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Dangerous File Extensions

While file extensions serve an important role as a part of file system component of the operating system they also provide an easy way for certain malicious application to enter and infect the computer. Every operating system has a database of matches between file extensions and applications which could be used to open them. When user tries to open a file operating system will scan this database and select a matching application which can open it. On one hand it makes it very easy to open any file by just clicking on it. On the other hand if user clicks on a file with malicious content operating system will immediately open it without even looking inside it.

File extension is everything which will be needed to find a corresponding application and launch it. Many viruses were distributed by e-mail with files attached with extensions which were either executable, batch scripts, or office files. Name of the file lures users into opening the file without looking into file extensions and computer becomes infected. That’s why nowadays most e-mail hosting providers completely disable attachment of files with potentially dangerous file extensions. For certain less dangerous file extensions like archives or document files automatic virus scan is ran on a file while e-mail is being sent.

Below are file extensions which are considered dangerous as attachments:

• EXE, COM, SCR – executable files. File with any of these extensions could contain a virus.
• APP – Mac OS application files. Could contain a virus inside.
• DLL – dynamic link libraries in portable executable file format.
• BAT, CMD – script files. Can either perform dangerous actions on the computer or launch executable files with viruses.
• VBS – Visual Basic script files. Similarly to script files can either perform dangerous actions on the computer or launch executable files with viruses.
• XLS, DOC, XLSX, DOCX – Microsoft Office files which may contain macro viruses.