From Wikipedia, the free encyclopedia
A computer virus is a computer program written to alter the way a computer operates, without the permission or knowledge of the user, by hiding in other program files. Though the term is commonly used to refer to a range of malware, a true virus must do these two things:
- replicate itself
- execute itself
The latter criteria are often met by a virus which replaces existing executable files with a vi and VINCE virus).
Comparison with biological viruses
How viruses work
A computer virus will pass from one computer to another like a real life biological virus passes from person to person. For example, it is estimated by experts that the Mydoom worm infected a quarter-million computers in a single day in January 2004. Another example is the ILOVEYOU virus, which occurred in 2000 and had a similar effect. There Bold text--126.96.36.199 19:30, 27 December 2006 (UTC) DRUGS R EVERYONE!!!!!!!!!!!!!!!!!!!!!!! ariety of infection or spreading patterns, there are broad categories commonly used to describe various types of viruses.
Basic types of viruses
File viruses, also known as parasitic or executable viruses, are pieces of code that attach themselves to executable files, driver files or compressed files, and are activated when the host program is run. After activation, the virus may spread itself by attaching itself to other programs in the system, and also carry out the malevolent activity it was programmed for. Most file viruses spread by loading themselves in system memory and looking for any other programs located on the drive. If it finds one, it modifies the program’s code so that it contains and activates the virus the next time it’s run. It keeps doing this over and over until it spreads across the system, and possibly to other systems that the infected program may be shared with. Besides spreading themselves, these viruses also carry some type of destructive constituent that can be activated immediately or by a particular ‘trigger’. The trigger could be a specific date, or the number of times the virus has been replicated, or anything equally trivial. Some examples of file viruses are Randex, Meve and MrKlunky.
Boot sector viruses
A boot sector virus affects the boot sector of a hard disk, which is a very crucial part. The boot sector is where all information about the drive is stored, along with a program that makes it possible for the operating system to boot up. By inserting its code into the boot sector, a virus guarantees that it loads into memory during every boot sequence. A boot virus does not affect files; instead, it affects the disks that contain them. Perhaps this is the reason for their downfall. During the days when programs were carried around on floppies, the boot sector viruses used to spread like wildfire. However, with the CD-ROM revolution, it became impossible to infect pre-written data on a CD, which eventually stopped such viruses from spreading. Though boot viruses still exist, they are rare compared to new-age malicious software. Another reason why they’re not so prevalent is that operating systems today protect the boot sector, which makes it difficult for them to thrive. Examples of boot viruses are Polyboot.B and AntiEXE.
Multipartite viruses are a combination of boot sector viruses and file viruses. These viruses come in through infected media and reside in memory. They then move on to the boot sector of the hard drive. From there, the virus infects executable files on the hard drive and spreads across the system. There aren’t too many multipartite viruses in existence today, but in their heyday, they accounted for some major problems due to their capacity to combine different infection techniques. A well-known multipartite virus is Ywinz.
Macro viruses infect files that are created using certain applications or programs that contain macros. These include Microsoft Office documents such as Word documents, Excel spreadsheets, PowerPoint presentations, Access databases, and other similar application files such as Corel Draw, AmiPro, etc. Since macro viruses are written in the language of the application, and not in that of the operating system, they are known to be platform-independent—they can spread between Windows, Mac, and any other system, so long as they’re running the required application. With the ever-increasing capabilities of macro languages in applications, and the possibility of infections spreading over net-works, these viruses are major threats. The first macro virus was written for Microsoft Word and was discovered back in August 1995. Today, there are thousands of macro viruses in existence—some examples are Relax, Melissa.A and Bablas.
This kind of virus is proficient in quickly spreading across a Local Area Network (LAN) or even over the Internet. Usually, it propagates through shared resources, such as shared drives and folders. Once it infects a new system, it searches for potential targets by searching the network for other vulnerable systems. Once a new vulnerable system is found, the network virus infects the other system, and thus spreads over the network. Some of the most notorious network viruses are Nimda and SQLSlammer.
An e-mail virus could be a form of a macro virus that spreads itself to all the contacts located in the host’s email address book. If any of the e-mail recipients open the attachment of the infected mail, It spreads to the new host’s address book contacts, and then proceeds to send itself to all those contacts as well. These days, e-mail viruses can infect hosts even if the infected e-mail is previewed in a mail client.
