Buffer Overflow: A Beginner’s Guide

TL;DR

A buffer overflow happens when a program tries to put more data into a storage area (a ‘buffer’) than it can hold. This can overwrite other important information, potentially letting attackers take control of your computer. We’ll show you how this works and some basic ways to prevent it.

What is a Buffer Overflow?

Imagine you have a box that can only fit 10 sweets. If someone tries to put 15 sweets in, they’ll overflow out of the box. A buffer overflow is similar – a program has a limited space for data, and if it receives more than it expects, the extra data spills over into other parts of memory.

How Does It Happen?

1. Fixed-Size Buffers: Many older programs use fixed-size buffers to store input. For example:

   char buffer[10]; // A buffer that can hold 9 characters + null terminator

2. Unchecked Input: If the program doesn’t check how much data it’s receiving before copying it into the buffer, a malicious user can send more data than the buffer can handle.

   strcpy(buffer, userInput); // Dangerous! No length check.

3. Overwriting Memory: The extra data overwrites adjacent memory locations. This could overwrite other variables, program code, or even important system information.

Example Scenario (C)

Let’s look at a simple C 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 than strcpy, but still needs care.

  strcpy(buffer, userInput); // Vulnerable! No check on input length.

  printf("You entered: %sn", buffer);
  return 0;
}

If you enter more than 9 characters in the ‘userInput’ field, the `strcpy` function will write past the end of the `buffer`, causing a buffer overflow.

How to Prevent Buffer Overflows

1. Use Safe Functions: Avoid functions like strcpy, strcat, and sprintf. Instead, use safer alternatives:
   • strncpy: Copies a limited number of characters.

     strncpy(buffer, userInput, sizeof(buffer) - 1); // Safer, but still needs null termination check!

   • strncat: Appends a limited number of characters.
   • snprintf: Formats output with a maximum length.

     snprintf(buffer, sizeof(buffer), "%s", userInput); // Best for formatted output.

2. Check Input Length: Always validate the size of input before copying it into a buffer. Make sure the input is smaller than the buffer’s capacity.

   if (strlen(userInput) < sizeof(buffer)) {
     strcpy(buffer, userInput);
   } else {
     printf("Input too long!n");
   }
   

3. Use Compiler and Operating System Protections:
   • Stack Canaries: These are values placed on the stack to detect if a buffer overflow has occurred. If overwritten, the program will terminate.
   • Address Space Layout Randomization (ASLR): This randomizes the memory addresses of key program components, making it harder for attackers to predict where to overwrite data.
   • Data Execution Prevention (DEP) / NX Bit: Prevents code from being executed in areas of memory that are intended for data storage.

Further Learning

Buffer overflows can be complex, and there’s a lot more to learn. Here are some resources:

• OWASP: https://owasp.org/www-project-top-ten/
• SANS Institute: https://www.sans.org/