A computer virus is a computer program that can copy itself and
infect a computer without the permission or knowledge of the owner. The term
"virus" is also commonly but erroneously used to refer to other types
of malware, adware, and spyware programs that do not have the reproductive
ability. A true virus can only spread from one computer to another (in some
form of executable code) when its host is taken to the target computer; for
instance because a user sent it over a network or the Internet, or carried it
on a removable medium such as a floppy disk, CD, DVD, or USB drive. 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 another computer.
The term "computer virus" is sometimes used as a catch-all phrase
to include all types of. Malware includes computer viruses, worms, trojan
horses, most rootkits, spyware, dishonest adware, crimeware, and other
malicious and unwanted software), including true viruses. Viruses are sometimes
confused with computer worms and Trojan horses, which are technically
different. A worm can exploit security vulnerabilities to spread itself to
other computers without needing to be transferred as part of a host, and a Trojan
horse is a program that appears harmless but has a hidden agenda. Worms and
Trojans, like viruses, may cause harm to either a computer system's hosted
data, functional performance, or networking throughput, when they are executed.
Some viruses and other malware have symptoms noticeable to the computer user,
but many are surreptitious.
Most 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, Instant
Messaging, and file sharing systems to spread.
History About Computer Virus
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 in 1971. Creeper used the ARPANET to
infect DEC PDP-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.
A program called "Rother J" was the first computer virus to appear
"in the wild" — that is, outside the single computer or lab
where it was created.Witten in 1981 by Richard Skrenta, it attached
itself to the Apple DOS 3.3 operating system and spread via floppy disk.This
virus was created as a practical joke when Richard Skrenta was still in high
school. It was injected in a game on a floppy disk. On its 50th use the Elk
Cloner virus would be activated, infecting the computer and displaying a short
poem beginning "Elk Cloner: The program with a personality."
The first PC virus in the wild was a boot sector virus dubbed (c)Brain,
created in 1986 by the Farooq Alvi Brothers, 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, usually inadvertently. PCs of the era
would attempt to boot first from a floppy if one had been left in the drive.
Until floppy disks fell out of use, this was the most successful infection
strategy and boot sector viruses were the most common in the wild for many
years.
Traditional computer viruses emerged in the 1980s, driven by the spread of
personal computers and the resultant increase in BBS, 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 were easy targets for viruses.
macro viruses have become common since the mid-1990s. Most of these viruses
are written in the scripting languages for Microsoft programs such as Word and
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 e-mail, those viruses which did took advantage of
the Microsoft Outlook 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 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.
Cross-site scripting viruses emerged recently, and were academically
demonstrated in 2005.Since 2005 there have been multiple instances of the
cross-site scripting viruses in the wild, exploiting websites such as MySpace
and Yahoo.
Infection 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 attempts to launch an
infected program, the virus' code may be executed simultaneously. Viruses can
be divided into two types based on their behavior when they are executed.
Nonresident viruses immediately search for other hosts that can be infected,
infect those 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. This module, however, is not called by
a finder module. The virus loads the replication module into memory when it is
executed instead and ensures that this module is executed each time the
operating system is called to perform a certain operation. the replication
module can be called, for example, 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. A fast infector, for instance,
can infect every potential host file that is accessed. This poses a special
problem when using 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. Some slow
infectors, for instance, 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, however, does not seem very successful.
Vectors and hosts
Viruses have targeted various types of transmission media or hosts. This
list is not exhaustive:
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)
Cross-site scripting
vulnerabilities in web applications
Arbitrary computer files. An
exploitable buffer overflow, format string, race condition or other
exploitable bug in a program which reads the file could be used to trigger
the execution of code hidden within it. Most bugs of this type can be made
more difficult to exploit in computer architectures with protection
features such as an execute disable bit and/or address space layout randomization.
PDFs, like HTML, may link to malicious code. PDFs can also be
infected with malicious code.
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
named "picture.png.exe", in which the user sees only
"picture.png" and therefore assumes that this file is an image and
most likely is safe.
An additional method is to generate the virus code from parts of existing
operating system files by using the CRC16/CRC32 data. The initial code can be
quite small (tens of bytes) and unpack a fairly large virus. This is analogous
to a biological "prion" in the way it works but is vulnerable to
signature based detection.
This attack has not yet been seen "in the wild".
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 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.
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 which remain identical between infections, 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 which is part of the metamorphic engine.
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 and non-integrated Microsoft 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 own protected memory space.
An Internet based research revealed that there were cases when people
willingly pressed a particular button to download a virus. Security analyst
Didier Stevens ran a half year advertising campaign on Google AdWords which
said "Is your PC virus-free? Get it infected here!". The result was
409 clicks.
As of 2006[update],
there are relatively few security exploitstargeting Mac OS X (with
a Unix-based file system and kernel). 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. Virus
vulnerability between Macs and Windows is a chief selling point, one that Apple
uses in their Get a Mac advertising.
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. The Bliss virus may be considered
characteristic of viruses – as opposed to worms – on Unix systems.
Bliss requires that the user run it explicitly (so it is 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.
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 minimise 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 (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.
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,
the backup should remain safe from any Linux viruses (unless they are written
to specifically provide this capability). 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 Me, Windows XP and Windows 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 or also exists in previous restore
points. 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 (for example, to reduce potential damage from and the spread of
viruses). 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.
Users running a Microsoft operating system can access Microsoft's website to
run a free scan, provided they have their 20-digit registration number.
Operating system reinstallation
Reinstalling the operating system is another approach to virus removal. It
involves simply reformatting the OS partition and installing the OS from its
original media, or imaging the partition with a clean backup image (Taken with
Ghost or Acronis for example).
This method has the benefits of being simple to do,
being faster than running multiple antivirus scans, and is guaranteed to remove
any malware. Downsides include having to reinstall all other software,
reconfiguring, restoring user preferences. User data can be backed up by
booting off of a Live CD or putting the hard drive into another computer and
booting from the other computer's operating system (though care must be taken
not to transfer the virus to the new computer).