Proj 1: Linux Buffer Overflow: Command Injection (10 pts. + 15 pts. extra credit)

buffer-overflow
bof
command-injection
#1

What You Need

A 32-bit x86 Kali 2 Linux machine, real or virtual.

Purpose

To develop a very simple buffer overflow exploit in Linux, using injected shell commands.

Creating a Vulnerable Program

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.

Challenge 1: Long List (5 pts. extra credit)

Execute the “ls -l” command by entering a crafted name, so it shows file details, as shown below.

Hint

If spaces are annoying you, try using $IFS to replace them.

Saving a Screen Image

Make sure you can see the “long list”, with file permissions and creation dates for files, 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 1b ", replacing “YOUR NAME” with your real name.

Challenge 2: Exploit a Remote Server (10 pts. extra credit)

Execute this command to connect to a remote server running this program:

nc attack32direct.samsclass.info 1055

Then put your name in this file on that server:

/home/p1x/winners

Create this file:

/home/p1x/updatenow

After one minute, your name will appear on the WINNERS page here:

http://attack32direct.samsclass.info/p1x-winners.html

Hint

The injected commands run in the Bourne shell (sh), not the bash shell. You can test commands on your local Mac or Linux box by using the sh command to open a Bourne shell.

Troubleshooting

If you have network problems, you can check the local network connections at this page:

http://attack32direct.samsclass.info/netstat.htm
That page is updated every 5 seconds.

Source: https://samsclass.info/127/proj/p1-lbci.htm