Monday, June 6, 2011

Defcon 19 Quals - Pwntent Pwnables 200 Writeup

First, I checked the binary with various commands

$ file pp200_64625bc51c5b8dc75b
pp200_64625bc51c5b8dc75b: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked (uses shared libs), stripped
$ strings pp200_64625bc51c5b8dc75b
$ objdump -s -j .comment pp200_64625bc51c5b8dc75b

pp200_64625bc51c5b8dc75b:     file format elf32-i386

Contents of section .comment:
 0000 00402823 2953756e 4f532035 2e313020  .@(#)SunOS 5.10
 0010 47656e65 72696320 4a616e75 61727920  Generic January
 0020 32303035 00004028 23295375 6e4f5320  2005..@(#)SunOS
 0030 352e3130 2047656e 65726963 204a616e  5.10 Generic Jan
 0040 75617279 20323030 35000040 28232953  uary 2005..@(#)S
 0050 756e4f53 20352e31 30204765 6e657269  unOS 5.10 Generi
 0060 63204a61 6e756172 79203230 30350000  c January 2005..
 0070 4743433a 2028474e 55292033 2e342e33  GCC: (GNU) 3.4.3
 0080 20286373 6c2d736f 6c323130 2d335f34   (csl-sol210-3_4
 0090 2d627261 6e63682b 736f6c5f 72706174  -branch+sol_rpat
 00a0 68290000 4743433a 2028474e 55292033  h)..GCC: (GNU) 3

From output, we know this binary is for Solaris x86. The "file" command tell the binary is stripped but it is not. When I open it in IDA, all function name is resolved. So it is easy to read/guess the code.

The challenge is so straightforward. The program receives input from client then jump to the received buffer+1. Just send the shellcode.

I had no knowledge about writing shellcode for Solaris. So I tried with metasploit "solaris/x86/shell_reverse_tcp". The size is 91 bytes but the BUFSIZE is 0x49=73 bytes. The metasploit payload is too big for this challenge.

I planned to sending small shellcode to receiving big shellcode. My trick is reusing the code. Pushing the size to be received then jump to address 0x080516b4 in order to make program call recvAll with my size then jump to received buffer+1 again. So the first shellcode is

sc = "\x6a\x5c"   # push 92
sc += "\xb8\xb4\x16\x05\x08" # mov eax,0x080516b4 
sc += "\xff\xe0"  # jmp eax

The full python code is

The above method, I failed to do it in CTF because I put the wrong BUFSIZE in python code. Then, I learnt writing shellcode for Solaris x86 and tried connection reuse. I failed again because of my stupid mistake :[. I ended up with writing shellcode to read key file. Here is the python code for reading key file

After loading OpenSolaris VMWare image as someguy (sorry I cannot remember the name) in irc gave me the link, I know why I failed to get the shell. Thanks for the link.

The key, I cannot remember. :P

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