Sep 30, 2016. The difference between these two virus types is where the viral code is located. Master boot record infectors normally save a legitimate copy of the master boot record in an different location. Windows NT computers that become infected by either boot sector viruses or master boot sector viruses will not boot.

Of the, showing a message left for co-founder by the worm's programmer A computer virus is a type of malicious software program (') that, when executed, itself by modifying other and inserting its own code. Infected computer programs can include, as well, data, or the of the. When this replication succeeds, the affected areas are then said to be 'infected' with a computer virus. Virus writers use and exploit detailed knowledge of to initially infect systems and to spread the virus. The vast majority of viruses target systems running, employing a variety of mechanisms to infect new hosts, and often using complex anti-detection/stealth strategies to evade. Motives for creating viruses can include seeking (e.g., with ), desire to send a political message, personal amusement, to demonstrate that a vulnerability exists in software, for and, or simply because they wish to explore issues, and.

Computer viruses currently cause billions of dollars' worth of economic damage each year, due to causing system failure, wasting computer resources, corrupting data, increasing maintenance costs, etc. In response, antivirus tools have been developed, and an industry of has cropped up, selling or freely distributing virus protection to users of various. As of 2005, even though no currently existing antivirus software was able to uncover all computer viruses (especially new ones), computer security researchers are actively searching for new ways to enable antivirus solutions to more effectively detect emerging viruses, before they have already become widely distributed. The term 'virus' is also commonly, but erroneously, used to refer to other types of. 'Malware' encompasses computer viruses along with many other forms of malicious software, such as,,,,,,,, malicious (BHOs) and other malicious software. The majority of active malware threats are actually trojan horse programs or computer worms rather than computer viruses. The term computer virus, coined by in 1985, is a misnomer.

Viruses often perform some type of harmful activity on infected host computers, such as acquisition of space or (CPU) time, accessing private information (e.g., numbers), corrupting data, displaying political or humorous messages on the user's screen, their e-mail contacts,, or even rendering the computer useless. However, not all viruses carry a destructive 'payload' and attempt to hide themselves—the defining characteristic of viruses is that they are self-replicating computer programs which modify other software without user consent. The virus 'Universal Peace', as displayed on a Mac in March 1988 The was first detected on, the forerunner of the, in the early 1970s. Creeper was an experimental self-replicating program written by Bob Thomas at in 1971. Creeper used the ARPANET to infect computers running the. 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. In fiction, the 1973 movie made an early mention of the concept of a computer virus, being a central plot theme that causes to run amok. 's character summarizes the problem by stating that '.there's a clear pattern here which suggests an analogy to an infectious disease process, spreading from one.area to the next.' To which the replies are stated: 'Perhaps there are superficial similarities to disease' and, 'I must confess I find it difficult to believe in a disease of machinery.'

In 1982, a program called ' was the first personal computer virus to appear 'in the wild'—that is, outside the single computer or [computer] lab where it was created. Written in 1981 by while in the ninth grade at near, it attached itself to the 3.3 operating system and spread via.

This virus, created as a when Skrenta was still in high school, was injected in a game on a floppy disk. On its 50th use the virus would be activated, infecting the personal computer and displaying a short poem beginning 'Elk Cloner: The program with a personality.' In 1984 from the wrote his paper 'Computer Viruses – Theory and Experiments'. It was the first paper to explicitly call a self-reproducing program a 'virus', a term introduced by Cohen's mentor. In 1987, Fred Cohen published a demonstration that there is no that can perfectly detect all possible viruses.

Fred Cohen's theoretical was an example of a virus which was not malicious software (), but was putatively benevolent (well-intentioned). However, antivirus professionals do not accept the concept of 'benevolent viruses', as any desired function can be implemented without involving a virus (automatic compression, for instance, is available under the at the choice of the user). Any virus will by definition make unauthorised changes to a computer, which is undesirable even if no damage is done or intended. On page one of Dr Solomon's Virus Encyclopaedia, the undesirability of viruses, even those that do nothing but reproduce, is thoroughly explained.

