Computer virus

A computer virus is a computer program that can copy itself and infect a computer without permission or knowledge of the user. However, the term "virus" is commonly used, albeit erroneously, to refer to many different types of malware programs. The original virus 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 the Internet, or by carrying it on a removable medium such as a floppy disk, CD, or USB drive. Meanwhile 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 can spread itself to other computers without needing to be transferred as part of a host, and a Trojan horse is a file that appears harmless until executed.
Most personal computers are now connected to the Internet and to local area networks, facilitating the spread of malicious code. 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.
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.

History

The Creeper virus was first detected on ARPANET, the forerunner of the Internet in the early 1970s. It propagated via the TENEX operating system and could make use of any connected modem to dial out to remote computers and infect them. It would display the message "I'M THE CREEPER : CATCH ME IF YOU CAN.". It is rumored[attribution needed] that the Reaper program, which appeared shortly after and sought out copies of the Creeper and deleted them, may have been written by the creator of the Creeper in a fit of regret.[original research?]
A program called "Elk Cloner" is commonly credited[attribution needed] with being the first computer virus to appear "in the wild" — that is, outside the single computer or lab where it was created, but that claim is false. See the Timeline of notable computer viruses and worms for other earlier viruses. It was however the first virus to infect computers "in the home". Written in 1982 by Richard Skrenta, it attached itself to the Apple DOS 3.3 operating system and spread by floppy disk. This virus was originally a joke, created by a high school student and put onto a game. The disk could only be used 49 times. The game was set to play, but release the virus on the 50th time of starting the game.The first PC virus in the wild was a boot sector virus called (c)Brain[, created in 1986 by the Farooq Alvi Brothers, operating out of Lahore, Pakistan. The brothers reportedly created the virus to deter pirated copies of software they had written. However, analysts have claimed that the Ashar virus, a variant of Brain, possibly predated it based on code within the virus.
Before computer networks became widespread, most viruses spread on removable media, particularly floppy disks.[citation needed] 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.
Traditional computer viruses emerged in the 1980s, driven by the spread of personal computers and the resultant increase in BBS and 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 and games were easy targets for viruses.[original research?]
Since the mid-1990s, macro viruses have become common. Most of these viruses are written in the scripting languages for Microsoft programs such as Word and Excel. These viruses spread in Microsoft Office by infecting documents and spreadsheets. Since Word and Excel were also available for Mac OS, most of these viruses were able to spread on Macintosh computers as well. Most of these viruses did not have the ability to send infected e-mail. Those viruses which did spread through e-mail took advantage of the Microsoft Outlook COM interface.[citation needed]
Macro viruses pose unique problems for detection software[citation needed]. For example, some versions of Microsoft Word allowed macros to replicate themselves with additional blank lines. The virus behaved identically but would be misidentified as a new virus. In another example, 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".
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.
The newest species of the virus family is the cross-site scripting virus[citation needed]. The virus emerged from research and was academically demonstrated in 2005. This virus utilizes cross-site scripting vulnerabilities to propagate. Since 2005 there have been multiple instances of the cross-site scripting viruses in the wild, most notable sites affected have been MySpace and Yahoo.

Replication 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 tries to start an infected program, the virus' code may be executed first. Viruses can be divided into two types, on the basis of their behavior when they are executed. Nonresident viruses immediately search for other hosts that can be infected, infect these 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. However, this module is not called by a finder module. Instead, the virus loads the replication module into memory when it is executed and ensures that this module is executed each time the operating system is called to perform a certain operation. For example, the replication module can be called 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. For instance, a fast infector can infect every potential host file that is accessed. This poses a special problem to 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. For instance, some slow infectors 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 does not seem very successful, however.

Methods to avoid detection

In order to avoid detection by users, some viruses employ different kinds of deception. Some old viruses, especially on the MS-DOS platform, make sure that the "last modified" date of a host file stays the same when the file is infected by the virus. This approach does not fool anti-virus software, however, especially that which maintains and dates Cyclic redundancy check on file changes.Some viruses can infect files without increasing their sizes or damaging the files. They accomplish this by overwriting unused areas of executable files. These are called cavity viruses. For example the CIH virus, or Chernobyl Virus, infects Portable Executable files. Because those files had many empty gaps, the virus, which was 1 KB in length, did not add to the size of the file.Some viruses try to avoid detection by killing the tasks associated with antivirus software before it can detect them.As computers and operating systems grow larger and more complex, old hiding techniques need to be updated or replaced. Defending a computer against viruses may demand that a file system migrate towards detailed and explicit permission for every kind of file access.

Avoiding bait files and other undesirable hosts

A virus needs to infect hosts in order to spread further. In some cases, it might be a bad idea to infect a host program. For example, many anti-virus programs perform an integrity check of their own code. Infecting such programs will therefore increase the likelihood that the virus is detected. For this reason, some viruses are programmed not to infect programs that are known to be part of anti-virus software. Another type of host that viruses sometimes avoid is bait files. Bait files (or goat files) are files that are specially created by anti-virus software, or by anti-virus professionals themselves, to be infected by a virus. These files can be created for various reasons, all of which are related to the detection of the virus:Anti-virus professionals can use bait files to take a sample of a virus (i.e. a copy of a program file that is infected by the virus). It is more practical to store and exchange a small, infected bait file, than to exchange a large application program that has been infected by the virus.Anti-virus professionals can use bait files to study the behavior of a virus and evaluate detection methods. This is especially useful when the virus is polymorphic. In this case, the virus can be made to infect a large number of bait files. The infected files can be used to test whether a virus scanner detects all versions of the virus. Some anti-virus software employs bait files that are accessed regularly. When these files are modified, the anti-virus software warns the user that a virus is probably active on the system. Since bait files are used to detect the virus, or to make detection possible, a virus can benefit from not infecting them. Viruses typically do this by avoiding suspicious programs, such as small program files or programs that contain certain patterns of 'garbage instructions'.A related strategy to make baiting difficult is sparse infection. Sometimes, sparse infectors do not infect a host file that would be a suitable candidate for infection in other circumstances. For example, a virus can decide on a random basis whether to infect a file or not, or a virus can only infect host files on particular days of the week.