Security Notes | Integer Cyber Security: Linux Buffer Overflow (Command Injection)

 আপনার যা প্রয়োজন

একটি ৩২-বিট (x86) কালি লিনাক্স অপারেটিং সিস্টেম (রিয়েল কিংবা ভার্চুয়াল)

উদ্দেশ্য

ইনঞ্জেক্টেড শেল কমান্ড ব্যবহার করে খুব সিম্পল একটা বাফার ওভারফ্লো এক্সপ্লইট তৈরি করতে হবে।

একটি ভালনারাবল প্রোগ্রাম তৈরি

This program inputs a name from the user and prints out a "Goodbye" message. It then calls system() to print out the Linux version. It uses two buffers in a subroutine to do that in an unsafe manner, allowing the name buffer to overflow into the command buffer.

In a Terminal window, execute this command:

nano buf.c

Copy and paste in this code:

#include <string.h>
#include <stdio.h>

main(){
        char name[200];
        printf("What is your name?\n");
        scanf("%s", name);
        bo(name, "uname -a");
}

int bo(char *name, char *cmd){
        char c[40];
        char buffer[40];
        printf("Name buffer address:    %x\n", buffer);
        printf("Command buffer address: %x\n", c);
        strcpy(c, cmd);
        strcpy(buffer, name);
        printf("Goodbye, %s!\n", buffer);
        printf("Executing command: %s\n", c);
        fflush(stdout);
        system(c);
}

Save the file with Ctrl+X, Y, Enter.

Execute this command to compile the code without modern protections against stack overflows, and with debugging symbols:

gcc -g -fno-stack-protector -z execstack -o buf buf.c

You should see compiler warnings, but no errors.

Troubleshooting If you see this error: fatal error: string.h: No such file or directory That means gcc is not properly installed, which was the case on my Kali 2017.3 machine. Execute this command to fix gcc: apt install build-essential -y

Running the Program Normally

Execute this command:

./buf

Enter your first name when prompted to.

The program prints out the location of the Name buffer and the command buffer, says "Goodbye", and excutes the command "uname -a", as shown below.

Observing a Crash

Execute this command:

./buf

Enter fifty 'A' characters instead of your name.

The program attempts to execute the command AAAAAAA, as shown below.

Finding the Code Injection Point

Execute this command:

./buf

Enter:

• Ten 'A' characters, then
• Ten 'B' characters, then
• Ten 'C' characters, then
• Ten 'D' characters, then
• Ten 'E' characters.

The program attempts to execute the command EEEEEEEEEE, as shown below. So any text we put in place of EEEEEEEEEE will execute.

Executing the "ls" command

Execute this command:

./buf

Enter ten 'A' characters, then ten 'B' characters, then ten 'C' characters, then ten 'D' characters, then ls

The program executes the "ls" command, showing the files in your working directory, as shown below.

Saving a Screen Image

Make sure you can see "Executing command: ls, as shown above.

Press the PrintScrn key to copy the whole desktop to the clipboard.

YOU MUST SUBMIT A FULL-SCREEN IMAGE FOR FULL CREDIT!

Paste the image into Paint.

Save the document with the filename "YOUR NAME Proj 1a", replacing "YOUR NAME" with your real name.

ক্রেডিটঃ SamClass