There are many ways in which a virus can infect or stay dormant on your PC. However, whether active or dormant, it’s dangerous to let one loose on your system, and should be dealt with immediately.
Other malicious software
Earlier, the only way a computer was at risk was when you inserted an infected floppy. With the new age of technology, almost every computer is interconnected to the rest of the world at some point or the other, so it’s difficult to pinpoint the source and/or time of the infection. As if that weren’t bad enough, new-age computing has also brought about a new breed of malicious software. Today, the term ‘virus’ has become a generic term used for all the different ways that your computer can be attacked by malicious software. Besides the type of viruses we mentioned here’s a look at some of the newer problems we face today.
The only function of these Trojans is to destroy and delete files. They can automatically delete all the core system files on your machine. The Trojan could be controlled by the attacker or could be programmed to strike like a logic bomb, starting on a specific day or hour.
The main idea behind Denial of Service (DoS) Attack Trojans is to generate a lot of Internet traffic on the victim’s machine, to the extent that the Internet connection is too overloaded to let the user visit a website or download anything. Another variation of a DoS Trojan is the mail-bomb Trojan, whose main aim is to infect as many machines as possible and simultaneously attack specific e-mail addresses with random subjects and contents that cannot be filtered.
These types of trojan turn the victim’s computer into a proxy/wingate server. That way, the infected computer is available to the whole world to be used for anonymous access to various risky Internet services. The attacker can register domains or access pornographic Web sites with stolen credit cards or do similar illegal activities without being traced.
These trojans are probably the most simple, and are outdated. The only thing they do is open port 21 — the port for FTP transfers — and let everyone connect to your machine. Newer versions are password-protected, so only the attacker can connect to your computer.
Software Detection Killers
These trojans kill popular anti-virus/firewall programs that protect your machine to give the attacker access to the victim’s machine. A trojan could have any one or a combination of the above mentioned functionalities.
Computer Worms are programs that reproduce and run independently, and travel across network connections. The main difference between viruses and worms is the method in which they reproduce and spread. A virus is dependent upon a host file or boot sector, and the transfer of files between machines to spread, while a worm can run completely independently and spread of its own accord through network connections. The security threat of worms is equivalent to that of a virus. Worms are capable of doing a whole range of damage such as destroying essential files in your system, slowing it down to a great extent, or even causing some essential programs to crash. Two famous examples of worms are the MS-Blaster and Sesser worms.
Spyware is another term for advertising-supported software (Adware). Advertising in shareware products is a way for shareware authors to make money, other than by selling it to the user. There are several large media companies that offer to place banner ads in their products in exchange for a portion of the revenue from banner sales. If the user finds the banners annoying, there is usually an option to get rid of them by paying the licensing fee. The advertising companies often also install additional tracking software on your system, which is continuously using your Internet connection to send statistical data back to the advertisers. While the privacy policies of the companies claim there will be no sensitive or identifying data collected from your system and that you shall remain anonymous, the fact remains that your PC is acting as a server, sending information about you and your surfing habits to a remote location, using your bandwidth.
Spyware has been known to slow down computers with their semi-intensive usage of processing power, bringing up annoying pop-up windows at the most inappropriate times and changing your Internet browsing settings such as your home page or default search engine to their own services. Even if many do not consider this illegal, it is still is a major security threat, and the fact that there’s no way to get rid of them makes them as much of a nuisance as viruses.
Viruses can be subdivided into a number of types, the main ones being:
- Boot sector viruses
- Companion viruses
- Email viruses
- Logic bombs and time bombs
- Macro viruses
- Cross-site scripting virus
Two other types of malware are often classified as viruses, but are actually forms of distributing malware:
- Trojan horses
Boot sector virus
A boot sector virus alters or hides in the boot sector, usually the 1st sector, of a bootable disk or hard drive. Boot sector viruses were prevalent in the 1980s.
