Not to be confused with computer worm or Trojan horse (computing).
A computer virus is a type of malicious software program ("malware") that, when executed, replicates itself by modifying other computer programs and inserting its own code. When this replication succeeds, the affected areas are then said to be "infected" with a computer virus.
Virus writers use social engineeringdeceptions and exploit detailed knowledge of security vulnerabilities to initially infect systems and to spread the virus. The vast majority of viruses target systems running Microsoft Windows, employing a variety of mechanisms to infect new hosts, and often using complex anti-detection/stealth strategies to evade antivirus software. Motives for creating viruses can include seeking profit (e.g., with ransomware), desire to send a political message, personal amusement, to demonstrate that a vulnerability exists in software, for sabotage and denial of service, or simply because they wish to explore cybersecurity issues, artificial life and evolutionary algorithms.
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, free, open-source antivirus tools have been developed, and an industry of antivirus software has cropped up, selling or freely distributing virus protection to users of various operating systems. 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. "Malware" encompasses computer viruses along with many other forms of malicious software, such as computer "worms", ransomware, spyware, adware, trojan horses, keyloggers, rootkits, bootkits, malicious Browser Helper Object (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 Fred Cohen in 1985, is a misnomer. Viruses often perform some type of harmful activity on infected host computers, such as acquisition of hard disk space or central processing unit (CPU) time, accessing private information (e.g., credit card numbers), corrupting data, displaying political or humorous messages on the user's screen, spamming their e-mail contacts, logging their keystrokes, 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.
See also: Timeline of notable computer viruses and worms
Early academic work on self-replicating programs
The first academic work on the theory of self-replicating computer programs was done in 1949 by John von Neumann who gave 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". In his essay von Neumann described how a computer program could be designed to reproduce itself. Von Neumann's design for a self-reproducing computer program is considered the world's first computer virus, and he is considered to be the theoretical "father" of computer virology. In 1972, Veith Risak, directly building on von Neumann's work on self-replication, published his article "Selbstreproduzierende Automaten mit minimaler Informationsübertragung" (Self-reproducing automata with minimal information exchange). The article describes a fully functional virus written in assembler programming 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. In his work Kraus postulated that computer programs can behave in a way similar to biological viruses.
The Creeper virus was first detected on ARPANET, the forerunner of the Internet, in the early 1970s. Creeper was an experimental self-replicating program written by Bob Thomas at BBN Technologies in 1971. Creeper used the ARPANET to infect DECPDP-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. In fiction, the 1973 Michael Crichtonsci-fi movie Westworld made an early mention of the concept of a computer virus, being a central plot theme that causes androids to run amok.Alan Oppenheimer'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 "Elk Cloner" 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 Richard Skrenta while in the ninth grade at Mount Lebanon High School near Pittsburgh, it attached itself to the Apple DOS 3.3 operating system and spread via floppy disk. 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 personal computer and displaying a short poem beginning "Elk Cloner: The program with a personality." In 1984 Fred Cohen from the University of Southern California 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 Leonard Adleman. In 1987, Fred Cohen published a demonstration that there is no algorithm that can perfectly detect all possible viruses. Fred Cohen's theoretical compression virus was an example of a virus which was not malicious software (malware), 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 Windows operating system 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 J. B. Gunn under the title "Use of virus functions to provide a virtual APL interpreter under user control" in 1984. The first IBM PC virus in the "wild" was a boot sector virus dubbed (c)Brain, created in 1986 by the Farooq Alvi Brothers in Lahore, Pakistan, reportedly to deter unauthorized copying of the software they had written. The first virus to specifically target Microsoft Windows, WinVir was discovered in April 1992, two years after the release of Windows 3.0. The virus did not contain any Windows APIcalls, instead relying on DOS interrupts. 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 Windows 95. In late 1997 the encrypted, memory-resident stealth virus Win32.Cabanas was released—the first known virus that targeted Windows NT (it was also able to infect Windows 3.0 and Windows 9x hosts).
Even home computers were affected by viruses. The first one to appear on the Commodore Amiga was a boot sector virus called SCA virus, which was detected in November 1987. The first social networking virus, Win32.5-0-1, was created by Matt Larose on August 15, 2001. The virus specifically targeted users of MSN Messenger and online bulletin boards. 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 IP address and email addresses, contacts, website browsing history, and commonly used phrases. In 2008, larger websites used part of the Win32.5-0-1 code to track web users advertising-related interests.
Operations and functions
A viable computer virus must contain a search routine, 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 (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.
