Hiding In Plain Sight

By default, Windows hides file extensions for known file types. Your average Windows user when he or she opens up their "My Documents" folder, doesn't see the ".doc" extensions that accompany Microsoft Word files, or the ".pdf" extension that is appended to Adobe PDF files. Let's say I'm a virus writer, and the computer worm I want to spam out to the rest of the world is called nice.exe. If I rename that program to "nice.txt.exe," the file will appear to Windows recipients who haven't changed the default settings as "nice.txt."

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