Class: Rex::Encoder::Alpha2::AlphaMixed

Inherits:
Generic
  • Object
show all
Defined in:
lib/rex/encoder/alpha2/alpha_mixed.rb

Class Method Summary collapse

Methods inherited from Generic

add_terminator, default_accepted_chars, encode, encode_byte, gen_second

Class Method Details

.gen_decoder(reg, offset, modified_registers = []) ⇒ String

Generates the decoder stub

Parameters:

  • reg (String) —

    the register pointing to the encoded payload

  • offset (Fixnum) —

    the offset to reach the encoded payload

  • modified_registers (Array) (defaults to: []) —

    accounts the registers modified by the stub

Returns:

  • (String) —

    the alpha mixed decoder stub



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# File 'lib/rex/encoder/alpha2/alpha_mixed.rb', line 93

def self.gen_decoder(reg, offset, modified_registers = [])
  mod_registers = []

  decoder =
     gen_decoder_prefix(reg, offset, mod_registers) +
     "jA" +          # push 0x41
     "X" +           # pop eax
     "P" +           # push eax
     "0A0" +         # xor byte [ecx+30], al
     "A" +           # inc ecx                        <---
     "kAAQ" +        # imul eax, [ecx+42], 51 -> 10       |
     "2AB" +         # xor al, [ecx + 42]                 |
     "2BB" +         # xor al, [edx + 42]                 |
     "0BB" +         # xor [edx + 42], al                 |
     "A" +           # inc ecx                            |
     "B" +           # inc edx                            |
     "X" +           # pop eax                            |
     "P" +           # push eax                           |
     "8AB" +         # cmp [ecx + 42], al                 |
     "uJ" +          # jnz short -------------------------
     "I"             # first encoded char, fixes the above J

  mod_registers.concat(
    [
      Rex::Arch::X86::ESP,
      Rex::Arch::X86::EAX,
      Rex::Arch::X86::ECX,
      Rex::Arch::X86::EDX
    ])

  mod_registers.uniq!
  modified_registers.concat(mod_registers)

  decoder
end

.gen_decoder_prefix(reg, offset, modified_registers = []) ⇒ String

Generates the decoder stub prefix

Parameters:

  • reg (String) —

    the register pointing to the encoded payload

  • offset (Fixnum) —

    the offset to reach the encoded payload

  • modified_registers (Array) (defaults to: []) —

    accounts the registers modified by the stub

Returns:

  • (String) —

    the alpha mixed decoder stub prefix



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# File 'lib/rex/encoder/alpha2/alpha_mixed.rb', line 17

def self.gen_decoder_prefix(reg, offset, modified_registers = [])
  if offset > 32
    raise 'Critical: Offset is greater than 32'
  end

  mod_registers = []
  nop_regs = []
  mod_regs = []
  edx_regs = []

  # use inc ebx as a nop here so we still pad correctly
  if offset <= 16
    nop = 'C' * offset
    nop_regs.push(Rex::Arch::X86::EBX) unless nop.empty?

    mod = 'I' * (16 - offset) + nop + '7QZ'    # dec ecx,,, push ecx, pop edx
    mod_regs.push(Rex::Arch::X86::ECX) unless offset == 16
    mod_regs.concat(nop_regs)
    mod_regs.push(Rex::Arch::X86::EDX)

    edxmod = 'J' * (17 - offset)
    edx_regs.push(Rex::Arch::X86::EDX) unless edxmod.empty?
  else
    mod = 'A' * (offset - 16)
    mod_regs.push(Rex::Arch::X86::ECX) unless mod.empty?

    nop = 'C' * (16 - mod.length)
    nop_regs.push(Rex::Arch::X86::EBX) unless nop.empty?

    mod << nop + '7QZ'
    mod_regs.concat(nop_regs)
    mod_regs.push(Rex::Arch::X86::EDX)

    edxmod = 'B' * (17 - (offset - 16))
    edx_regs.push(Rex::Arch::X86::EDX) unless edxmod.empty?
  end

  regprefix = {
    'EAX'   => 'PY' + mod,                         # push eax, pop ecx
    'ECX'   => 'I' + mod,                          # dec ecx
    'EDX'   =>  edxmod + nop + '7RY',			   # dec edx,,, push edx, pop ecx
    'EBX'   => 'SY' + mod,                         # push ebx, pop ecx
    'ESP'   => 'TY' + mod,                         # push esp, pop ecx
    'EBP'   => 'UY' + mod,                         # push ebp, pop ecx
    'ESI'   => 'VY' + mod,                         # push esi, pop ecx
    'EDI'   => 'WY' + mod,                         # push edi, pop ecx
  }

  reg.upcase!

  unless regprefix.keys.include?(reg)
    raise ArgumentError.new('Invalid register name')
  end

  case reg
  when 'EDX'
    mod_registers.concat(edx_regs)
    mod_registers.concat(nop_regs)
    mod_registers.push(Rex::Arch::X86::ECX)
  else
    mod_registers.push(Rex::Arch::X86::ECX)
    mod_registers.concat(mod_regs)
  end

  mod_registers.uniq!
  modified_registers.concat(mod_registers)

  return regprefix[reg]
end