MCAFEE VIRUSSCAN SECURITY SOFTWARE
Protection Against Viruses, Trojans, Malicious Spyware
McAfee VirusScan Powerful AntiVirus Software - Computer Virus Facts
In computer security, a computer virus is a self-replicating computer program that spreads by inserting copies of itself into other executable code 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 a virus into the program is termed as an "infection", and the infected file, or executable code that is not part of a file, is called a "host". Viruses are one of the several types of malicious software or malware. In common parlance, the term virus is often extended to refer to worms, trojan horses and other sorts of malware; viruses in the narrow sense of the word are less common than they used to be, compared to other forms of malware. 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. The predominant negative effect of viruses is their uncontrolled self-reproduction, which wastes or overwhelms computer resources. Today, 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, identity theft and adware.
Definition of a computer virus
A virus is a type of program that can replicate itself by making copies of itself, which can be possibly modified (or modify themselves, as occurs in a metamorphic virus). The main criterion for classifying a piece of executable code as a virus is that it spreads itself by means of 'hosts'. 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. 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. A worm, however, can spread itself to other computers without needing to be transferred as part of a host. Many personal computers are now connected to the Internet and to local-area networks, facilitating their spread. 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. Viruses can infect different types of hosts. The most common targets are executable files that contain application software or parts of the operating system. Viruses have also infected the executable boot sectors of floppy disks, script files of application programs, and documents that can contain macro scripts. Additionally, viruses can infect files in other ways than simply inserting a copy of their code into the code of the host program. For example, a virus can overwrite its host with the virus code, or it can use a trick to ensure that the virus program is executed when the user wants to execute the (unmodified) host program. Viruses have existed for many different operating systems, including MS-DOS, AmigaOS, Linux and Mac OS; however, the vast majority of viruses affect Microsoft Windows.
Use of the word "virus"
The term "virus" is often used in common parlance to describe all kinds of malware (malicious software), including those that are more properly classified as worms or trojans. Most popular anti-virus software packages defend against all of these types of attack. The English plural of "virus" is "viruses". Some people use "virii" or "viri" as a plural, although computer professionals seldom use these words. For a discussion about whether "viri" and "virii" are correct alternatives for "viruses", see plural of virus. Sometimes the word "virus" is also considered an acronym which stands for "Vital Information Resource Under Siege", but this only started relatively recently (meaning it is a backronym instead) and is unrelated to the initial use of the word "virus" for computer malware. [2]
History
A program called "Elk Cloner" is credited with being the first computer virus to appear "in the wild" -- that is, outside the single computer or lab where it was created. Written in 1982 by Rich Skrenta, it attached itself to the Apple DOS 3.3 operating system and spread by floppy disk. The first PC virus was a boot sector virus called (c)Brain, created in 1986 by two brothers, Basit and Amjad Farooq Alvi, operating out of Lahore, Pakistan. The brothers reportedly created the virus to deter pirated copies of software they had written.[3] 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. In the early days of personal computers, 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 were first widely seen in the late 1980s, and they came about because of several factors. The first factor was the spread of personal computers. Prior to the 1980s, home computers were nearly non-existent or they were toys. Real computers were rare, and they were locked away for use by "experts." During the 1980s, real computers started to spread to businesses and homes because of popularity. By the late 1980s, PCs were widespread in businesses, homes and college campuses. The second factor was the use of computer bulletin boards. People could dial up a bulletin board with a modem and download programs of all types. Games were extremely popular, and so were simple word processors, spreadsheets, etc. Bulletin boards led to the precursor of the virus known as the Trojan horse. The third factor that led to the creation of viruses was the floppy disk. In the 1980s, programs were small, and you could fit the operating system, a word processor (plus several other programs) and some documents onto a floppy disk or two. Many computers did not have hard disks, so you would turn on your machine and it would load the operating system and everything else off of the floppy disk. Viruses took advantage of these three facts to create the first self-replicating programs. As bulletin board systems and online software exchange became popular in the late 1980s and early 1990s, more viruses were written to infect popularly traded software. Shareware and bootleg software were equally common vectors for viruses on BBSes. Within the "pirate scene" of hobbyists trading illicit copies of commercial software, traders in a hurry to obtain the latest applications and games were easy targets for viruses. 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. Numerically, most of these viruses did not have the ability to send infected e-mail. The ones that did usually worked by accessing the Microsoft Outlook COM interface. Macro viruses pose very unique problems for detection software. For example, some versions of Microsoft Word caused 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 computer virus may also be transmitted through instant messaging. A virus may 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) and follows the link to the website, the virus hosted at the site may be able to infect this new computer and continue propagating.
