Blog | G5 Cyber Security

Fix Buffer Overreads

G5 Cyber Security

TL;DR

Buffer overreads happen when your program tries to read data from memory beyond the allocated size of a buffer. This can lead to crashes, unexpected behaviour, or even security vulnerabilities. We’ll cover how to identify and fix them.

What causes Buffer Overreads?

A buffer overread occurs when code attempts to access data outside the boundaries of an allocated memory region (the ‘buffer’). Common causes include:

How to Identify Buffer Overreads

Identifying these issues can be tricky, but here are some techniques:

  1. Code Review: Carefully examine code that handles buffers, especially loops and size calculations.
  2. Static Analysis Tools: Use tools like Coverity, SonarQube or cppcheck to automatically detect potential overreads during development.
  3. Dynamic Analysis (Debugging): Run your program in a debugger (like GDB) and watch memory access patterns. Breakpoints can be set on read operations to check buffer boundaries.
  4. Fuzzing: Provide unexpected or very large inputs to the program to trigger overreads. Tools like AFL are useful for this.

Fixing Buffer Overreads

Here’s a step-by-step guide to fixing buffer overreads:

  1. Verify Buffer Sizes: Double-check all calculations involving buffer sizes. Ensure they are correct and account for null terminators if needed (e.g., strings).
  2. Check Loop Conditions: Make sure loops don’t iterate beyond the allocated buffer size. Use appropriate loop conditions.
  3. Use Safe Functions: Replace unsafe functions with safer alternatives that perform bounds checking.

Example Fix (C)

Let’s look at a simple example in C and how to fix it:

Unsafe Code

#include <stdio.h>
#include <string.h>

int main() {
  char buffer[10];
  char input[] = "This is a long string";

  strcpy(buffer, input); // Potential buffer overread!
  printf("%sn", buffer);
  return 0;
}

In this example, strcpy doesn’t check the size of input before copying it into buffer. If input is larger than 9 characters (plus null terminator), a buffer overread will occur.

Safe Code

#include <stdio.h>
#include <string.h>

int main() {
  char buffer[10];
  char input[] = "This is a long string";

  strncpy(buffer, input, sizeof(buffer) - 1); // Safer: limits the copy size
buffer[sizeof(buffer) - 1] = ' '; // Ensure null termination
  printf("%sn", buffer);
  return 0;
}

Here, we use strncpy which takes a maximum length argument. We also explicitly add the null terminator to ensure the string is properly terminated.

Further Considerations

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