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GitHub Repository: rapid7/metasploit-framework
 Path: blob/master/docs/metasploit-framework.wiki/How-to-Use-the-FILEFORMAT-mixin-to-create-a-file-format-exploit.md
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Msf::Exploit::FILEFORMAT is the mixin to use to create a file format exploit. There actually isn't much in the mixin, but the most important method is this: file_create:

Usage for file_create

As the name implies, the file_create method allows you to create a file. You should be using this method because it does more than just writing data to disk. One of the important things it does is it will report the file creation to the database in the format of #{ltype}.localpath, and the file will always be written to Metasploit's local directory defined in Msf::Config.local_directory (by default this path is ~/.msf4/local), which keep files nice and organized.

To use the mixin, first include Msf::Exploit::FILEFORMAT under the scope of your Metasploit3 class:

include Msf::Exploit::FILEFORMAT

And here's an example of using file_create to build an imaginary exploit:

# This is my imaginary exploit
buf = ""
buf << "A" * 1024
buf << [0x40201f01].pack("V")
buf << "\x90" * 10
buf << payload.encoded

file_create(buf)

Custom filename

The Msf::Exploit::FILENAME mixin by default has a registered FILENAME datastore option, and it is actually optional. If there's no filename provided, the mixin will set the name in this format: "exploit.fileformat.#{self.shortname}", where self.shortname means the shorter version of the module name.

If you wish to set a default one (but still changeable by the user), then you simply register it again in the module, like this:

register_options(
  [
    OptString.new('FILENAME', [true, 'The malicious file name',  'msf.jpg'])
  ], self.class)

Fixed filename

Occasionally, you might not want your user to change the filename at all. A lazy trick to do that is by modifying the FILENAME datastore option at runtime, but this is very much not recommended. In fact, if you do this, you will not pass [[msftidy|./Guidelines-for-Accepting-Modules-and-Enhancements.md]]. Instead, here's how it's done properly:

1 - Deregister the FILENAME option

deregister_options('FILENAME')

2 - Next, override the file_format_filename method, and make it return the filename you want:

def file_format_filename
  'something.jpg'
end

3 - Finally, please leave a note about this in the module description.

References