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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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class MetasploitModule < Msf::Encoder::Xor
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  def initialize
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    super(
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      'Name'             => 'Single-byte XOR Countdown Encoder',
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      'Description'      => %q{
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        This encoder uses the length of the payload as a position-dependent
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        encoder key to produce a small decoder stub.
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      },
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      'Author'           => 'vlad902',
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      'Arch'             => ARCH_X86,
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      'License'          => MSF_LICENSE,
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      'Decoder'          =>
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        {
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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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    # Sanity check that saved_registers doesn't overlap with modified_registers
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    if (modified_registers & saved_registers).length > 0
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      raise BadGenerateError
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    end
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    begin
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      decoder =
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        Rex::Arch::X86.set(
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          Rex::Arch::X86::ECX,
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          state.buf.length - 1,
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          state.badchars) +
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        "\xe8\xff\xff\xff" +  # call $+4
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        "\xff\xc1" +          # inc ecx
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        "\x5e" +              # pop esi
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        "\x30\x4c\x0e\x07" +  # xor_loop: xor [esi + ecx + 0x07], cl
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        "\xe2\xfa"            # loop xor_loop
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      # Initialize the state context to 1
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      state.context = 1
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    rescue RuntimeError => e
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      raise BadcharError if e.message == "No valid set instruction could be created!"
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    end
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    return decoder
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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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    state.context += 1
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    [ block.unpack('C')[0] ^ (state.context - 1) ].pack('C')
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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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  # A list of registers always touched by this encoder
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  def modified_registers
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    [ Rex::Arch::X86::ECX, Rex::Arch::X86::ESI ]
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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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