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##
# This module requires Metasploit: https://metasploit.com/download
# Current source: https://github.com/rapid7/metasploit-framework
##
class MetasploitModule < Msf::Encoder::Xor
def initialize
super(
'Name' => 'Single-byte XOR Countdown Encoder',
'Description' => %q{
This encoder uses the length of the payload as a position-dependent
encoder key to produce a small decoder stub.
},
'Author' => 'vlad902',
'Arch' => ARCH_X86,
'License' => MSF_LICENSE,
'Decoder' =>
{
'BlockSize' => 1,
})
end
#
# Returns the decoder stub that is adjusted for the size of the buffer
# being encoded.
#
def decoder_stub(state)
# Sanity check that saved_registers doesn't overlap with modified_registers
if (modified_registers & saved_registers).length > 0
raise BadGenerateError
end
begin
decoder =
Rex::Arch::X86.set(
Rex::Arch::X86::ECX,
state.buf.length - 1,
state.badchars) +
"\xe8\xff\xff\xff" + # call $+4
"\xff\xc1" + # inc ecx
"\x5e" + # pop esi
"\x30\x4c\x0e\x07" + # xor_loop: xor [esi + ecx + 0x07], cl
"\xe2\xfa" # loop xor_loop
# Initialize the state context to 1
state.context = 1
rescue RuntimeError => e
raise BadcharError if e.message == "No valid set instruction could be created!"
end
return decoder
end
#
# Encodes a one byte block with the current index of the length of the
# payload.
#
def encode_block(state, block)
state.context += 1
[ block.unpack('C')[0] ^ (state.context - 1) ].pack('C')
end
# Indicate that this module can preserve some registers
def can_preserve_registers?
true
end
# A list of registers always touched by this encoder
def modified_registers
[ Rex::Arch::X86::ECX, Rex::Arch::X86::ESI ]
end
# Convert the SaveRegisters to an array of x86 register constants
def saved_registers
Rex::Arch::X86.register_names_to_ids(datastore['SaveRegisters'])
end
end
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