Why people create computer viruses
Unlike biological viruses, computer viruses do not simply evolve by themselves. Computer viruses cannot come into existence spontaneously, nor can they be created by bugs in regular programs. They are deliberately created by programmers, or by people who use virus creation software. It is possible that copying errors and recombination may lead to the actual evolution of a computer virus; however, the possibility of this type of 'digital evolution' is extremely remote. Virus writers can have various reasons for creating and spreading malware. Viruses have been written as research projects, pranks, vandalism, to attack the products of specific companies, to distribute political messages, and financial gain from identity theft, spyware, and cryptoviral extortion. Some virus writers consider their creations to be works of art, and see virus writing as a creative hobby. Additionally, many virus writers oppose deliberately destructive payload routines. Some viruses were intended as "good viruses". They spread improvements to the programs they infect, or delete other viruses. These viruses are, however, quite rare, still consume system resources, may accidentally damage systems they infect, and, on occasion, have become infected and acted as vectors for malicious viruses. A poorly-written "good virus" can also inadvertantly become a virus in and of itself (for example, such a 'good virus' may misidentify its target file and delete an innocent system file by mistake). Moreover, they normally operate without asking for permission of the owner of the computer. Since self-replicating code causes many complications, it is questionable if a well-intentioned virus can ever solve a problem in a way which is superior to a regular program that does not replicate itself.
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 get 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.
Host types
Viruses have targeted various types of hosts. This is a non-exhaustive list:
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Binary executable files (such as COM-
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Boot sectors of floppy disks and hard disk partitions
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The master boot record of a harddisk
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Application-
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Documents that can contain macros (such as Microsoft Word documents, Microsoft Excel spreadsheets, AmiPro documents, and Microsoft Access database files)
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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. 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. Recent viruses avoid any kind of detection attempt by attempting to forcefully kill the tasks associated with the virus scanner before it can detect them. As computers and operating systems grow larger and more complex, old hiding techniques need to be updated or replaced.
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 hosts 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:
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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.
Viruses and legitimate software
The vulnerability of operating systems to viruses
Another analogy to biological viruses: just as genetic diversity in a population decreases the chance of a single disease wiping out a population, the diversity of software systems on a network similarly limits the destructive potential of viruses. This became a particular concern in the 1990s, when Microsoft gained market dominance in desktop operating systems and office suites. Users who use Microsoft software (especially networking software such as Microsoft Outlook and Internet Explorer) are especially vulnerable to the spread of viruses. Microsoft software is targeted by virus writers due to their desktop dominance, and is often criticized for including many errors and holes for virus writers to exploit. Integrated applications, applications with scripting languages with access to the file system (for example Visual Basic Script (VBS), and applications with networking features) are also particularly vulnerable. Although Windows is by far the most popular operating system for virus writers, some viruses also exist on other platforms. Any operating system that allows third-party programs to run can theoretically run viruses. Some operating systems are less secure than others. Unix-based OSes (and NTFS-aware applications on Windows NT based platforms) only allow their users to run executables within their protected space in their own directories. Windows and Unix have similar scripting abilities, but while Unix natively blocks normal users from having access to make changes to the operating system environment, Windows does not. In 1997, when a virus for Linux was released – known as "Bliss" – leading antivirus vendors issued warnings that Unix-like systems could fall prey to viruses just like Windows. [6] The Bliss virus may be considered characteristic of viruses – as opposed to worms – on Unix systems. Bliss requires that the user run it explicitly, and it can only infect programs that the user has the access to modify. Unlike Windows users, most Unix users do not log in as the administrator user except to install or configure software; as a result, even if a user ran the virus, it could not harm their operating system. The Bliss virus never became widespread, and remains chiefly a research curiosity. Its creator later posted the source code to Usenet, allowing researchers to see how it worked.
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 which produce large numbers of bugs will generally also produce potential exploits. Closed-source software development, as practiced by Microsoft and other proprietary software companies, is seen by many as a security weakness. Open source software such as Linux, for example, allows all users to look for and fix security problems without relying on a single vendor. Some advocate that proprietary software makers practice vulnerability disclosure to improve this weakness. On the other hand, some claim that open source development exposes potential security problems to virus writers, hence increases in the prevalance of exploits. They counter claims that popular closed source systems such as Windows are often exploited by claiming that these systems are only commonly exploited due to their popularity and the potential widespread effect such an exploit will have.
Anti-virus software and other countermeasures
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 contents of the computer's memory (its RAM, and boot sector) 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. There have been attempts to do this but adoption of such anti-virus solutions can void the warranty for the host software. Users must therefore 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 and hoaxes.
Virus extensions
@mm is an extension commonly appended to the end of a mass mailing computer virus. This model is used by security firm Symantec, and follows any variant letter. Examples include:
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