A computer virus is a computer program that can copy itself and infect a
computer without permission or knowledge of the user. The original virus may modify the copies, or the copies may modify
themselves, as occurs in a metamorphic virus. A virus can only spread from one computer
to another when its host is taken to the uninfected computer, for instance by a user sending it over a network or the Internet,
or by carrying it on a removable medium such as a floppy disk, CD, or USB drive. Additionally, viruses can spread to other
computers by infecting files on a network file system or a file system that is
accessed by another computer. Viruses are sometimes confused with computer worms and
Trojan horses. A worm can spread itself to other computers without needing to
be transferred as part of a host, and a Trojan horse is a file that appears harmless until executed.
Many personal computers are now connected to the Internet and to local area
networks, facilitating the spread of malicious code. Today's viruses may also take advantage of network services such as
the World Wide Web, e-mail, and file sharing systems to spread, blurring the line between viruses and worms. Furthermore, some sources use
an alternative terminology in which a virus is any form of self-replicating malware.
Some viruses are programmed to damage the computer by damaging programs, deleting files, or reformatting the hard disk. Others
are not designed to do any damage, but simply replicate themselves and perhaps make their presence known by presenting text,
video, or audio messages. Even these benign viruses can create problems for the computer
user. They typically take up computer memory used by legitimate programs.
As a result, they often cause erratic behavior and can result in system crashes. In addition, many viruses are bug-ridden, and these bugs may lead to system crashes and data loss.
History
The Creeper virus was first detected on ARPANET, the forerunner of the Internet in the early 1970s. It propagated via the
TENEX operating system and could make use of any connect modem to dial out to remote computers
and infect them. It would display the message "I'M THE CREEPER : CATCH ME IF YOU CAN.". It is rumored that the Reaper
program that appeared shortly after and sought out copies of the Creeper and deleted them, may have been written by the creator
of the Creeper in a fit of regret.
A program called "Elk Cloner" is commonly credited with being the first computer virus to
appear "in the wild" — that is, outside the single computer or lab where it was created, but that claim is false. See the
Timeline of notable computer viruses and worms for other
earlier viruses. It was however the first virus to infect computers "in the home". Written in 1982 by Richard Skrenta, it attached itself to the Apple DOS 3.3 operating
system and spread by floppy disk.[1] This virus was originally a joke, created by a high school student and put onto a game. The disk
could only be used 49 times. The game was set to play, but release the virus on the 50th time of starting the game. 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 poem
that showed up on the screen is as follows: It will get on all your disks. It will infiltrate your chips. Yes it's Cloner! It
will stick to you like a fly on a glue stick. It will modify RAM too. Send in the 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
retail 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".[2]
A virus may also send a web address link as an instant message to all the contacts on an infected machine. If the recipient, thinking the link is
from a friend (a trusted source) follows the link to the website, the virus hosted at the site may be able to infect this new
computer and continue propagating.
The newest species of the virus family is the cross-site scripting virus. The virus emerged from research and was academically
demonstrated in 2005 [3]. This virus utilizes
cross-site scripting vulnerabilities to propagate. Since 2005 there have been
multiple instances of the cross-site scripting viruses in the wild, most notable sites affected have been MySpace and Yahoo.
Replication strategies
In order to replicate itself, a virus must be permitted to execute code and write to memory. For this reason, many viruses
attach themselves to executable files that may be part of legitimate programs. If a user tries to start an infected program, the
virus' code may be executed first. Viruses can be divided into two types, on the basis of their behavior when they are executed.
Nonresident viruses immediately search for other hosts that can be infected, infect these targets, and finally transfer control
to the application program they infected. Resident viruses do not search for hosts
when they are started. Instead, a resident virus loads itself into memory on execution and transfers control to the host program.
The virus stays active in the background and infects new hosts when those files are accessed by other programs or the operating
system itself.
Nonresident viruses
Nonresident viruses can be thought of as consisting of a finder module and a replication module. The finder
module is responsible for finding new files to infect. For each new executable file the finder module encounters, it calls the
replication module to infect that file.
