E-mail attachments -- educating the end user (Addendum)

SearchSecurity member Rick Pierides brings to light an important fact about file extensions.

E-mail attachments -- educating the end user (Addendum)

This tip was submitted to the searchSecurity Tip Exchange by user Rick Pierides. Let other users know how useful it is by rating the tip below.

Robert Vibert, in this month's excellent online searchSecurity Web cast on new virus tactics, brings up an interesting point that completely escaped me in an earlier tip I submitted. When Windows encounters a file WITHOUT AN EXTENSION it will first attempt to match the structure of the file to certain common file formats used within Windows. The structure of a Windows EXE native executable is very predictable because of the required headers and sections... Windows will recognize and merrily execute the file as a program.

Mr. Vibert also mentioned that an Excel Workbook with or without an embedded macro, but minus the XLS extension will be recognized by the Windows operating system as a bona fide Excel Workbook and open it. By extension, we can assume that the other well-known, registered Office document file formats are known intimately by the OS and will be opened properly without the extension. I have tried this with a Word document and was surprised when Word started up and opened the extension-less document. Additionally, the icons no longer point to the proper owning application. Windows does all this checking and, if appropriate, automatic execution, before it slaps up the "Open With" dialog box as a last resort.

That was an eye-opener for me. Therefore, I must emphatically add 'No Extension' to my list of file types to be very wary of. The end-user has no way to determining what the nature of a file is when there is no extension. Windows sometimes does and that can lead to trouble!

Acknowledgements to Mr. Robert Vibert for his insight.

(Editor's Note: An archive of Robert Vibert's Web cast is available here.)


This was first published in August 2001

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