A companion virus does not have host files per se, but exploits MS-DOS. A companion virus creates new files (typically .COM but can also use other extensions such as ".EXD") that have the same file names as legitimate .EXE files. When a user types in the name of a desired program, if a user does not type in ".EXE" but instead does not specify a file extension, DOS will assume he meant the file with the extension that comes first in alphabetical order and run the virus. For instance, if a user had "(filename).COM" (the virus) and "(filename).EXE" and the user typed "filename", he will run "(filename).COM" and run the virus. The virus will spread and do other tasks before redirecting to the legitimate file, which operates normally. Some companion viruses are known to run under Windows 95 and on DOS emulators on Windows NT systems. Path companion viruses create files that have the same name as the legitimate file and place new virus copies earlier in the directory paths. These viruses have become increasingly rare with the introduction of Windows XP,which does not use the MS-DOS command prompt.
An E-mail virus is a virus which uses e-mail messages as a mode of transport. These viruses often copy themselves by automatically mailing copies to hundreds of people in the victim's address book.
A logic bomb employs code that lies inert until specific conditions are met. The resolution of the conditions will trigger a certain function (such as printing a message to the user and/or deleting files). Logic bombs may reside within standalone programs, or they may be part of worms or viruses. An example of a logic bomb would be a virus that waits to execute until it has infected a certain number of hosts. A time bomb is a subset of logic bomb, which is set to trigger on a particular date and/or time. An example of a time bomb is the infamous ‘Friday the 13th’ virus.
A macro virus, often written in the scripting languages for programs such as Word and Excel, is spread by infecting documents and spreadsheets.
Cross-site scripting virus
A cross-site scripting virus (XSSV) is a type of virus that utilizes cross-site scripting vulnerabilities to replicate. A XSSV is spread between vulnerable web applications and web browsers creating a symbiotic relationship.
Trojan Horses are impostor files that claim to be something desirable but, in fact, are malicious. Rather than insert code into existing files, a Trojan horse appears to do one thing (install a screen saver, or show a picture inside an e-mail, for example) when in fact it does something entirely different, and potentially malicious, such as erase files. Trojans can also open back doors so that computer hackers can gain access to passwords and other personal information stored on a computer.
Although often referred to as such, Trojan horses are not viruses in the strict sense because they cannot replicate automatically. For a Trojan horse to spread, it must be invited onto a computer by the user opening an email attachment or downloading and running a file from the Internet, for example.
A worm is a piece of software that uses computer networks and security flaws to create copies of itself. A copy of the worm will scan the network for any other machine that has a specific security flaw. It replicates itself to the new machine using the security flaw, and then begins scanning and replicating a new worm.
Worms are programs that replicate themselves from system to system without the use of a host file. This is in contrast to viruses, which requires the spreading of an infected host file. Although worms generally exist inside of other files, often Word or Excel documents, there is a difference between how worms and viruses use the host file. Usually the worm will release a document that already has the "worm" macro inside the document. The entire document will travel from computer to computer, so the entire document should be considered the worm. Mydoom or ILOVEYOU are two examples of worms.
Effects of computer viruses
Some viruses are programmed to damage the computer by damaging programs, deleting files, or reformatting the hard disk. Others are not designed to do any damage, but simply replicate themselves and make their presence known by presenting text, video, or audio messages. Even these benign viruses can create problems for the computer user. They typically take up computer memory used by legitimate programs. As a result, they often cause erratic behavior and can result in system crashes. In addition, many viruses are bug-ridden, and these bugs may lead to system crashes and data loss.
Use of the word "virus"
The word virus is derived from and used in the same sense as the biological equivalent. 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. In some technical communities, the term "virus" is also extended to include the authors of malware, in an insulting sense. The English plural of "virus" is "viruses". Some people use "virii" or "viri" as a plural, but this is rare. For a discussion about whether "viri" and "virii" are correct alternatives of "viruses", see plural of virus.
The term "virus" was first used in an academic publication by Fred Cohen in his 1984 paper Experiments with Computer Viruses, where he credits Len Adleman with coining it. However, a 1972 science fiction novel by David Gerrold, When H.A.R.L.I.E. Was One, includes a description of a fictional computer program called "VIRUS" that worked just like a virus (and was countered by a program called "VACCINE"). The term "computer virus" with current usage also appears in the comic book Uncanny X-Men #158, written by Chris Claremont and published in 1982. Therefore, although Cohen's use of "virus" may, perhaps, have been the first "academic" use, the term had been used earlier.