An article that describes 'useful virus functionalities' was published by under the title 'Use of virus functions to provide a virtual interpreter under user control' in 1984. The first virus in the 'wild' was a virus dubbed, created in 1986 by the Farooq Alvi Brothers in, reportedly to deter unauthorized copying of the software they had written. The first virus to specifically target, was discovered in April 1992, two years after the release of. The virus did not contain any, instead relying on.

A few years later, in February 1996, Australian hackers from the virus-writing crew VLAD created the Bizatch virus (also known as 'Boza' virus), which was the first known virus to target. In late 1997 the encrypted, memory-resident stealth virus was released—the first known virus that targeted (it was also able to infect Windows 3.0 and Windows 9x hosts). Even were affected by viruses. The first one to appear on the was a boot sector virus called, which was detected in November 1987. The first virus,, was created by on August 15, 2001. The virus specifically targeted users of and.

Users would be required to click on a link to activate the virus, which would then send an email containing user data to an anonymous email address, which was later found to be owned by Larose. Data sent would contain items such as user and email addresses, contacts, website browsing history, and. In 2008, larger websites used part of the Win32.5-0-1 code to track web users advertising-related interests. Operations and functions [ ] Parts [ ] A viable computer virus must contain a, which locates new files or new disks which are worthwhile targets for infection. Secondly, every computer virus must contain a routine to copy itself into the program which the search routine locates. The three main virus parts are: Infection mechanism [ ] Infection mechanism (also called 'infection vector'), is how the virus spreads or propagates.

A virus typically has a search routine, which locates new files or new disks for infection. Trigger [ ] The trigger, which is also known as, is the that could be activated any time an with the virus is run that determines the event or condition for the malicious ' to be activated or delivered such as a particular date, a particular time, particular presence of another program, capacity of the disk exceeding some limit, or a that opens a particular file. Payload [ ] The is the actual body or data that perform the actual malicious purpose of the virus. Payload activity might be noticeable (e.g., because it causes the system to slow down or 'freeze'), as most of the time the 'payload' itself is the harmful activity, or some times non-destructive but distributive, which is called. Phases [ ] Virus phases is the of the computer virus, described by using an analogy to. This life cycle can be divided into four phases: Dormant phase [ ] The virus program is idle during this stage. The virus program has managed to access the target user's computer or software, but during this stage, the virus does not take any action.

The virus will eventually be activated by the 'trigger' which states which event will execute the virus, such as a date, the presence of another program or file, the capacity of the disk exceeding some limit or the user taking a certain action (e.g., double-clicking on a certain icon, opening an e-mail, etc.). Not all viruses have this stage. Propagation phase [ ] The virus starts propagating, that is multiplying and replicating itself. The virus places a copy of itself into other programs or into certain system areas on the disk. The copy may not be identical to the propagating version; viruses often 'morph' or change to evade detection by IT professionals and anti-virus software. Each infected program will now contain a of the virus, which will itself enter a propagation phase.

Triggering phase [ ] A dormant virus moves into this phase when it is activated, and will now perform the function for which it was intended. The triggering phase can be caused by a variety of system events, including a count of the number of times that this copy of the virus has made copies of itself.

Execution phase [ ] This is the actual work of the virus, where the 'payload' will be released. It can be destructive such as deleting files on disk, crashing the system, or corrupting files or relatively harmless such as popping up humorous or political messages on screen. Infection targets and replication techniques [ ] Computer viruses infect a variety of different subsystems on their host computers and software. One manner of classifying viruses is to analyze whether they reside in (such as or ), data files (such as documents or ), or in the of the host's (or some combination of all of these).

