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From: full-disclosure () lists netsys com (Nick FitzGerald)
Date: Fri, 26 Jul 2002 18:34:32 +1200

Jeff Kell <jeff-kell () utc edu> replied to http-equiv () malware com:

[I thought I replied to "http-equiv"'s message earlier, but on 
checking I sent it direct, not to the lists...]

Just tested something here. Typically IE can or will open files
depending what the contents are regardless of the extension that it
is: <html> tag in a gif or some other file type should or can be
rendered by IE for what the contents are, not the extension.

The Windows run function (IE viewer) ignores the extension (sort of) if
the file is in a portable OLE-type format.  For example, go in Word and
create "foo.doc".  Exit and rename "foo.doc" to "foo.fubar".  Double
click "foo.fubar" and Word opens up.  Same for Excel and other things.

If the extension is known, it appears to try and use it.  If not, it
will look for OLE-extensions and launch what matches.

It's the other way around -- if a file's extension is not registered
on the system trying to "run" (or "open") the file, depending on how 
it is being "opened", some further checks than just "what is 
registered to handle this extension" are made.  One of those checks 
determines whether the file is apparently internally an OLE2 file, 
and if so the application registered to handle the CLSID of the root 
directory entry in the OLE2 file is directed to open the file.  If 
that CLSID is also not registered then the usual "Open With..." 
dialog appears.  Another file type tested for in this process is the 
DOS ("MZ") EXE format, which can be run "as normal", depending on the 
"open" method used, depsite having been renamed to a non-EXE 

Thus, "http-equiv"'s discovery that a non-extensioned EXE could be 
launched through one of these code execution holes is not all that

Nick FitzGerald
Computer Virus Consulting Ltd.
Ph/FAX: +64 3 3529854

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