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. This virus was originally a joke, created by the high school student and put onto a game. The game was set to play, but release the virus on the 50th time of starting the game. Only this time, instead of playing the game, it would change to a blank screen that read a poem about the virus named Elk Cloner. The computer would then be infected.
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. 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 the personal computer, many users regularly exchanged information and programs on floppies. Some viruses spread by infecting programs stored on these disks, while others installed themselves into the disk boot sector, ensuring that they would be run when the user booted the computer from the disk.
Traditional computer viruses emerged in the 1980s, driven by the spread of personal computers and the resultant increase in BBS and modem use, and software sharing. Bulletin board driven software sharing contributed directly to the spread of Trojan horse programs, and viruses were written to infect popularly traded software. Shareware and bootleg software were equally common vectors for viruses on BBS's. 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. Most of these viruses did not have the ability to send infected e-mail. Those viruses which did spread through e-mail took advantage of the Microsoft Outlook COM interface.
Macro viruses pose unique problems for detection software. For example, some versions of Microsoft Word allowed macros to replicate themselves with additional blank lines. The virus behaved identically but would be misidentified as a new virus. In another example, if two macro viruses simultaneously infect a document, the combination of the two, if also self-replicating, can appear as a "mating" of the two and would likely be detected as a virus unique from the "parents".
A 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.
The newest species of the virus family is the cross-site scripting virus. The virus emerged from research and was academically demonstrated in 2005. This virus utilizes cross-site scripting vulnerabilities to propagate. Since 2005 there have been multiple instances of the cross-site scripting viruses in the wild, most notable sites affected have been MySpace and Yahoo.
Why people create computer viruses
Unlike biological viruses, computer viruses do not simply evolve by themselves. Computer viruses do not come into existence spontaneously, nor are they likely to be created by bugs in regular programs. They are deliberately created by programmers, or by people who use virus creation software. Computer viruses can only do what the programmers have programmed them to do.
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. Many writers consider the systems they attack an intellectual challenge or a logical problem to be solved; this multiplies when a cat-and-mouse game is anticipated against anti-virus software. 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 inadvertently 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 the permission of the computer owner. Since self-replicating code causes many complications, it is questionable if a well-intentioned virus can ever solve a problem in a way that is superior to a regular program that does not replicate itself. In short, no single answer is likely to cover the broad demographic of virus writers.
Releasing computer viruses (as well as worms) is a crime in most jurisdictions.
See also the BBC News article.
In order to replicate itself, a virus must be permitted to execute code and write to memory. For this reason, many viruses attach themselves to executable files that may be part of legitimate programs. If a user tries to start an infected program, the virus' code may be executed first. Viruses can be divided into two types, on the basis of their behavior when they are executed. Nonresident viruses immediately search for other hosts that can be infected, infect these targets, and finally transfer control to the application program they infected. Resident viruses do not search for hosts when they are started. Instead, a resident virus loads itself into memory on execution and transfers control to the host program. The virus stays active in the background and infects new hosts when those files are accessed by other programs or the operating system itself.
Nonresident viruses can be thought of as consisting of a finder module and a replication module. The finder module is responsible for finding new files to infect. For each new executable file the finder module encounters, it calls the replication module to infect that file.
For simple viruses the replicator's tasks are to:
- Open the new file
- Check if the executable file has already been infected (if it is, return to the finder module)
- Append the virus code to the executable file
- Save the executable's starting point
- Change the executable's starting point so that it points to the start location of the newly copied virus code
- Save the old start location to the virus in a way so that the virus branches to that location right after its execution.
- Save the changes to the executable file
- Close the infected file
- Return to the finder so that it can find new files for the replicator to infect.
Resident viruses contain a replication module that is similar to the one that is employed by nonresident viruses. However, this module is not called by a finder module. Instead, the virus loads the replication module into memory when it is executed and ensures that this module is executed each time the operating system is called to perform a certain operation. For example, the replication module can be called each time the operating system executes a file. In this case, the virus infects every suitable program that is executed on the computer.