Non-resident viruses [ ] A memory-resident virus (or simply 'resident virus') installs itself as part of the when executed, after which it remains in from the time the computer is booted up to when it is shut down. Resident viruses overwrite code or other, and when the operating system attempts to access the target file or disk sector, the virus code intercepts the request and redirects the to the replication module, infecting the target. In contrast, a non-memory-resident virus (or 'non-resident virus'), when executed, scans the disk for targets, infects them, and then exits (i.e. It does not remain in memory after it is done executing).

Macro viruses [ ] Many common applications, such as and, allow programs to be embedded in documents or emails, so that the programs may be run automatically when the document is opened. A macro virus (or 'document virus') is a virus that is written in a, and embedded into these documents so that when users open the file, the virus code is executed, and can infect the user's computer. This is one of the reasons that it is dangerous to open unexpected or suspicious in. While not opening attachments in e-mails from unknown persons or organizations can help to reduce the likelihood of contracting a virus, in some cases, the virus is designed so that the e-mail appears to be from a reputable organization (e.g., a major bank or credit card company). Boot sector viruses [ ] Boot sector viruses specifically target the and/or the (MBR) of the host's or removable storage media (,, etc.). Email virus [ ] Email virus – A virus that intentionally, rather than accidentally, uses the email system to spread.

While virus infected files may be accidentally sent as, email viruses are aware of email system functions. They generally target a specific type of email system (Microsoft’s Outlook is the most commonly used), harvest email addresses from various sources, and may append copies of themselves to all email sent, or may generate email messages containing copies of themselves as attachments.

Stealth techniques [ ] In order to avoid detection by users, some viruses employ different kinds of. Some old viruses, especially on the 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 antivirus software, however, especially those which maintain and date 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, or Chernobyl Virus, infects files.

Because those files have many empty gaps, the virus, which was 1 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 (for example, ). In the 2010s, 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.

Read request intercepts [ ] While some kinds of antivirus software employ various techniques to counter stealth mechanisms, once the infection occurs any recourse to 'clean' the system is unreliable. In Microsoft Windows operating systems, the is proprietary. This leaves antivirus software little alternative but to send a 'read' request to Windows OS files that handle such requests. Some viruses trick antivirus software by intercepting its requests to the (OS). A virus can hide by intercepting the request to read the infected file, handling the request itself, and returning an uninfected version of the file to the antivirus software.

The interception can occur by of the actual operating system files that would handle the read request. Thus, an antivirus software attempting to detect the virus will either not be given permission to read the infected file, or, the 'read' request will be served with the uninfected version of the same file.

The only reliable method to avoid 'stealth' viruses is to 'reboot' from a medium that is known to be 'clear'. Security software can then be used to check the dormant operating system files. Most security software relies on virus signatures, or they employ. Security software may also use a database of file ' for Windows OS files, so the security software can identify altered files, and request Windows installation media to replace them with authentic versions. In older versions of Windows, file of Windows OS files stored in Windows—to allow file integrity/authenticity to be checked—could be overwritten so that the would report that altered system files are authentic, so using file hashes to scan for altered files would not always guarantee finding an infection. Self-modification [ ]. See also: Most modern antivirus programs try to find virus-patterns inside ordinary programs by scanning them for so-called virus signatures.

Unfortunately, the term is misleading, in that viruses do not possess unique signatures in the way that human beings do. Such a virus 'signature' is merely a sequence of bytes that an antivirus program looks for because it is known to be part of the virus. A better term would be 'search '. Different antivirus programs will employ different search strings, and indeed different search methods, when identifying viruses.

If a virus scanner finds such a pattern in a file, it will perform other checks to make sure that it has found the virus, and not merely a coincidental sequence in an innocent file, before it notifies the user that the file is infected. The user can then delete, or (in some cases) 'clean' or 'heal' the infected file. Some viruses employ techniques that make detection by means of signatures difficult but probably not impossible. These viruses modify their code on each infection. That is, each infected file contains a different variant of the virus. [ ] Encrypted viruses [ ] One method of evading signature detection is to use simple to encipher (encode) the body of the virus, leaving only the encryption module and a static in which does not change from one infection to the next.

