Computer
virus
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Wikipedia, the free encyclopedia
(Redirected from Computer
viruses)
In computer security
technology, a computer virus is a self-replicating or
self-reproducing-automation
computer program that
spreads by inserting copies of itself into other executable code or documents.
A
computer virus behaves in
a way similar to a biological virus, which spreads by inserting itself into
living
cells. Extending the
analogy, the insertion of a virus into the program is termed as an
"infection", and the
infected file, or
executable code that is not part of a file, is called a "host".
Viruses are one of the several
types of malicious
software or malware. In a common parlance, the term virus is often
extended to refer to
worms, trojan horses and
other sorts of malware; however, this can confuse computer users, since viruses
in
the narrow sense of the
word are less common than they used to be, compared to other forms of malware.
This confusion can have
serious consequences, because it may lead to a focus on preventing one genre of
malware over another,
potentially leaving computers vulnerable to future damage. However, a basic
rule is
that computer viruses
cannot directly damage hardware, but only software.
While viruses can be
intentionally destructive, for example, by destroying data, many other viruses
are fairly
benign or merely annoying.
Some viruses have a delayed payload, which is sometimes called a bomb.
For
example, a virus might
display a message on a specific day or wait until it has infected a certain
number of
hosts. A time bomb
occurs during a particular date or time, and a logic bomb occurs when
the user of a
computer takes an action
that triggers the bomb. However, the predominant negative effect of viruses is
their
uncontrolled self-reproduction,
which wastes or overwhelms computer resources.
Today, viruses are
somewhat less common than network-borne worms, due to the popularity of the
Internet.
Anti-virus software,
originally designed to protect computers from viruses, has in turn expanded to
cover
worms and other threats
such as spyware,identity theft and adware.
Definition
A virus is a type of
program that can replicate itself by making copies of itself, which can be
possibly
modified. The main
criterion for classifying a piece of executable code as a virus is that it
spreads itself by
means of 'hosts'. 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 carrying it on a removable
medium. 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. A
worm, however, can spread
itself to other computers without needing to be transferred as part of a host.
Many personal computers
are now connected to the Internet and to local-area networks, facilitating
their
spread. 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.
Viruses can infect
different types of hosts. The most common targets are executable files that
contain
application software or
parts of the operating system. Viruses have also infected the executable boot
sectors
of floppy disks, script
files of application programs, and documents that can contain macro scripts.
Additionally, viruses can
infect files in other ways than simply inserting a copy of their code into the
code of
the host program. For
example, a virus can overwrite its host with the virus code, or it can use a
trick to
ensure that the virus
program is executed when the user wants to execute the (unmodified) host
program.
Viruses have existed for
many different operating systems, including MS-DOS, AmigaOS, Linux and Mac
OS; however, the vast
majority of viruses affect Microsoft Windows.
A legitimate application
program that can copy itself as a side effect of its normal function (e.g.
backup
software) is not
considered a virus. Some programs that were apparently intended as viruses
cannot
self-replicate, because
the infection routine contains bugs. For example, a buggy virus can insert
copies of
itself into host programs,
but these copies never get executed and are thus unable to spread the virus. As
long
as at least some of the
copies are able to make copies of themselves, they are still considered
viruses,
otherwise they are
referred to as intended viruses.
Some people incorrectly
argue that malware is only classified as a virus if it both meets the above
definition
and can infect a computer
without user activation. By this definition, malware that requires user
activation to
run would be classified as
a trojan or a worm. But, before computers were networked together, the only way
a virus would
activate(excluding boot sector viruses) was by user activation, so this never
was part of the
definition of a virus.
