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##
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# This module requires Metasploit: https://metasploit.com/download
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# Current source: https://github.com/rapid7/metasploit-framework
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##
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require 'rex/encoder/alpha2/alpha_mixed'
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class MetasploitModule < Msf::Encoder::Alphanum
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  Rank = LowRanking
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  def initialize
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    super(
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      'Name' => 'Alpha2 Alphanumeric Mixedcase Encoder',
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      'Description' => %q{
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        Encodes payloads as alphanumeric mixedcase text.  This encoder uses
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        SkyLined's Alpha2 encoding suite.
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        A pure alpha encoder is impossible without having a register that points at or near the shellcode.
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        In a default configuration the first few bytes at the beginning are an fnstenv getpc stub (the same as used in shikata_ga_nai) and thus are not alphanumeric.
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        You can set BufferRegister for full alpha (see Encoder options for details).
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      },
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      'Author' => [ 'pusscat', 'skylined' ],
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      'Arch' => ARCH_X86,
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      'License' => BSD_LICENSE,
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      'EncoderType' => Msf::Encoder::Type::AlphanumMixed,
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      'Decoder' => {
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        'BlockSize' => 1
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      })
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  end
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  #
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  # Returns the decoder stub that is adjusted for the size of the buffer
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  # being encoded.
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  #
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  def decoder_stub(state)
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    modified_registers = []
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    reg = datastore['BufferRegister']
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    off = (datastore['BufferOffset'] || 0).to_i
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    buf = ''
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    # We need to create a GetEIP stub for the exploit
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    if !reg
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      if datastore['AllowWin32SEH'] && datastore['AllowWin32SEH'].to_s =~ (/^(t|y|1)/i)
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        buf = 'VTX630VXH49HHHPhYAAQhZYYYYAAQQDDDd36FFFFTXVj0PPTUPPa301089'
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        reg = 'ECX'
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        off = 0
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        modified_registers.concat(
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          [
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            Rex::Arch::X86::ESP,
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            Rex::Arch::X86::EDI,
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            Rex::Arch::X86::ESI,
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            Rex::Arch::X86::EBP,
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            Rex::Arch::X86::EBX,
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            Rex::Arch::X86::EDX,
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            Rex::Arch::X86::ECX,
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            Rex::Arch::X86::EAX
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          ]
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        )
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      else
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        res = Rex::Arch::X86.geteip_fpu(state.badchars, modified_registers)
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        if !res
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          raise EncodingError, 'Unable to generate geteip code'
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        end
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        buf, reg, off = res
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      end
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    else
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      reg.upcase!
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    end
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    stub = buf + Rex::Encoder::Alpha2::AlphaMixed.gen_decoder(reg, off, modified_registers)
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    # Sanity check that saved_registers doesn't overlap with modified_registers
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    modified_registers.uniq!
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    if !(modified_registers & saved_registers).empty?
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      raise BadGenerateError
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    end
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    stub
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  end
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  #
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  # Encodes a one byte block with the current index of the length of the
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  # payload.
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  #
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  def encode_block(state, block)
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    Rex::Encoder::Alpha2::AlphaMixed.encode_byte(block.unpack('C')[0], state.badchars)
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  end
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  #
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  # Tack on our terminator
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  #
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  def encode_end(state)
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    state.encoded += Rex::Encoder::Alpha2::AlphaMixed.add_terminator
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  end
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  # Indicate that this module can preserve some registers
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  def can_preserve_registers?
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    true
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  end
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  # Convert the SaveRegisters to an array of x86 register constants
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  def saved_registers
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    Rex::Arch::X86.register_names_to_ids(datastore['SaveRegisters'])
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  end
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end
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