The trigger, which is also known as logic bomb, is the compiled version that could be activated any time an executable file with the virus is run that determines the event or condition for the malicious "payload" 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 double-click that opens a particular file.
The "payload" 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 Virus hoax.
Virus phases is the life cycle of the computer virus, described by using an analogy to biology. This life cycle can be divided into four phases:
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.
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 clone of the virus, which will itself enter a propagation 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.
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 binary executables (such as .EXE or .COM files), data files (such as Microsoft Word documents or PDF files), or in the boot sector of the host's hard drive (or some combination of all of these).
A memory-resident virus (or simply "resident virus") installs itself as part of the operating system when executed, after which it remains in RAM from the time the computer is booted up to when it is shut down. Resident viruses overwrite interrupt handling code or other functions, and when the operating system attempts to access the target file or disk sector, the virus code intercepts the request and redirects the control flow 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).
Many common applications, such as Microsoft Outlook and Microsoft Word, allow macro 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 macro language, 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 attachments in e-mails. 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 boot sector and/or the Master Boot Record (MBR) of the host's hard drive or removable storage media (flash drives, floppy disks, etc.).
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 attachments, 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.
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 antivirus software, however, especially those which maintain and date cyclic redundancy checks 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 have 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 (for example, Conficker). 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 NTFS file system 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 Operating system (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 code injection 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 heuristics. Security software may also use a database of file "hashes" 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 cryptographic hash functions of Windows OS files stored in Windows—to allow file integrity/authenticity to be checked—could be overwritten so that the System File Checker would report that altered system files are authentic, so using file hashes to scan for altered files would not always guarantee finding an infection.
See also: Self-modifying code
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 strings". 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.
One method of evading signature detection is to use simple encryption to encipher (encode) the body of the virus, leaving only the encryption module and a static cryptographic key in cleartext 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 self-modifying code 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 XORing, 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 rebooted. This is called cryptovirology. At said times, the executable will decrypt the virus and execute its hidden runtimes, infecting the computer and sometimes disabling the antivirus software.
Polymorphic code was the first technique that posed a serious threat to virus scanners. Just like regular encrypted viruses, a polymorphic virus infects files with an encrypted copy of itself, which is decoded by a decryption 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 emulator, or by statistical pattern analysis of the encrypted virus body. To enable polymorphic code, the virus has to have a polymorphic engine (also called "mutating engine" or "mutation engine") somewhere in its encrypted body. See polymorphic code 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.
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 metamorphic code. To enable metamorphism, a "metamorphic engine" is needed. A metamorphic virus is usually very large and complex. For example, W32/Simile consisted of over 14,000 lines of assembly language code, 90% of which is part of the metamorphic engine.
Vulnerabilities and infection vectors
As software is often designed with security features to prevent unauthorized use of system resources, many viruses must exploit and manipulate security bugs, which are security defects in a system or application software, to spread themselves and infect other computers. Software development strategies that produce large numbers of "bugs" will generally also produce potential exploitable "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 executable files that may be part of legitimate programs (see code injection). If a user attempts to launch an infected program, the virus' code may be executed simultaneously. In operating systems that use file extensions 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 digital image 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 Microsoft Windows. This is due to Microsoft's large market share of desktop computer users. The diversity of software systems on a network limits the destructive potential of viruses and malware.Open-source operating systems such as Linux allow users to choose from a variety of desktop environments, 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 operating system 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 administrator accounts in contemporary versions like Windows XP. In 1997, researchers created and released a virus for Linux—known as "Bliss". 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 log in as an administrator, or "root 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.
See also: Vulnerability to malware, Anti-malware strategies, and Browser hardening
Many users install antivirus software that can detect and eliminate known viruses when the computer attempts to download or run the executable file (which may be distributed as an email attachment, or on USB flash drives, 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 patchsecurity vulnerabilities ("holes"). Antivirus software also needs to be regularly updated in order to recognize the latest threats. This is because malicious hackers and other individuals are always creating new viruses. The German AV-TEST Institute publishes evaluations of antivirus software for Windows and Android.
Examples of Microsoft Windows anti virus and anti-malware software include the optional Microsoft Security Essentials (for Windows XP, Vista and Windows 7) for real-time protection, the Windows Malicious Software Removal Tool (now included with Windows (Security) Updates on "Patch Tuesday", the second Tuesday of each month), and Windows Defender (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 security vulnerabilities are assigned CVE IDs and listed in the US National Vulnerability Database. Secunia PSI 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. Ransomware and phishingscam alerts appear as press releases on the Internet Crime Complaint Center noticeboard. 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 ransom payment is made. Phishing 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 passwords 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 antivirus software article. The first, and by far the most common method of virus detection is using a list of virus signature definitions. This works by examining the content of the computer's memory (its Random Access Memory (RAM), and boot sectors) and the files stored on fixed or removable drives (hard drives, floppy drives, or USB flash drives), and comparing those files against a database 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 "zero-day attack").
