Buffer Overflow Vulnerability Experiment

First, the experimental description

A buffer overflow is a scenario in which a program attempts to write to a buffer beyond the pre-allocated fixed-length data. This vulnerability could be exploited by malicious users to alter the flow control of a program, or even execute arbitrary fragments of code. This vulnerability occurs because of a temporary shutdown of the data buffer and the return address, which causes the return address to be rewritten.

II. Preparation of the experiment

System User name Shiyanlou, password Shiyanlou

The lab building provides 64-bit Ubuntu Linux, and in this experiment we need to operate in 32-bit environments to facilitate the observation of assembly statements, so we need to do some preparation before the experiment.

1. Enter a command to install something that compiles a 32-bit C program:

sudo apt-get update

sudo apt-get install lib32z1 libc6-dev-i386

sudo apt-get install Lib32readline-gplv2-dev

2. Enter the command "linux32" into the 32-bit Linux environment.

At this point you will find that the command line is not as good as the tab completion, so enter "/bin/bash" Using bash:

Iii. Experimental Step 3.1 initial Setup

In Ubuntu and some other Linux systems, the initial address of random heap (heap) and stack (stack) is randomized using address space, which makes it difficult to guess the exact memory address, and guessing the memory address is the key to the buffer overflow attack. So in this experiment, we use the following command to turn off this feature:

sudo sysctl-w kernel.randomize_va_space=0

In addition, in order to further protect against buffer overflow attacks and other attacks using shell programs, many shell programs automatically abandon their privileges when called. Therefore, even if you can trick a set-uid program into invoking a shell, you cannot maintain root privileges in the shell, which is implemented in/bin/bash.

In a Linux system,/bin/sh is actually a symbolic link to/bin/bash or/bin/dash. To reproduce the situation before this protective measure was implemented, we used another shell program (zsh) instead of/bin/bash. The following instructions describe how to set up the ZSH program:

3.2 Shellcode

In general, a buffer overflow can cause a program to crash, and in the program, the overflow data overwrites the return address. And if the data that overwrites the return address is another address, then the program jumps to that address, and if the address is a piece of well-designed code to implement other functions, this code is shellcode.

Observe the following code:

#include <stdio.h>

int main()

{

Char *name[2];

name[0] = "/bin/sh";

name[1] = NULL;

Execve (name[0], name, NULL);

}

The shellcode of this experiment is the compiled version of the code just now:

\x31\xc0\x50\x68 "//sh" \x68"/bin" \x89\xe3\x50\x53\x89\xe1\x99\xb0\x0b\xcd\x80

3.3 Vulnerability Procedures

Save the following code as a "stack.c" file and save it to the/tmp directory. The code is as follows:

The code lets you know that the program reads a file named "Badfile" and loads the contents of the file into "buffer".

Compile the program, and set the Set-uid. The command is as follows:

sudo su

Gcc-m32-g-Z execstack-fno-stack-protector-o stack stack.c

chmod u+s Stack

Exit

The GCC compiler has a stack protection mechanism to prevent buffer overflows, so we need to use –fno-stack-protector to close this mechanism when compiling the code.

The-Z execstack is used to allow execution of the stack.

3.4 Attack Program

Our goal is to attack the vulnerability program just now and gain root access through the attack.

Save the following code as a "exploit.c" file and save it to the/tmp directory.

Notice the above code, "\x??" \x?? \x?? \x?? " Need to add shellcode to the address stored in memory because the location can overwrite the return address just after an overflow occurs.

and strcpy (Buffer+100,shellcode); This sentence tells us again that Shellcode is stored in the buffer+100 position.

Now we're going to get shellcode in-memory address, enter the command:

GDB Stack

Disass Main

Results

Next action: Set breakpoints to get the STR address

According to the statement strcpy (Buffer+100,shellcode); We calculate the address of Shellcode as 0xffffd1b0 (hex) +100 (decimal) =0xffffd214 (hexadecimal)

Modify EXPLOIT.C file Now! Will \x?? \x?? \x?? \x?? Modify to \x14\xd2\xff\xff

Then, compile the EXPLOIT.C program:

Gcc-m32-o Exploit exploit.c

3.5 Attack Results

Run the attack program exploit before running the vulnerability stack and observe the results:

Exercise one: Follow the experimental steps to attack the vulnerability program and gain root privileges.

Visible, through the attack, get the root permission!

If the attack succeeds and the "segment error" is indicated, re-use GDB disassembly to compute the memory address.

Iv. Practice

2, through the command "sudo sysctl-w kernel.randomize_va_space=2" to open the system's address space randomization mechanism, repeated use of exploit program to attack the stack program, to see if the attack succeeds, can gain root authority.

Found unable to attack successfully, can not get root permissions.

3, the/bin/sh to/bin/bash (or/bin/dash), to observe whether the attack succeeds, can gain root privileges.

Five, the experiment encountered problems and experience:

1.sudo command, enter the password does not show the password length, did not pay attention to the font size, appeared several times the error, fortunately in time found changed.

2. Exercise 31 began to not understand the meaning, with the help of classmates to do it again, more consolidation of the understanding of the experiment.

Experiment Experience:

The main experiment is to use the vulnerability function in the execution without checking the characteristics of the buffer length, by writing to the program buffer beyond its length of content, causing buffer overflow, thereby destroying the program stack, causing the program to crash or to enable the program to execute other instructions to achieve the purpose of the attack. The most critical point of an attack is to overwrite the stack with the data from the buffer overflow section, replacing the return address in the stack with a specific address, so that when the function call returns, it pops the return address of our replacement to the current base, and proceeds to the address where we want to jump to execute the pre-set code snippet.

Buffer Overflow Vulnerability Experiment