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Chapter 2 – Introduction to Computer Viruses
compared to thousands for Windows. But now that macro viruses are prevalent,
the danger is present.
Many programs provide the ability to create macro commands. These simple
programs use either the internal functions of an application to "record" and "play
back" commonly used sequences of commands. Other applications provide a more
powerful macro language, which includes both menu commands and a
programming language. Programs such as Microsoft Word and Excel base their
macro functions on Visual Basic, which is similar to the Basic programming
language.
One of the reasons that macro virus writers target Microsoft programs is that these
applications allow users to embed macros in data files. In the past, one worried
only about viruses coming through applications, since, for a virus to act, it has to
execute, and only applications could execute. But the Microsoft Visual Basic
approach is different—if you wish to use a macro, you can either run it from your
template, or add it to a data file. This surprised users at first, since they thought
that nothing was "executed" when opening a word processor or spreadsheet file.
But these files can indeed contain "programs", and do things you would never
expect.
If the macro language provides the possibility to modify files, a macro virus will be
able to copy itself into other files used by the same application. This then allows
the virus to spread when you open other files, create new files, or pass files on to
someone else.
Most macro viruses that target Microsoft Word files use commands such as
AutoOpen, AutoClose, AutoExec and AutoExit. These are commands that are
executed when a certain event occurs to the file, and these four events always
occur when you work with a file. If, for example, a macro were written to copy
Intego VirusBarrier X4 User's Manual
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