A second method to find viruses is to use a heuristicalgorithm 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 false positives 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 backups 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), read-only or not accessible for other reasons, such as using different file systems. 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 optical media like CD and DVD 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 CD/DVD). Likewise, an operating system on a bootable 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 flash drives.
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 Google subsidiary VirusTotal.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 Microsoft Security Essentials, a Windows Malicious Software Removal Tool that is updated as part of the regular Windows update regime, and an older optional anti-malware (malware removal) tool Windows Defender that has been upgraded to an antivirus product in Windows 8.
Some viruses disable System Restore and other important Windows tools such as Task Manager and CMD. An example of a virus that does this is CiaDoor. Many such viruses can be removed by rebooting the computer, entering Windows "safe mode" with networking, and then using system tools or Microsoft Safety Scanner.System Restore on Windows Me, Windows XP, Windows Vista and Windows 7 can restore the registry 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 System File Checker (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 cloned disk, a disk image, or a backup 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 live CD, 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 executable files.
Viruses and the Internet
See also: Computer worm
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. 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 bulletin board system (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 BBSs. 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 other computers.
Macro viruses have become common since the mid-1990s. Most of these viruses are written in the scripting languages for Microsoft programs such as Microsoft Word and Microsoft 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 email messages, those viruses which did take advantage of the Microsoft OutlookComponent Object Model (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 web address link as an instant message 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 cross-site scripting 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 MySpace (with the Samy worm) and Yahoo!.
It was Fred Cahen who incidentally coined the expression ‘Computer Virus’. The term ‘Virus’ and ‘Worm’ were being used in science fiction novels in the early 1970’s. Around the same period, researchers at Xerox Corp, created and demonstrated a self replicating code, called virus.
A virus is a program that can modify another program which is deemed infected. This cart-also become an evolved copy of the original virus program. Every program that gets infected may also act as a virus and thus the infection multiplies. The key property of a virus is its ability to infect other programs. Every general purpose system currently in use is open to viral attack. In some secure systems, virus tends to spread further when created by some user of the system. A virus has the potential to spread throughout any system which allows sharing. The virus can be generated and introduced by a hacker. The perpetrator gets the satisfaction of demonstrating human superiority over a cybernetic system.
With the advent of internet a haven has been created for virus mongers. An important ongoing research involves determining how quickly a virus could spread to a large percentage of computers in the world. Studies through simplified mathematical models of virus spreading in typical computer networks have been going on. Obviously virus like programs have to be written, injected into systems and the effect has to be studied. In a simulated environment, the extent, speed and effect of infection is studied. Several experiments have been systematically carried out. The anti-virus program writers must be doing similar experiments before eventually bringing out their anti-virus packages.
Virus study indicates a set of ‘undecidable detection problems’. A list could be as follows
- Detection of a virus by its appearance.
- Detection of a virus by its behaviour.
- Detection of evolution of a known virus.
- Detection of a triggering mechanism by its appearance.
- Detection of a triggering mechanism by its behaviour.
- Detection of evolution of a known triggering mechanism.
- Detection of a virus detector by its behaviour.
- Detection of evolution of a known virus detector.
- Safety of a protection scheme.
With networking becoming the order of the day, a virus may get initiated only through a particular node or through a few nodes and may give an appearance of having originated from some other node. A virus may also get kindled at some stages of a program in an executable file and not necessarily whenever the program is called for.
Experts say that viruses need not to be used only for evil purposes. A very interesting theory in compression through virus has been developed. It can be explained that a simple virus can be written to find uninfected executable files, compress them and insert itself into them. Upon execution the infected program decompresses itself and executes normally. Studies indicate that such a virus could save over 50% of the space taken up by the executable files in an average system. The performance of infected programs decreases slightly as they are decompressed, and then the ‘compression virus’ implements a particular ‘time space trade off’.
Another example could be that a virus program can be written in such a way as to find ‘uninfected’ executable. It will plant itself at their beginning. After a given date and time the virus would cause the executable to ‘refuse service’ by going into an indefinite loop. And in modern networking with the level of sharing that is prevalent, the entire system would become unusable after that moment. Anti-virus operators might find a great deal of hard work that is required to treat/undo the damage caused by such a virus.