Resident viruses
Resident viruses contain a replication module that is similar to the one that is employed by nonresident viruses. However, this
module is not called by a finder module. Instead, the virus loads the replication module into memory when it is executed and
ensures that this module is executed each time the operating system is called to perform a certain operation. For example, the
replication module can be called each time the operating system executes a file. In this case, the virus infects every suitable
program that is executed on the computer.
Resident viruses are sometimes subdivided into a category of fast infectors and a category of slow infectors.
Fast infectors are designed to infect as many files as possible. For instance, a fast infector can infect every potential host
file that is accessed. This poses a special problem to anti-virus software, since a virus scanner will access every potential
host file on a computer when it performs a system-wide scan. If the virus scanner fails to notice that such a virus is present in
memory, the virus can "piggy-back" on the virus scanner and in this way infect all files that are scanned. Fast infectors rely on
their fast infection rate to spread. The disadvantage of this method is that infecting many files may make detection more likely,
because the virus may slow down a computer or perform many suspicious actions that can be noticed by anti-virus software. Slow
infectors, on the other hand, are designed to infect hosts infrequently. For instance, some slow infectors only infect files when
they are copied. Slow infectors are designed to avoid detection by limiting their actions: they are less likely to slow down a
computer noticeably, and will at most infrequently trigger anti-virus software that detects suspicious behavior by programs. The
slow infector approach does not seem very successful, however.
Vectors and hosts
Viruses have targeted various types of transmission media or hosts. This list is not exhaustive:
Inhospitable vectors
It is difficult, but not impossible, for viruses to tag along in source files, seeing that computer languages are built for
human eyes and experienced operators. With the notable exception of WMF,
it is almost impossible for viruses to tag along in data files like MP3s, MPGs, OGGs, JPGs, GIFs, PNGs, MNGs,
PDFs, and DVI files
(this is not an exhaustive list of generally trusted file types). Even if a virus were to 'infect' such a file, it would be
inoperative since there would be no way for the viral code to be executed. A caveat must be mentioned from PDFs, that like
HTML, may link to malicious code. Further, an exploitable buffer overflow in a program which reads the data files could be used to trigger the execution of code
hidden within the data file, but this attack is substantially mitigated in computer
architectures with an execute disable bit.
It is worth noting that some virus authors have written an .EXE extension on the end of .PNG (for example), hoping that users
would stop at the trusted file type without noticing that the computer would start with the final type of file. See
Trojan horse (computing).
Methods to avoid detection
In order to avoid detection by users, some viruses employ different kinds of deception. Some old viruses, especially on the
MS-DOS platform, make sure that the "last modified" date of a host file stays the same when the file is infected by the virus.
This approach does not fool anti-virus software, however, especially that which maintains and dates Cyclic Redundancy Codes on file changes.
Some viruses can infect files without increasing their sizes or damaging the files. They accomplish this by overwriting unused
areas of executable files. These are called cavity viruses. For example the CIH
virus, or Chernobyl Virus, infects Portable Executable files. Because those files had many empty gaps, the virus, which was 1
KB in length, did not add to the size of the file.
Some viruses try to avoid detection by killing the tasks associated with antivirus software before it can detect them.
As computers and operating systems grow larger and more complex, old hiding techniques need to be updated or replaced.
Defending a computer against viruses may demand that a file system migrate towards detailed and explicit permission for every
kind of file access.
Avoiding bait files and other undesirable hosts
A virus needs to infect hosts in order to spread further. In some cases, it might be a bad idea to infect a host program. For
example, many anti-virus programs perform an integrity check of their own code. Infecting such programs will therefore increase
the likelihood that the virus is detected. For this reason, some viruses are programmed not to infect programs that are known to
be part of anti-virus software. Another type of host that viruses sometimes avoid is bait files. Bait files (or goat
files) are files that are specially created by anti-virus software, or by anti-virus professionals themselves, to be infected
by a virus. These files can be created for various reasons, all of which are related to the detection of the virus:
- Anti-virus professionals can use bait files to take a sample of a virus (i.e. a copy of a program file that is infected by
the virus). It is more practical to store and exchange a small, infected bait file, than to exchange a large application program
that has been infected by the virus.
- Anti-virus professionals can use bait files to study the behavior of a virus and evaluate detection methods. This is
especially useful when the virus is polymorphic. In this case, the virus can be made to
infect a large number of bait files. The infected files can be used to test whether a virus scanner detects all versions of the
virus.
