Fix Buffer Overflow

TL;DR

A buffer overflow happens when a program tries to write more data into a memory area than it’s allowed. This can cause crashes or, worse, let attackers take control of your system. We’ll show you how to identify and fix a simple buffer overflow vulnerability.

Understanding the Problem

Imagine a box that can only hold 10 items. If you try to put 15 items in it, some will spill over – that’s similar to a buffer overflow. In programming, ‘buffers’ are areas of memory set aside for specific data. If a program doesn’t check the size of incoming data before writing it into a buffer, an attacker can send more data than expected, overwriting other important parts of the program’s memory.

Fixing a Simple Buffer Overflow

1. Identify the Vulnerable Code: Let’s assume you have C code like this (this is a simplified example):

   #include <stdio.h>
   #include <string.h>
   
   int main() {
     char buffer[10];
     strcpy(buffer, "This is a long string!"); // Vulnerable line
     printf("%sn", buffer);
     return 0;
   }
   

   The problem is the use of strcpy. It copies data without checking if the source string is larger than the destination buffer.

2. Compile with Warnings: Use compiler flags to detect potential issues. For GCC, try:

   gcc -Wall -Wextra -o overflow_example overflow_example.c
   

   This will often highlight strcpy as a dangerous function.

3. Use Safer Alternatives: Replace strcpy with functions that limit the number of bytes copied:
   • strncpy: Copies a maximum number of characters. Be careful, it might not null-terminate the string if the source is longer than the buffer size.

     strncpy(buffer, "This is a long string!", sizeof(buffer) - 1);
     buffer[sizeof(buffer) - 1] = ''; // Ensure null termination
     

   • snprintf: Formats and writes to a buffer with size limits.

     snprintf(buffer, sizeof(buffer), "%s", "This is a long string!");
     

4. Input Validation (Important for User Input): If the data comes from user input, always check its length before copying it into a buffer. For example:

   #include <stdio.h>
   #include <string.h>
   
   int main() {
     char buffer[10];
     char userInput[20];
   
     printf("Enter some text: ");
     fgets(userInput, sizeof(userInput), stdin); // Safer input function
   
     // Remove trailing newline if present
     size_t len = strlen(userInput);
     if (len > 0 && userInput[len-1] == 'n') {
       userInput[len-1] = '';
     }
   
     if (strlen(userInput) < sizeof(buffer)) {
       strcpy(buffer, userInput); // Now safe because of the check
       printf("%sn", buffer);
     } else {
       printf("Input too long!n");
     }
   
     return 0;
   }
   

5. Test Your Fix: Try providing input longer than the buffer size to confirm that your program no longer crashes or behaves unexpectedly. Consider using fuzzing tools (automated testing with random inputs) for more thorough testing.

Further Security Considerations

Buffer overflows are a serious cyber security risk. Always prioritize safe coding practices, especially when dealing with user input. Modern compilers and operating systems often have features to help mitigate buffer overflow attacks (like Address Space Layout Randomization – ASLR and Data Execution Prevention – DEP), but relying on these alone isn’t enough – you need secure code.