How does duqu implement its dll files in PNF file format?

Question

As you know files with PNF extension are associated with bitmap file type, from technically point of view how did authors of duqu implement this? as I concluded it is possible to make dlls in any file format, am I right?

Answer 1

As SteveS said in the comments, Windows doesn't care what file name you use - it's just a label. The "file extension" is only used as an indicator for how a user can interact with the file. Many applications also use it as a way to indicate that the file is of a particular type, but ultimately it's nothing more than a naming convention and shouldn't be relied on.

As long as the file is a PE format module, it can be loaded into a process as a module, e.g. via kernel32.dll!LoadLibrary. In fact, Windows relies on this behaviour in many cases, since .exe, .dll, .cpl, .ocx, .sys, .scr and .drv files are all PE files.