory of computer viruses (although the term "computer virus" was not invented at that time) was done by John von Neumann in 1949 who held lectures at the University of Illinois about the "Theory and Organization of Complicated Automata". The work of von Neumann was later published as the "Theory of self-reproducing automata".^[4] In his essay von Neumann postulated that a computer program could reproduce.

In 1972 Veith Risak published his article "Selbstreproduzierende Automaten mit minimaler Informationsübertragung" (Self-reproducing automata with minimal information exchange).^[5] The article describes a fully functional virus written in assembler language for a SIEMENS 4004/35 computer system.

In 1980 Jürgen Kraus wrote his diplom thesis "Selbstreproduktion bei Programmen" (Self-reproduction of programs) at the University of Dortmund.^[6] In his work Kraus postulated that computer programs can behave in a way similar to biological viruses.

In 1984 Fred Cohen from the University of Southern California wrote his paper "Computer Viruses - Theory and Experiments".^[7] It was the first paper to explicitly call a self-reproducing program a "virus"; a term introduced by his mentor Leonard Adleman.

An article that describes "useful virus functionalities" was published by J.B. Gunn under the title "Use of virus functions to provide a virtual APL interpreter under user control" in 1984.^[8]

Science Fiction

The Terminal Man, a science fiction novel by Michael Crichton (1972), told (as a sideline story) of a computer with telephone modem dialing capability, which had been programmed to randomly dial phone numbers until it hit a modem that is answered by another computer. It then attempted to program the answering computer with its own program, so that the second computer would also begin dialing random numbers, in search of yet another computer to program. The program is assumed to spread exponentially through susceptible computers.

The actual term 'virus' was first used in David Gerrold's 1972 novel, When HARLIE Was One. In that novel, a sentient computer named HARLIE writes viral software to retrieve damaging personal information from other computers to blackmail the man who wants to turn him off.

Virus programs

The Creeper virus was first detected on ARPANET, the forerunner of the Internet, in the early 1970s.^[9] Creeper was an experimental self-replicating program written by Bob Thomas at BBN Technologies in 1971.^[10] Creeper used the ARPANET to infect DEC PDP-10 computers running the TENEX operating system. Creeper gained access via the ARPANET and copied itself to the remote system where the message, "I'm the creeper, catch me if you can!" was displayed. The Reaper program was created to delete Creeper.^[11]

A program called "Elk Cloner" was the first computer virus to appear "in the wild" — that is, outside the single computer or lab where it was created.^[12] Written in 1981 by Richard Skrenta, it attached itself to the Apple DOS 3.3 operating system and spread via floppy disk.^[12]^[13] This virus, created as a practical joke when Skrenta was still in high school, was injected in a game on a floppy disk. On its 50th use the Elk Cloner virus would be activated, infecting the computer and displaying a short poem beginning "Elk Cloner: The program with a personality."

The first PC virus in the wild was a boot sector virus dubbed (c)Brain^[14], created in 1986 by the Farooq Alvi Brothers in Lahore, Pakistan, reportedly to deter piracy of the software they had written^[15]. However, analysts have claimed that the Ashar virus, a variant of Brain, possibly predated it based on code within the virus.^{[original research?]}

Before computer networks became widespread, most viruses spread on removable media, particularly floppy disks. In the early days of the personal computer, many users regularly exchanged information and programs on floppies. Some viruses spread by infecting programs stored on these disks, while others installed themselves into the disk boot sector, ensuring that they would be run when the user booted the computer from the disk, usually inadvertently. PCs of the era would attempt to boot first from a floppy if one had been left in the drive. Until floppy disks fell out of use, this was the most successful infection strategy and boot sector viruses were the most common in the wild for many years.^[1]

Traditional computer viruses emerged in the 1980s, driven by the spread of personal computers and the resultant increase in BBS, modem use, and software sharing. Bulletin board-driven software sharing contributed directly to the spread of Trojan horse programs, and viruses were written to infect popularly traded software. Shareware and bootleg software were equally common vectors for viruses on BBS's.^{[citation needed]} Within the "pirate scene" of hobbyists trading illicit copies of retail software, traders in a hurry to obtain the latest applications were easy targets for viruses.^{[original research?]}Macro viruses have become common since the mid-1990s. Most of these viruses are written in the scripting languages for Microsoft programs such as Word and Excel and spread throughout Microsoft Office by infecting documents and spreadsheets. Since Word and Excel were also available for Mac OS, most could also spread to Macintosh computers. Although most of these viruses did not have the ability to send infected e-mail, those viruses which did take advantage of the Microsoft Outlook COM interface.^{[citation needed]}

Some old versions of Microsoft Word allow macros to replicate themselves with additional blank lines. If two macro viruses simultaneously infect a document, the combination of the two, if also self-replicating, can appear as a "mating" of the two and would likely be detected as a virus unique from the "parents".^[16]

A virus may also send a web address link as an instant message to all the contacts on an infected machine. If the recipient, thinking the link is from a friend (a trusted source) follows the link to the website, the virus hosted at the site may be able to infect this new computer and continue propagating.

Viruses that spread using cross-site scripting were first reported in 2002^[17], and were academically demonstrated in 2005.^[18] There have been multiple instances of the cross-site scripting viruses in the wild, exploiting websites such as MySpace and Yahoo.

Infection strategies

In order to replicate itself, a virus must be permitted to execute code and write to memory. For this reason, many viruses attach themselves to executable files that may be part of legitimate programs. If a user attempts to launch an infected program, the virus' code may be executed simultaneously. Viruses can be divided into two types based on their behavior when they are executed. Nonresident viruses immediately search for other hosts that can be infected, infect those targets, and finally transfer control to the application program they infected. Resident viruses do not search for hosts when they are started. Instead, a resident virus loads itself into memory on execution and transfers control to the host program. The virus stays active in the background and infects new hosts when those files are accessed by other programs or the operating system itself.

Nonresident viruses

Nonresident viruses can be thought of as consisting of a finder module and a replication module. The finder module is responsible for finding new files to infect. For each new executable file the finder module encounters, it calls the replication module to infect that file.

Resident viruses

Resident viruses contain a replication module that is similar to the one that is employed by nonresident viruses. This module, however, is not called by a finder module. The virus loads the replication module into memory when it is executed instead and ensures that this module is executed each time the operating system is called to perform a certain operation. The replication module can be called, for example, each time the operating system executes a file. In this case the virus infects every suitable program that is executed on the computer.

Resident viruses are sometimes subdivided into a category of fast infectors and a category of slow infectors. Fast infectors are designed to infect as many files as possible. A fast infector, for instance, can infect every potential host file that is accessed. This poses a special problem when using anti-virus software, since a virus scanner will access every potential host file on a computer when it performs a system-wide scan. If the virus scanner fails to notice that such a virus is present in memory the virus can "piggy-back" on the virus scanner and in this way infect all files that are scanned. Fast infectors rely on their fast infection rate to spread. The disadvantage of this method is that infecting many files may make detection more likely, because the virus may slow down a computer or perform many suspicious actions that can be noticed by anti-virus software. Slow infectors, on the other hand, are designed to infect hosts infrequently. Some slow infectors, for instance, only infect files when they are copied. Slow infectors are designed to avoid detection by limiting their actions: they are less likely to slow down a computer noticeably and will, at most, infrequently trigger anti-virus software that detects suspicious behavior by programs. The slow infector approach, however, does not seem very successful.