Buffer Overflow: Pre-Return Address Payloads

TL;DR

Stack clearing doesn’t always prevent pre-return address payloads from working in buffer overflows. This is because the return address is often overwritten *before* other stack data, and even if the stack is cleared later, the corrupted return address remains valid until it’s used during function exit.

Understanding the Stack

Before diving into payloads, let’s quickly recap how the stack works. When a function is called:

• Local variables are allocated on the stack.
• Arguments are pushed onto the stack.
• The return address (where to go after the function finishes) is pushed onto the stack.

The stack grows downwards in memory.

Buffer Overflows and Return Address Control

A buffer overflow happens when you write more data into a buffer than it can hold. If this overwrites the return address, you can redirect execution to an arbitrary location – typically malicious code.

Why Stack Clearing Doesn’t Always Help

Many modern compilers and operating systems attempt to mitigate buffer overflows by clearing the stack before a function returns. However, this doesn’t always prevent exploitation. Here’s why:

1. Timing Matters

The return address is usually overwritten *before* other parts of the stack are filled with data. Stack clearing happens after the buffer overflow has already occurred and the return address has been modified.

2. Return Address Persistence

Once the return address is corrupted, it remains corrupted until the function actually returns. Stack clearing doesn’t undo this change. The corrupted value is still there, waiting to be used when the function tries to go back to its caller.

3. Limited Clearing Scope

Stack clearing often only affects local variables and arguments. It typically doesn’t clear the area where the return address resides (the stack frame).

Pre-Return Address Payloads: A Step-by-Step Example

Let’s illustrate with a simplified C example:

#include 
#include 

void vulnerable_function(char *input) {
  char buffer[16];
  strcpy(buffer, input);
}

int main() {
  char long_string[] = "AAAAAAAAAAAAAAAAAAAAAAAAAAAA"; // Longer than 16 bytes
  vulnerable_function(long_string);
  printf("Function returned successfully.n");
  return 0;
}

1. The Overflow: The `strcpy` function copies the contents of `long_string` into `buffer`. Because `long_string` is longer than 16 bytes, it overflows the buffer.
2. Return Address Overwrite: This overflow overwrites data on the stack beyond `buffer`, including the return address.
3. Stack Clearing (Hypothetical): Imagine the compiler inserts code to clear the stack after the `strcpy` call but before returning from `vulnerable_function`.
4. The Problem: The corrupted return address is already set *before* the clearing happens. Stack clearing won’t restore it. When `vulnerable_function` returns, execution jumps to the address you overwrote.

Exploitation Scenario

To exploit this, you would craft a payload that includes:

• Padding to fill the buffer.
• The desired return address (e.g., an address containing shellcode or a useful function like `system`).

Mitigation Techniques

Several techniques can help prevent these attacks:

• Use Safe Functions: Replace unsafe functions like strcpy with safer alternatives like strncpy, which limit the number of bytes copied.
• Stack Canaries: A random value is placed on the stack before the return address. If this canary is modified during an overflow, it indicates a problem and terminates the program.
• Address Space Layout Randomization (ASLR): Randomizes the base addresses of libraries and other memory regions, making it harder to predict where shellcode or useful functions are located.
• Data Execution Prevention (DEP) / NX Bit: Marks certain memory regions as non-executable, preventing shellcode from running in those areas.

Conclusion

Stack clearing is a helpful defense, but it’s not foolproof. Understanding the timing of stack operations and how return addresses are overwritten is crucial for both exploiting and defending against buffer overflow attacks.