Types of Viruses
Non-TSR File Virus
This is the simplest form of virus to write and the least effective, and so one is unlikely to be troubled by them. When an infected program is first run, the virus code carries out its task checking that an executable file is not infected, and then attaches a copy to it. It then runs the original program to which it is attached. In contrast, TSR viruses load themselves into memory when they are executed and are able to infect any executable program they can reach from that point.
Boot Sector Virus
This is the other major type of virus. Most of the boot sector viruses consist of a simple, small programme that is used to start DOS, or whatever operating system is installed. Boot sector viruses replace this with virus code and typically move the boot sector to another part of the disc. When the PC booted, the virus code is executed first. Then, the virus runs the real boot sector. A very slow boot from an infected floppy with an excess of floppy disc activity is a common symptom of an infected machine.
These combine both techniques. They can infect both boot sectors and files. The file version of ‘Tequila’, e.g., infects the Master Boot Record. Once the PC has been booted from an infected MBR, the virus goes memory resident and infects all accessed EXE flies.
Companion viruses create a .COM companion to an .exe file. Because the DOS executes .Com files before .EXEs, the virus is run before the .EXE file of the same name. The virus then runs the original .EXE.
These aim to foil anti-virus packages that search for a specific strain by looking for a known sequence of bytes. No two copies of a true polymorphic virus are alike. When polymorphic viruses run they first decrypt themselves and then behave like any other virus. Programmes such as the ‘Nuke Encryption Device’ (NED) and the ‘Trident Polymorphic Engine’ have been written that turn a standard virus into a polymorphic virus. Fortunately, once measures have been taken by an anti-virus company to defeat each ‘engine’, all viruses processed by it are detectable.
Stealth covers a variety of techniques that viruses use to disguise their presence from anything as simple as hiding the increase in files size of executable to full-blown detection of the tools used to detect the virus and the taking of appropriate action to fool them.
Trojans are not viruses at all. They are programmes that hide a malevolent code within a seemingly innocuous programme but they do not replicate! For this reason the chances of being caught out accidentally by Trojans are, low.
Macro viruses have been predicted for a while. It had recently appeared when it was sent out accidentally by Microsoft on a CD-ROM to OEMs. They called it a ‘Prank Macro’. It is the first virus that will run on both PCs and Macs. It replicates using an auto-executing Word Basic macro embedded in a document. When the document is loaded, it copies the macro to Word’s settings file NORMAL.DOT. and replaces the File Save command with a routine that also saves a copy of the macro in each document.
The most significant computer viruses of the last 10 years are
- Code Red
In 2004, Code Red targetted Web servers and it would automatically spread by exploiting vulnerability in Microsoft IIS servers. It took less than a week for more than 400000 servers to be infected by Code Red.
Fortinet’s Lovet said 2004’s Sasser virus spread without anyone’s help and exploited vulnerability in Microsoft Windows. A bug in the worm’s code also caused it to shut down infected systems every couple of minutes. The damage caused by Sasser was estimated at more than $ 18 billion.
And Microsoft wanted answers, offering a $ 2500000 bounty for Sasser’s author, who turned out to be an 18-year-old German student who said he created the virus as a way to help his mother find a job in computer security. Lovet said Sasser was the first virus to really a tract attention from the traditional Press.
MyTob was a mass-mailed worm that included its own SMTP engine to spread itself to other PCs after hijacking addresses from an infected system.
Appearing in 2005, MyTob combined features of a bot and a mass-mailer. Along with heralding the era of cybercrime MyTob also introduced the botnet.
The Storm botnet launched in 2007 and took what seemed like ages to get under control. Storm left a legacy as one of the most destructive bots in history, infecting millions of computers around the world its wake. At its height, Storm infected between 1 million and 50 million systems and accounted for 8% of all malware.
Koobface, launched in 2008, marked the first botnet to recruit its Zombie computers across various social networks, including Facebook, MySpace, hi5, Bebo and more. Currently, it is estimated that at any time more than 500000 Koobface zombies are online simultaneously.
In 2009, Conficker targetted the Microsoft Windows OS and used Windows flaws and dictionary attacks on admin passwords to co-opt machines and link them to a computer that can be commanded remotely by the authors.
Some hospitals and military bases were also impacted and it’s estimated that roughly 7 million systems were infected globally. Oddly enough, no
Ukrainian IPs or machines were infected by Conficker, suggesting that the authors were not targeting their own country.