Resident viruses are sometimes subdivided into a category of fast infectors and a category of slow infectors. Fast infectors are designed to infect as many files as possible. For instance, a fast infector can infect every potential host file that is accessed. This poses a special problem to anti-virus software, since a virus scanner will access every potential host file on a computer when it performs a system-wide scan. If the virus scanner fails to notice that such a virus is present in memory, the virus can "piggy-back" on the virus scanner and in this way infect all files that are scanned. Fast infectors rely on their fast infection rate to spread. The disadvantage of this method is that infecting many files may make detection more likely, because the virus may slow down a computer or perform many suspicious actions that can be noticed by anti-virus software. Slow infectors, on the other hand, are designed to infect hosts infrequently. For instance, some slow infectors only infect files when they are copied. Slow infectors are designed to avoid detection by limiting their actions: they are less likely to slow down a computer noticeably, and will at most infrequently trigger anti-virus software that detects suspicious behavior by programs. The slow infector approach does not seem very successful however.
Viruses have targeted various types of hosts. This is a non-exhaustive list:
- Binary executable files (such as COM files and EXE files in MS-DOS, Portable Executable files in Microsoft Windows, and ELF files in Linux)
- Volume Boot Records of floppy disks and hard disk partitions
- The master boot record (MBR) of a hard disk
- General-purpose script files (such as batch files in MS-DOS and Microsoft Windows, VBScript files, and shell script files on Unix-like platforms).
- Application-specific script files (such as Telix-scripts)
- Documents that can contain macros (such as Microsoft Word documents, Microsoft Excel spreadsheets, AmiPro documents, and Microsoft Access database files)
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.
Some viruses try to avoid detection by killing the tasks associated with antivirus software before it can detect them.
As computers and operating systems grow larger and more complex, old hiding techniques need to be updated or replaced.
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:
- Anti-virus professionals can use bait files to take a sample of a virus (i.e. a copy of a program file that is infected by the virus). It is more practical to store and exchange a small, infected bait file, than to exchange a large application program that has been infected by the virus.
- Anti-virus professionals can use bait files to study the behavior of a virus and evaluate detection methods. This is especially useful when the virus is polymorphic. In this case, the virus can be made to infect a large number of bait files. The infected files can be used to test whether a virus scanner detects all versions of the virus.
- Some anti-virus software employs bait files that are accessed regularly. When these files are modified, the anti-virus software warns the user that a virus is probably active on the system.
Since bait files are used to detect the virus, or to make detection possible, a virus can benefit from not infecting them. Viruses typically do this by avoiding suspicious programs, such as small program files or programs that contain certain patterns of 'garbage instructions'.
A related strategy to make baiting difficult is sparse infection. Sometimes, sparse infectors do not infect a host file that would be a suitable candidate for infection in other circumstances. For example, a virus can decide on a random basis whether to infect a file or not, or a virus can only infect host files on particular days of the week.
Some viruses try to trick anti-virus software by intercepting its requests to the operating system. A virus can hide itself by intercepting the anti-virus software’s request to read the file and passing the request to the virus, instead of the OS. The virus can then return an uninfected version of the file to the anti-virus software, so that it seems that the file is "clean". Modern anti-virus software employs various techniques to counter stealth mechanisms of viruses. The only completely reliable method to avoid stealth is to boot from a medium that is known to be clean.
Most modern antivirus programs try to find virus-patterns inside ordinary programs by scanning them for so-called virus signatures. A signature is a characteristic byte-pattern that is part of a certain virus or family of viruses. If a virus scanner finds such a pattern in a file, 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 or impossible. These viruses modify their code on each infection. That is, each infected file contains a different variant of the virus.
In the past, some viruses modified themselves only in simple ways. For example, they regularly exchanged subroutines in their code for others that would perform the same action - for example, 2+2 could be swapped for 1+3. This poses no problems to a somewhat advanced virus scanner.
Encryption with a variable key
A more advanced method is the use of simple encryption to encipher the virus. 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.
Mostly, the decryption techniques that these viruses employ are simple and mostly done by just XORing each byte with a randomized key that was saved by the parent virus. The use of XOR-operations has the additional advantage that the encryption and decryption routine are the same (a XOR b = c, c XOR b = a.)