In this case, the virus consists of a small decrypting module and an encrypted copy of the virus code. If the virus is encrypted with a different key for each infected file, the only part of the virus that remains constant is the decrypting module, which would (for example) be appended to the end. In this case, a virus scanner cannot directly detect the virus using signatures, but it can still detect the decrypting module, which still makes indirect detection of the virus possible. Since these would be symmetric keys, stored on the infected host, it is entirely possible to decrypt the final virus, but this is probably not required, since is such a rarity that it may be reason for virus scanners to at least 'flag' the file as suspicious. An old but compact way will be the use of arithmetic operation like addition or subtraction and the use of logical conditions such as, where each byte in a virus is with a constant, so that the exclusive-or operation had only to be repeated for decryption. It is suspicious for a code to modify itself, so the code to do the encryption/decryption may be part of the signature in many virus definitions. A simpler older approach did not use a key, where the encryption consisted only of operations with no parameters, like incrementing and decrementing, bitwise rotation, arithmetic negation, and logical NOT.

Some viruses will employ a means of encryption inside an executable in which the virus is encrypted under certain events, such as the virus scanner being disabled for updates or the computer being. This is called. At said times, the executable will decrypt the virus and execute its hidden, infecting the computer and sometimes disabling the antivirus software. [ ] Polymorphic code [ ] was the first technique that posed a serious to virus scanners. Just like regular encrypted viruses, a polymorphic virus infects files with an encrypted copy of itself, which is decoded by a module. In the case of polymorphic viruses, however, this decryption module is also modified on each infection.

A well-written polymorphic virus therefore has no parts which remain identical between infections, making it very difficult to detect directly using 'signatures'. Antivirus software can detect it by decrypting the viruses using an, or by of the encrypted virus body. To enable polymorphic code, the virus has to have a (also called 'mutating engine' or ' engine') somewhere in its encrypted body. See for technical detail on how such engines operate. Some viruses employ polymorphic code in a way that constrains the mutation rate of the virus significantly.

For example, a virus can be programmed to mutate only slightly over time, or it can be programmed to refrain from mutating when it infects a file on a computer that already contains copies of the virus. The advantage of using such slow polymorphic code is that it makes it more difficult for antivirus professionals and investigators to obtain representative samples of the virus, because 'bait' files that are infected in one run will typically contain identical or similar samples of the virus. This will make it more likely that the detection by the virus scanner will be unreliable, and that some instances of the virus may be able to avoid detection. Metamorphic code [ ] To avoid being detected by emulation, some viruses rewrite themselves completely each time they are to infect new executables. Viruses that utilize this technique are said to be in. To enable metamorphism, a 'metamorphic engine' is needed.

A metamorphic virus is usually very large and complex. For example, consisted of over 14,000 lines of code, 90% of which is part of the metamorphic engine. Vulnerabilities and infection vectors [ ] Software bugs [ ] As software is often designed with security features to prevent unauthorized use of system resources, many viruses must exploit and manipulate, which are in a system or application software, to spread themselves and infect other computers. Strategies that produce large numbers of 'bugs' will generally also produce potential 'holes' or 'entrances' for the virus.

Social engineering and poor security practices [ ] 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 that may be part of legitimate programs (see ). If a user attempts to launch an infected program, the virus' code may be executed simultaneously.

In operating systems that use to determine program associations (such as Microsoft Windows), the extensions may be hidden from the user by default. This makes it possible to create a file that is of a different type than it appears to the user.

For example, an executable may be created and named 'picture.png.exe', in which the user sees only 'picture.png' and therefore assumes that this file is a and most likely is safe, yet when opened, it runs the executable on the client machine. Vulnerability of different operating systems [ ] The vast majority of viruses target systems running. This is due to Microsoft's large market share of users. The diversity of software systems on a network limits the destructive potential of viruses and malware. Operating systems such as allow users to choose from a variety of, packaging tools, etc., which means that malicious code targeting any of these systems will only affect a subset of all users.

