79 lines
2.1 KiB
Ruby
79 lines
2.1 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' => '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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