Too many, is considered the first shot fired in a cyber war that will soon emerge. With Stuxnet targeting nuclear plants, it is said the virus landscape is changing dramatically. Stuxnet first emerged on the public radar in September 2010, when researchers found traces of code on Siemens industrial software systems that operate Iran’s Bushehr nuclear reactor.
- SQL Slammer (2003)
Appeared first in 27th January, 2003 and soon got a high rank in the list of most dangerous worms of the year. Slammer spread to over 90% of all vulnerable hosts in 10 minutes and infected around 359000 computers total and according to London-based market intelligence the worm caused between $ 950 million and $ 1.2 billion in lost productivity in its first five days worldwide.
- Bandook (2005)
ALIAS: Bandook Rat
Bandook Rat is a backdoor trojan horse that infects Windows NT family systems (Windows 2000, XP, 2003, Vista). It uses process hijacking/Kernel Patching to bypass the firewall, and allow the server component to hijack processes and gain rights for accessing the internet. It is somehow very similar to Beast Trojan (2002).
- Sadmind Worm (2001)
ALIAS: Sadmind/IIS, Unix/Sadmind, Solaris/Sadmind Worm
The Sadmind worm was a self-propagating piece of malware; it was first discovered on 8th May, 2001 in China. It affected some big operating systems like Sun Micro systems Solaris (OS) and Microsoft’s Internet Information Services (IIS) which is the world’s second most popular web server in terms of overall websites behind the industry leader Apache HTTP Server.
- Beast Trojan (2002)
Beast is a Windows-based backdoor trojan horse more commonly known in the underground cracker community as a RAT (Remote Administration Tool). It was created in Delphi and released first by its author Tataye in 2002. Beast was one of the first trojans to feature a ‘reverse connection’ to its victims and once established, it gave the attacker complete control over the infected computer.
Prevention from a Virus Attack
The most fundamental precaution against virus attacks is to limit access to a machine to avoid tampering with the system. In case of floppy discs, the simplest form of protection is to place write-protect tabs on all discs so that any attempt by a virus to write to the disc would result in an error message. The write-protect tab should be removed only when data has to be expressly written to the floppy.
It should be remembered that even the simple act of inserting a floppy disc and getting a directory listing can be enough to infect a machine. Though, write-protect facilities are generally not available for hard discs, hardware products have started appearing in the market offering users the ability to write-protect hard discs. But, being expensive, these are not likely to be used widely.
Software products to write-protect hard discs are also available. But, these render themselves vulnerable to virus attack also. In network environments, the use of disk-less or hard-disc-only systems is becoming popular. Control of software is then restricted to the file server and network administrators only.
Tips for Prevention of Virus Infection
Even if one buys and uses several anti-virus applications, the best defence is to avoid infection in the first place. There is no absolute guarantee against infection. But, the risk can be minimised by following the guidelines listed below
- Boot the system with a write-protected and already scanned floppy disc, which has the boot and system files and set of files of a qualified virus scanned program.
- Even, if there is a hard disc and the PC normally boots from that disc, start by first booting the system with the uninfected and write-protected disc boot floppy in the ‘A’ drive.
- All floppies should be scanned individually and periodically by using a qualified and uninfected virus scanning (or detection) program.
- Discourage the use of floppies of other users unless these are individually scanned and checked for any virus.
- Do not use previously formatted floppies brought by others even if these are apparently empty. Reformat all empty floppies with your uninfected system before further use.
- Avoid lending floppies.
- The most popular carriers of dangerous viruses are floppies containing different popular computer games, horoscopes, astrological predictions These should be avoided.
- Use of pirated software should be completely avoided as most of them are virus carriers.
- Take back-ups regularly. A full back-up once a week, with incremental back-ups daily, if necessary, is advisable. Uninfected back-ups allow overwriting infected files. Even infected back-ups permit recovery from logic bombs. Disinfect restored files right away.
- Write-protect and back-up the installation discs before installing any new software. If it is not done and the system already has virus infection, the original program discs could be permanently infected during installation.
- Scan network drives used regularly. The files attached to E-mail messages may be infected.
- Use the memory-resident, virus spotting portion of the anti-virus application at all times. If an infection is suspected, turn off the system immediately. Reboot from a clean floppy (one without an AUTOEXEC.BAT or a CONFIG.SYS file). Then disinfect the system using a disc-based copy of the anti-virus program.
User should also have some basic knowledge about viruses, their prevention and cure. Use of good anti-virus software for scanning files regularly should invariably be used by each and every user.
But, a single software cannot be depended upon to eliminate infection from ill strains of viruses. The battle against virus infection will be long and perhaps, lasting one.