Many Windows users are running the same set of applications, enabling viruses to rapidly spread among Microsoft Windows systems by targeting the same exploits on large numbers of hosts. While Linux and Unix in general have always natively prevented normal users from making changes to the environment without permission, Windows users are generally not prevented from making these changes, meaning that viruses can easily gain control of the entire system on Windows hosts. This difference has continued partly due to the widespread use of accounts in contemporary versions like. In 1997, researchers created and released a virus for Linux—known as '. Bliss, however, 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 as an administrator, or, 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, allowing researchers to see how it worked.

Countermeasures [ ]. Screenshot of the running in on Many users install that can detect and eliminate known viruses when the computer attempts to or run the executable file (which may be distributed as an email attachment, or on, for example). Some antivirus software blocks known malicious websites that attempt to install malware. Antivirus software does not change the underlying capability of hosts to transmit viruses. Users must update their software regularly to ('holes').

Antivirus software also needs to be regularly updated in order to recognize the latest. This is because malicious and other individuals are always creating new viruses. The German Institute publishes evaluations of antivirus software for Windows and Android. Examples of Microsoft Windows and anti-malware software include the optional (for Windows XP, Vista and Windows 7) for real-time protection, the (now included with on ', the second Tuesday of each month), and (an optional download in the case of Windows XP).

Additionally, several capable antivirus software programs are available for free download from the Internet (usually restricted to non-commercial use). Some such free programs are almost as good as commercial competitors. Common are assigned and listed in the US. Is an example of software, free for personal use, that will check a PC for vulnerable out-of-date software, and attempt to update it. And alerts appear as press releases on the.

Ransomware is a virus that posts a message on the user's screen saying that the screen or system will remain locked or unusable until a payment is made. Is a deception in which the malicious individual pretends to be a friend, computer security expert, or other benevolent individual, with the goal of convincing the targeted individual to reveal or other personal information. Other commonly used preventative measures include timely operating system updates, software updates, careful Internet browsing (avoiding shady websites), and installation of only trusted software.

Certain browsers flag sites that have been reported to Google and that have been confirmed as hosting malware by Google. There are two common methods that an antivirus software application uses to detect viruses, as described in the article. The first, and by far the most common method of virus detection is using a list of definitions. This works by examining the content of the computer's memory (its (RAM), and ) and the files stored on fixed or removable drives (hard drives, floppy drives, or USB flash drives), and comparing those files against a of known virus 'signatures'. Virus signatures are just strings of code that are used to identify individual viruses; for each virus, the antivirus designer tries to choose a unique signature string that will not be found in a legitimate program.

Different antivirus programs use different 'signatures' to identify viruses. The disadvantage of this detection method is that users are only protected from viruses that are detected by signatures in their most recent virus definition update, and not protected from new viruses (see '). A second method to find viruses is to use a based on common virus behaviors. This method has the ability to detect new viruses for which antivirus security firms have yet to define a 'signature', but it also gives rise to more than using signatures. False positives can be disruptive, especially in a commercial environment, because it may lead to a company instructing staff not to use the company computer system until IT services has checked the system for viruses. This can slow down productivity for regular workers. Recovery strategies and methods [ ] One may reduce the damage done by viruses by making regular of data (and the operating systems) on different media, that are either kept unconnected to the system (most of the time, as in a hard drive), or not accessible for other reasons, such as using different.

