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Heap Overflow Stack Corruption

G5 Cyber Security

TL;DR

Yes, a heap buffer overflow can overwrite the stack, but it’s not guaranteed and depends on several factors including memory layout, compiler optimizations, operating system protections (like ASLR), and the relative positions of the heap and stack in virtual memory. Exploiting this requires careful planning and often relies on predictable memory arrangements.

Understanding the Problem

The heap is used for dynamic memory allocation (e.g., using malloc in C). The stack, on the other hand, stores local variables, function call information, and return addresses. A buffer overflow occurs when you write data beyond the allocated boundaries of a buffer.

How Stack Corruption Can Happen

  1. Memory Layout: If the heap grows towards the stack (a common arrangement), an overflow can directly overwrite stack data.
  2. Stack Metadata: The stack often contains important metadata like saved frame pointers and return addresses. Overwriting these is a classic exploitation technique.
  3. Function Pointers: If function pointers are stored on the stack, overwriting them allows control of program execution.

Steps to Test for Stack Corruption

  1. Allocate Heap Memory: Use malloc (or similar) to allocate a buffer on the heap. 
    char *buffer = malloc(10);
  2. Overflow the Buffer: Write more data into the buffer than it can hold. A simple example in C: 
    strcpy(buffer, "AAAAAAAAAAAAAAAAAAAAAAA"); // Overflow!
  3. Check Stack Contents: After the overflow, examine the stack to see if it has been modified. This is usually done with a debugger (like GDB).
    • GDB Example: Set a breakpoint after the strcpy call and inspect the stack using commands like x/20wx $sp (examine 20 words starting from the stack pointer).
  4. Look for Return Address Changes: Specifically, check if the return address of the current function has been overwritten. This is a strong indicator of successful stack corruption.
    • GDB Example: x/wx $sp to view the return address on the stack.

Factors Affecting Success

  1. Address Space Layout Randomization (ASLR): ASLR randomizes the base addresses of the heap and stack, making it harder to predict their locations. Disable ASLR for testing purposes (but never in production!).
  • Compiler Optimizations: Compilers can reorder code and optimize memory access, potentially changing the layout. Compile with minimal optimizations (e.g., -O0).
  • Stack Canaries: Stack canaries are values placed on the stack to detect overflows. They will cause a program crash if overwritten. Disable them during testing.
    • GCC/Clang: Use the -fno-stack-protector flag.
  • Heap Protections: Modern heap implementations often include protections to prevent overflows from easily corrupting other memory regions.

    • Example Scenario (Simplified)

    Imagine a simple program where the stack contains a local variable and a return address, and the heap is allocated immediately before it.

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

int main() {
  char *heap_buffer = malloc(16);
  char stack_variable[8];
  strcpy(heap_buffer, "AAAAAAAAAAAAAAAAAAAAAAA"); // Overflow!
  printf("Stack variable: %sn", stack_variable); // May crash or print garbage.
  return 0;
}

    If the overflow is large enough and ASLR isn’t enabled, it could overwrite stack_variable or the return address.

    Mitigation

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