- Some anti-virus software employs bait files that are accessed regularly. When these files are modified, the anti-virus
software warns the user that a virus is probably active on the system.
Since bait files are used to detect the virus, or to make detection possible, a virus can benefit from not infecting them.
Viruses typically do this by avoiding suspicious programs, such as small program files or programs that contain certain patterns
of 'garbage instructions'.
A related strategy to make baiting difficult is sparse infection. Sometimes, sparse infectors do not infect a host file
that would be a suitable candidate for infection in other circumstances. For example, a virus can decide on a random basis
whether to infect a file or not, or a virus can only infect host files on particular days of the week.
Stealth
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.
Self-modification
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
but probably not impossible. These viruses modify their code on each infection. That is, each infected file contains a different
variant of the virus.
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. Since these would be symmetric
keys, stored on the infected host, it is in fact entirely possible to decrypt the final virus, but that probably isn't 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, encryption involves XORing each byte in a virus with a constant, so
that the exclusive-or operation had only to be repeated for decryption. It is suspicious code that modifies itself, so the code
to do the encryption/decryption may be part of the signature in many virus definitions.
Polymorphic code
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 very difficult
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.
Metamorphic code
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 is part of the metamorphic engine.[4]
Vulnerability and countermeasures
The vulnerability of operating systems to 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 (such as Microsoft Office) and 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[5] targeting Mac OS X (with a Unix-based file
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 targeted because of low market share and thus a Mac-specific virus could only infect a small
proportion of computers (making the effort less desirable). Virus vulnerability between Macs and Windows is a chief selling
point, one that Apple uses in their Get a Mac advertising.
That said Macs have also had significant critical security issues just as Microsoft Windows has.
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, older copies of Windows such as Windows 95 and 98 do not. In 1997, when a virus for
Linux was released – known as "Bliss" – leading antivirus vendors issued warnings that
Unix-like systems could fall prey to viruses just like Windows.[6] The Bliss virus may be considered characteristic of viruses – as opposed to worms
– on Unix systems. Bliss requires that the user run it explicitly (making it a trojan), 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.[7]
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.
Anti-virus software and other preventive measures
Many users install anti-virus software that can detect and eliminate known viruses
after the computer downloads or runs the executable. 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. This works by examining the content 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". 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.
Some anti-virus programs are able to scan opened files in addition to sent and received e-mails '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. Users must update their software regularly to patch
security holes. Anti-virus software also needs to be regularly updated in order to prevent 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
bootable 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.
Recovery methods
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.
Virus removal
One possibility on Windows XP and Vista is a tool known as System Restore, which restores the registry and critical system files to a previous checkpoint. Often a
virus will cause a system to hang, and a subsequent hard reboot will render a system restore point from the same day corrupt.
Restore points from previous days should work provided the virus is not designed to corrupt the restore files. Some viruses,
however, disable system restore and other important tools such as Task Manager and Command Prompt. An example of a virus that
does this is CiaDoor.
Administrators have the option to disable such tools from limited users for various reasons. The virus modifies the registry
to do the same, except, when the Administrator is controlling the computer, it blocks all users from accessing the tools.
When an infected tool activates it gives the message "Task Manager has been disabled by your administrator.", even if the user
trying to open the program is the administrator.
Operating system reinstallation
As a last ditch effort, if a virus is on your system and anti-viral software can't clean it, then reinstalling the operating
system may be required. To do this properly, the hard drive is completely erased (partition deleted and formatted, not
quick-formatted) and the operating system is reinstalled, and separately scanned for infection before erasing the original hard
drive and reinstalling installed from media known not to be infected. Important files should first be backed up, if possible.
See also
References
- ^ Anick Jesdanun. Prank starts 25 years of security woes.The anniversary of a nuisance.
- ^ Vesselin Bontchev. Macro Virus
Identification Problems. FRISK Software International.
- ^ Wade Alcorn. The Cross-site Scripting Virus.
- ^ Perriot, Fredrick; Peter Ferrie and Peter Szor (May 2002). Striking
Similarities (PDF). Retrieved on September 9, 2007.
- ^ 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.
External links
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