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Table Of Contents  CertiGuide to Security+
 9  Chapter 1:  General Security Concepts (Domain 1.0; 30%)
      9  1.5  Malicious Code

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1.5.2  Trojan Horses
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1.5.1  Viruses
(Page 1 of 2)

This is a catchall phrase for any man-made code that is anything from annoying (WAZZU100) to destructive, such as a boot sector virus101, which includes some way to attach itself to other files, called “host files,” and replicate itself. A virus can replicate either from file to file on a single system, or across a network (if it replicates automatically across the network without a user’s assistance, it is more precisely called a worm, another type of malicious code discussed in 1.5.4).

A Virus is…

A virus is a program that attaches itself to a host file and replicates itself on a system. Usually when run, unbeknownst to the user, a virus performs some action that is either malicious or simply annoying. For example, a virus may delete or modify system files, or just produce a joke message on the user’s screen.


Viruses move from computer to computer by some user’s actions, rather than spreading automatically across a network. A virus typically gets into a system from an external source, such as a floppy disk containing software given to you by an office-mate, a file on a network share being copied onto your system and then executed to install it or “just see what it does,” an email message sent to you by a friend containing a file that claims to be a greeting card (to get you to open it), or software that you download from the web or install from a very unlucky vendor’s CD distribution (the latter really has happened ).

The possibility of transmitting a virus by email is why many email systems feature virus scanning for in-bound email, trying to limit the number of ways known viruses can enter a network.

[spacer]Viruses are Getting Sneaky

Over the years, the IT world has seen several different kinds of viruses, including:

1. Boot sector viruses which place their code in the hard disk’s boot sector, which is loaded every time the machine powers on.

2. File infector viruses which attach themselves to legitimate executable programs, causing the virus to run each time one of those programs is launched.

3. Script viruses which, like file infectors, attach themselves to existing legitimate programs (in this case, scripts like DOS batch files or VBscript or java script batch files), causing the virus to run each time the infected script is launched.

4. Macro viruses that are embedded in files such as word processing documents and spreadsheets whose environments (such as Microsoft Word) support “macro” programming in documents to customize Word’s behavior when the document is open. These viruses are often attached to initialization macros that run whenever the document is opened, so that the user is not aware that they are “running” anything.



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100. http://open.jeffersonhospital.org/tju/dis/virus/desc/wazzu.html (I wonder if a U of W <Huskie> wrote this?)

101. http://www.sophos.com/virusinfo/analyses/index_dosexe.html.

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