The 9 Types of Computer Viruses To Watch Out For & What
They Do
Everyone
dreads being the recipient of a computer virus, but not everyone minds studying
them. There are researchers who spend a lot of time looking into different
types of computer viruses and related security threats in order to determine
how they’re programmed, how they do damage, and how they spread. Personally, I
find this field interesting, and I enjoy reading about the different types of
viruses in existence.
But even if you don’t know much
care, basic knowledge about security threats can be useful. It’s sometimes hard
to know how a risk must be dealt with before you know its consequences. With a
computer virus, however, the consequence is sometimes complete loss of your
data or identity theft – so it’s best to learn sooner rather than later!
1.
Boot Sector Virus
The term “boot sector” is a generic
name that seems to originally come from MS-DOS but is now applied generally to
the boot information used by any operating system. In modern computers this is
usually called the “master boot record,” and it is the first sector on a
partitioned storage device.
Boot sector viruses became popular
because of the use of floppy disks to boot a computer. The widespread usage of
the Internet and the death of the floppy has made other means of virus
transmission more effective.
2.
Browser Hijacker
This type of virus, which can spread
itself in numerous ways including voluntary download, effectively hijacks
certain browser functions, usually in the form of re-directing the user
automatically to particular sites. It’s usually assumed that this tactic is
designed to increase revenue from web advertisements.
There are a lot of such viruses, and
they usually have “search” included somewhere in their description. CoolWebSearch may be
the most well known example, but others are nearly as common.
3.
Direct Action Virus
This type of virus, unlike most,
only comes into action when the file containing the virus is executed. The
payload is delivered and then the virus essentially becomes dormant – it takes
no other action unless an infected file is executed again.
Most viruses do not use the direct
action method of reproduction simply because it is not prolific, but viruses of
this type have done damage in the past. The Vienna virus,
which briefly threatened computers in 1988, is one such example of a direct action
virus.
4.
File Infector Virus
Perhaps the most common type of
virus, the file infector takes root in a host file and then begins its
operation when the file is executed. The virus may completely overwrite the
file that it infects, or may only replace parts of the file, or may not replace
anything but instead re-write the file so that the virus is executed rather
than the program the user intended.
Although called a “file virus” the
definition doesn’t apply to all viruses in all files generally – for example,
the macro virus below is not referred to by the file virus. Instead, the
definition is usually meant to refer only to viruses which use an executable
file format, such as .exe, as their host.
5.
Macro Virus
A wide variety of programs,
including productivity applications like Microsoft Excel, provide support for
Macros – special actions programmed into the document using a specific macro
programming language. Unfortunately, this makes it possible for a virus to be
hidden inside a seemingly benign document.
Macro viruses very widely in terms
of payload. The most well known macro virus is probably Melissa,
a Word document supposedly containing the passwords to pornographic websites.
The virus also exploited Word’s link to Microsoft Outlook in order to automatically email copies of itself.
6.
Multipartite Virus
While some viruses are happy to
spread via one method or deliver a single payload, Multipartite viruses want it
all. A virus of this type may spread in multiple ways, and it may take
different actions on an infected computer depending on variables, such as the
operating system installed or the existence of certain files.
7.
Polymorphic Virus
Another jack-of-all-trades, the
Polymorphic virus actually mutates over time or after every execution, changing
the code used to deliver its payload. Alternatively, or in addition, a
Polymorphic virus may guard itself with an encryption algorithm that
automatically alters itself when certain conditions are met.
The goal of this trickery is
evasion. Antivirus programs often find viruses by the specific code used.
Obscuring or changing the code of a virus can help it avoid detection.
8.
Resident Virus
This broad virus definition applies
to any virus that inserts itself into a system’s memory. It then may take any
number of actions and run independently of the file that was originally infected.
A resident virus can be compared to
a direct payload virus, which does not insert itself into the system’s memory
and therefore only takes action when an infected file is executed.
9.
Web Scripting Virus
Many websites execute complex code
in order to provide interesting content. Displaying online video in
your browser, for example, requires the execution of a specific code language
that provides both the video itself and the player interface.
Of course, this code can sometimes
be exploited, making it possible for a virus to infect a computer or take
actions on a computer through a website. Although malicious sites are sometimes
created with purposely infected code, many such cases of virus exist because of
code inserted into a site without the webmaster’s knowledge.
Conclusion
I’ve included every type of computer
virus in this article that I felt was relevant, but this isn’t a list of every
type of virus known to man. Also, I did not include any worms or trojans, two
common threats that are similar to viruses but technically not the same. If
there is a specific type of virus that you feel was missed and is important,
let us know in the comments.