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 that stay the same on each infection, making it impossible to detect directly using signatures. Anti-virus 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 anti-virus professionals 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 use this technique are said to be metamorphic. To enable metamorphism, a metamorphic engine is needed. A metamorphic virus is usually very large and complex. For example, W32/Simile consisted of over 14000 lines of Assembly language code, 90% of it part of the metamorphic engine.
Vulnerability and countermeasures
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. The users of 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 OS's (and NTFS-aware applications on Windows NT based platforms) only allow their users to run executables within their protected space in their own directories.
As of 2006, there are relatively few security exploits  targeting Mac OS X (a Unix-based operating system); the known vulnerabilities fall under the classifications of worms and Trojans. The number of viruses for the older Apple operating systems, known as Mac OS Classic, varies greatly from source to source, with Apple stating that there are only four known viruses, and independent sources stating there are as many as 63 viruses. It is safe to say that Macs are less likely to be exploited due to their secure Unix base, and because a Mac-specific virus could only infect a small proportion of computers (making the effort less desirable). Virus vulnerability between Macs and Windows was/is a chief catalyst of the platform wars between Apple Computer and Microsoft.
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. 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 an 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 that 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 prevalence of exploits. They claim 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 preventative countermeasures
There are two common methods that an anti-virus software application uses to detect viruses. The first, and by far the most common method of virus detection is using a list of virus signature definitions. The disadvantage of this detection method is that users are only protected from viruses that pre-date their last virus definition update. The second method is to use a heuristic algorithm to find viruses based on common behaviors. This method has the ability to detect viruses that anti-virus security firms’ have yet to create a signature for.
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 content heuristics of the computer's memory (its RAM, and boot sectors) 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.
One may also prevent 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), 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 should preferably 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. Likewise, an Operating System on a live cd can be used to start the computer if the installed Operating Systems become unusable. Another method is to use different Operating Systems on different file systems. A virus is not likely to affect both. Data backups can also be put on different file systems. For example, Linux requires specific software to write to NTFS partitions, so if one does not install such software and uses a separate installation of MS Windows to make the backups on an ntfs partition (and preferably only for that reason), the backup should remain safe from any Linux viruses. Likewise, MS Windows can not read file systems like ext3, so if one normally uses MS Windows, the backups can be made on an ext3 partition using a Linux installation.
Once a computer has been compromised by a virus, it is usually unsafe to continue using the same computer without completely reinstalling the operating system. However, there are a number of recovery options that exist after a computer has a virus. These actions depend on severity of the type of virus.
Operating System Reinstallation
- Computer insecurity
- Virus hoax
- List of computer viruses
- List of computer virus hoaxes
- List of Linux computer viruses
- List of Trojan horses
- Timeline of notable computer viruses and worms
- Turing completeness
- Black hat
- Security through obscurity
- Palm OS Viruses
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- ^ Vesselin Bontchev. Macro Virus Identification Problems. FRISK Software International.
- ^ "Why people write computer viruses", BBC News, August 23, 2003.
- ^ Malware Evolution: MacOS X Vulnerabilities 2005 - 2006. Kaspersky Lab (July 24, 2006). Retrieved on August 19, 2006.
- ^ McAfee. McAfee discovers first Linux virus. news article.
- ^ Axel Boldt. Bliss, a Linux "virus". news article.
- Viruses at the Open Directory Project
- Article: "Can a virus lead an enterprise to court?"
- Article: "How Computer Viruses Work"
- Article: "Are 'Good' Computer Viruses Still a Bad Idea?"
- Article: "Protecting your Email from Viruses and Other MalWare"
- Article: "Hacking Away at the Counterculture" by Andrew Ross
- Article: Fred Cohen's 1984 paper on the first Virus
- Article: Keystroke logger and Trojan horse information and avoidance
- Article: "A Virus in Info-Space" by Tony Sampson
- Article: "Dr Aycock's Bad Idea" by Tony Sampson
- Article: "Digital Monsters, Binary Aliens" by Jussi Parikka
- Article: "The Universal Viral Machine" by Jussi Parikka
- Article: "Hypervirus: A Clinical Report" by Thierry Bardini
- Article: "The Cross-site Scripting Virus"
- RFC 1135 The Helminthiasis of the Internet
- Article: War of Virus Lotka-Volterra model based analysis of two fighting worms.
- Article: "The Virus Underground"