Certain file types, known as executable file types, open by themselves, i.e. they do not require or use a handling program. For example, Notepad.exe is known as a PE_EXE or portable executable file type. This means that it is a standalone executable, capable of running by itself on any Windows operating system. If I were to copy Notepad.exe from a Windows XP system to a Windows 95 system, it would work the same as if it were still on XP and requires no other files other than itself to operate normally because it has no other dependencies.
Conversely, other .EXE files are standard executables that do have dependencies on other program files (often .DLL or other .EXE files) in order to function normally. For example, WINWORD.EXE is the main program executable for Microsoft Word. It requires certain other files (several in fact) in order for it to function properly. If WINWORD.EXE were copied to another system, it would not execute. Instead, errors would be returned. It is not a PE_EXE file.
There are a large number of executable file types, most of which do not have a .EXE extension. To view a list of some of the more common executable file types, visit the Executable File Extensions Center.
File extension associations can also be manipulated via the System Registry. The Winevar virus exploited this ability and registered a completely innocuous and bogus file type, .CEO, as an .EXE file. This ruse allowed the Winevar virus to slip the virus past antivirus scanners and filtering products that scanned based on file extension alone, rather than checking the internal file structure to determine its true nature. This weakness can best be overcome by ensuring that “Scan all files” is selected as the scanning option in the antivirus product. However, even this method is not foolproof, as some vendors have persisted over the years in translating “Scan all files” to mean “Scan all files already listed on the extension list of files to be scanned”. Obviously, this has a dramatically different meaning. The antivirus products listed in the Top Picks all have valid “Scan all files” options.
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