Definition of computer virus – Type of computer
virus
A computer virus is a program or piece of code that is
loaded onto your computer without your knowledge and runs against your wishes.
Viruses can also replicate themselves. All computer viruses are man-made. A
simple virus that can make a copy of itself over and over again is relatively
easy to produce. Even such a simple virus is dangerous because it will quickly
use all available memory and bring the system to a halt. An even more dangerous
type of virus is one capable of transmitting itself across networks and
bypassing security systems.
In computers, a virus is a program or programming code that
replicates by being copied or initiating its copying to another program,
computer boot sector or document.
A computer virus is a program designed to harm or cause harm
on an infected computer. Its spreads through e-mail attachments, portable
devices, websites containing malicious scripts and file downloads. A computer
virus attaches itself to the host files and always activate whenever you open
the infected files. The virus can replicate itself and then infect the other
files on your computer causing more damage. Below is a list of different types
of computer viruses and what they do.
Types of computer virus
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.
Multipartite Viruses
Multipartite viruses are
distributed through infected media and usually hide in the memory. Gradually,
the virus moves to the boot sector of the hard drive and infects executable
files on the hard drive and later across the computer system.
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 belongs to this category, and can be classified depending on
the actions that they carry out.
Encrypted Viruses
This type of viruses consists of
encrypted malicious code, decrypted module. The viruses use encrypted code
technique which make antivirus software hardly to detect them. The antivirus
program usually can detect this type of viruses when they try spread by
decrypted themselves.
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
Network Virus
Network viruses rapidly spread
through a Local Network Area (LAN), and sometimes throughout the internet.
Generally, network viruses multiply through shared resources, i.e., shared
drives and folders. When the virus infects a computer, it searches through the
network to attack its new potential prey. When the virus finishes infecting
that computer, it moves on to the next and the cycle repeats itself.
The most dangerous network
viruses are Nimda and SQLSlammer.
Nonresident Viruses
This type of viruses is similar
to Resident Viruses by using replication of module. Besides that, Nonresident
Viruses role as finder module which can infect to files when it found one (it
will select one or more files to infect each time the module is executed).
Stealth Viruses
Stealth Viruses is some sort of
viruses which try to trick anti-virus software by intercepting its requests to
the operating system. It has ability to hide itself from some antivirus
software programs. Therefore, some antivirus program cannot detect them.
Sparse Infectors
In order to spread widely, a
virus must attempt to avoid detection. To minimize the probability of its being
discovered a virus could use any number of different techniques. It might, for
example, only infect every 20th time a file is executed; it might only infect
files whose lengths are within narrowly defined ranges or whose names begin
with letters in a certain range of the alphabet. There are many other
possibilities.
Spacefiller (Cavity) Viruses
Many viruses take the easy way
out when infecting files; they simply attach themselves to the end of the file
and then change the start of the program so that it first points to the virus
and then to the actual program code. Many viruses that do this also implement
some stealth techniques so you don't see the increase in file length when the
virus is active in memory.
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 technically not a
virus, but 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 (not a virus as well) 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 programs.
Their objective is to destroy
data on the computer once certain conditions have been met. Logic bombs go
undetected until launched, and the results can be destructive.
Use of Antivirus software
Antivirus or anti-virus software (often
abbreviated as AV), sometimes known as anti-malware software, is
computer software used to prevent, detect and remove malicious software.
Antivirus (or anti- virus) software is used to safeguard a
computer from malware, including viruses, computer worms, and Trojan horses.
Antivirus software may also remove or prevent spyware and
adware, along with other forms of malicious programs. Free antivirus software
generally only searches your computer using signature-based detection which
involves looking for patterns of data that are known to be related to
already-identified malware. Paid antivirus software will usually also include
heuristics to catch new, or zero-day threats, by either using genetic
signatures to identify new variants of existing virus code or by running the
file in a virtual environment (also called a sandbox), and watching what it
does to see if it has malicious intent.
Virus designers, however, usually test their malicious code
against the major antivirus types of malwares, specifically ransomware, use
polymorphic code to make it difficult to be detected by antivirus software.
Besides using antivirus software to keep your computer safe and running
smoothly, it is also always a good idea to be proactive: make sure your web
browser is updated to the latest version, use a firewall, only download
programs from websites you trust and always surf the web using a standard user
account, rather than your administrator one.
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