This way, if data is lost through a virus, one can start again using the backup (which will hopefully be recent). If a backup session on like and is closed, it becomes read-only and can no longer be affected by a virus (so long as a virus or infected file was not copied onto the /). Likewise, an operating system on a CD can be used to start the computer if the installed operating systems become unusable. Backups on removable media must be carefully inspected before restoration. The Gammima virus, for example, propagates via removable. Virus removal [ ] Many websites run by antivirus software companies provide free online virus scanning, with limited 'cleaning' facilities (after all, the purpose of the websites is to sell antivirus products and services). Some websites—like subsidiary.com—allow users to upload one or more suspicious files to be scanned and checked by one or more antivirus programs in one operation.

Additionally, several capable antivirus software programs are available for free download from the Internet (usually restricted to non-commercial use). Microsoft offers an optional free antivirus utility called, a that is updated as part of the regular Windows update regime, and an older optional anti-malware (malware removal) tool that has been upgraded to an antivirus product in Windows 8. Some viruses disable and other important Windows tools such as and. An example of a virus that does this is CiaDoor. Many such viruses can be removed by the computer, entering Windows ' with networking, and then using system tools. On,, and can restore the and critical system files to a previous checkpoint. Often a virus will cause a system to 'hang' or 'freeze', and a subsequent hard reboot will render a system restore point from the same day corrupted.

Restore points from previous days should work, provided the virus is not designed to corrupt the restore files and does not exist in previous restore points. Operating system reinstallation [ ] Microsoft's (improved in Windows 7 and later) can be used to check for, and repair, corrupted system files. Restoring an earlier 'clean' (virus-free) copy of the entire partition from a, a, or a copy is one solution—restoring an earlier backup disk 'image' is relatively simple to do, usually removes any malware, and may be faster than 'disinfecting' the computer—or reinstalling and reconfiguring the operating system and programs from scratch, as described below, then restoring user preferences. Reinstalling the operating system is another approach to virus removal. It may be possible to recover copies of essential user data by booting from a, or connecting the hard drive to another computer and booting from the second computer's operating system, taking great care not to infect that computer by executing any infected programs on the original drive. The original hard drive can then be reformatted and the OS and all programs installed from original media.

Once the system has been restored, precautions must be taken to avoid reinfection from any restored. Viruses and the Internet [ ]. See also: Before computer networks became widespread, most viruses spread on, particularly.

In the early days of the, 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, ensuring that they would be run when the user booted the computer from the disk, usually inadvertently.

Personal computers 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.

Traditional computer viruses emerged in the 1980s, driven by the spread of personal computers and the resultant increase in (BBS), use, and software sharing. Rankings On Mario Kart Wii. –driven software sharing contributed directly to the spread of programs, and viruses were written to infect popularly traded software. And software were equally common for viruses on BBSs.

Viruses can increase their chances of spreading to other computers by infecting files on a or a file system that is accessed by other computers. Have become common since the mid-1990s. Most of these viruses are written in the scripting languages for Microsoft programs such as and and spread throughout by infecting documents and.

Since Word and Excel were also available for, most could also spread to. Although most of these viruses did not have the ability to send infected, those viruses which did take advantage of the (COM) interface. 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'. A virus may also send a as an to all the contacts (e.g., friends and colleagues' e-mail addresses) stored 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 were first reported in 2002, and were academically demonstrated in 2005. There have been multiple instances of the cross-site scripting viruses in the 'wild', exploiting websites such as (with the Samy worm) and. See also [ ].

• Burger, Ralf (16 February 2010) [1991]. Computer Viruses and Data Protection. • Granneman, Scott (6 October 2003).. The Register. • Ludwig, Mark (1993).. Tucson, Arizona 85717: American Eagle Publications, Inc..

Archived from on July 4, 2008. Hot Soup Processor Game. • (November 2006).

Microsoft Corporation. Retrieved 24 July 2011. Digital Contagions. A Media Archaeology of Computer Viruses. Digital Formations. New York: Peter Lang..

External links [ ] Wikimedia Commons has media related to. • at Curlie (based on ) (DMOZ) • • • – The original paper by Fred Cohen, 1984 • by (On hacking, 1990).