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DIFFERENT TYPES OF COMPUTER VIRUSES
By: eba95 Date: July 25, 2010, 7:41 am
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. Let us have a look at
them …
Computer Virus is a kind of malicious
software written intentionally to
enter a computer without the user ’s
permission or knowledge, with an
ability to replicate itself, thus
continuing to spread. Some viruses
do little but replicate others can
cause severe harm or adversely
effect program and performance of
the system. A virus should never be
assumed harmless and left on a
system. Most common types of
viruses are mentioned below:
Resident Viruses
This type of virus is a permanent
which dwells in the RAM memory.
From there it can overcome and
interrupt all of the operations
executed by the system: corrupting
files and programs that are opened,
closed, copied, renamed etc.
Examples include: Randex, CMJ, Meve,
and MrKlunky.
Direct Action Viruses
The main purpose of this virus is to
replicate and take action when it is
executed. When a specific condition
is met, the virus will go into action
and infect files in the directory or
folder that it is in and in directories
that are specified in the
AUTOEXEC.BAT file PATH. This batch
file is always located in the root
directory of the hard disk and carries
out certain operations when the
computer is booted.
Overwrite Viruses
Virus of this kind is characterized by
the fact that it deletes the
information contained in the files
that it infects, rendering them
partially or totally useless once they
have been infected.
The only way to clean a file infected
by an overwrite virus is to delete the
file completely, thus losing the
original content.
Examples of this virus include: Way,
Trj.Reboot, Trivial.88.D.
Boot Virus
This type of virus affects the boot
sector of a floppy or hard disk. This is
a crucial part of a disk, in which
information on the disk itself is
stored together with a program that
makes it possible to boot (start) the
computer from the disk.
The best way of avoiding boot
viruses is to ensure that floppy disks
are write-protected and never start
your computer with an unknown
floppy disk in the disk drive.
Examples of boot viruses include:
Polyboot.B, AntiEXE.
Macro Virus
Macro viruses infect files that are
created using certain applications or
programs that contain macros.
These mini-programs make it
possible to automate series of
operations so that they are
performed as a single action,
thereby saving the user from having
to carry them out one by one.
Examples of macro viruses: Relax,
Melissa.A, Bablas, O97M/Y2K.
Directory Virus
Directory viruses change the paths
that indicate the location of a file. By
executing a program (file with the
extension .EXE or .COM) which has
been infected by a virus, you are
unknowingly running the virus
program, while the original file and
program have been previously
moved by the virus.
Once infected it becomes impossible
to locate the original files.
Polymorphic Virus
Polymorphic viruses encrypt or
encode themselves in a different
way (using different algorithms and
encryption keys) every time they
infect a system.
This makes it impossible for anti-
viruses to find them using string or
signature searches (because they are
different in each encryption) and
also enables them to create a large
number of copies of themselves.
Examples include: Elkern, Marburg,
Satan Bug, and Tuareg.
File Infectors
This type of virus infects programs
or executable files (files with an .EXE
or .COM extension). When one of
these programs is run, directly or
indirectly, the virus is activated,
producing the damaging effects it is
programmed to carry out. The
majority of existing viruses belong
to this category, and can be
classified depending on the actions
that they carry out.
Companion Viruses
Companion viruses can be
considered file infector viruses like
resident or direct action types. They
are known as companion viruses
because once they get into the
system they "accompany" the other
files that already exist. In other
words, in order to carry out their
infection routines, companion
viruses can wait in memory until a
program is run (resident viruses) or
act immediately by making copies of
themselves (direct action viruses).
Some examples include: Stator,
Asimov.1539, and Terrax.1069
FAT Virus
The file allocation table or FAT is the
part of a disk used to connect
information and is a vital part of the
normal functioning of the computer.
This type of virus attack can be
especially dangerous, by preventing
access to certain sections of the disk
where important files are stored.
Damage caused can result in
information losses from individual
files or even entire directories.
Worms
A worm is a program very similar to
a virus; it has the ability to self-
replicate, and can lead to negative
effects on your system and most
importantly they are detected and
eliminated by antiviruses.
Examples of worms include:
PSWBugbear.B, Lovgate.F, Trile.C,
Sobig.D, Mapson.
Trojans or Trojan Horses
Another unsavory breed of malicious
code are Trojans or Trojan horses,
which unlike viruses do not
reproduce by infecting other files,
nor do they self-replicate like worms.
Logic Bombs
They are not considered viruses
because they do not replicate. They
are not even programs in their own
right but rather camouflaged
segments of other program
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