What Is A Macro Virus?

The most common viruses that infect computers today--viruses such as Concept, Nuclear, Showoff, Adam, Wazzu, and Laroux--are macro viruses. They replicate by a completely different method than conventional viruses. We said earlier that a virus is a small computer program that needs to be executed by either running it or having it load from the boot sector of a disk. These types of viruses can spread through any program that they attach themselves to. Macro viruses can not attach themselves to just any program. Rather, each one can only spread through one specific program. The two most common types of macro viruses are Microsoft Word and Microsoft Excel viruses. These two programs are equipped with sophisticated macro languages so that many tasks can be automated with little or no input from the user. Virus writers quickly realized that it would be possible to construct self-replicating macros using these languages. The reason why this is possible is because Word documents and Excel spreadsheets can contain auto open macros. This means that when you open a Word Document in Word or an Excel spreadsheet in Excel any auto open macros contained within the document will execute automatically and you won't even know it's happening. In addition to auto open macros, both of these programs make use of a global macro template, which means that any macros stored in this global file will automatically execute whenever something is opened in that program. Macro viruses exploit these two aspects to enable themselves to replicate.
Here's how it works... You open an infected document in Microsoft Word. (Remember, Word documents can contain auto open macros). These macros, which in this example, contain a virus, execute when the document is opened and copy themselves into the global template that Word uses to store global macros. Now, since the infected macros are now part of your global template file they will automatically execute and copy themselves into other word documents whenever you open any document in Microsoft Word. Excel macro viruses work in relatively the same way. Because Word documents and Excel spreadsheets contain auto open macros it is important to think of them as computer programs in a sense. In other words, when you open Word documents in Word, or excel spreadsheets in Excel, you could be executing harmful code that is built right into the objects you're opening. They should be checked thoroughly for viruses before you open them in their respective programs. It is important to have an effective anti-virus strategy in place to prevent infection by these and all other kinds of viruses.