Prevent Zeroed Stack Addresses

TL;DR

Stack addresses containing zeros can be exploited in certain cyber security attacks. This guide shows how to modify your build process and code to avoid generating these vulnerable addresses, improving the robustness of your application.

Solution Guide

1. Understand the Problem
   • Stack addresses are used to store local variables and function call information.
   • If an address contains all zeros (e.g., 0x0), it can be easily predicted by attackers.
   • Attackers might use this predictability to overwrite return addresses or other critical data on the stack, leading to control of your program.
2. Compiler and Linker Options

   Modern compilers often have options to help mitigate this issue. The specific flags vary depending on your compiler.

   • GCC/Clang: Use Address Space Layout Randomization (ASLR). ASLR randomizes the base address of various memory regions, including the stack. This makes it harder for attackers to predict addresses.

     gcc -fstack-protector-all -D_FORTIFY_SOURCE=2 -pie -Wformat -Wformat-security your_code.c -o your_program

   • MSVC: Enable ASLR and Stack Canaries.

     cl /GS /DYNAMICBASE your_code.cpp

3. Code Modifications (if necessary)

   In some cases, you might need to adjust your code to avoid explicitly creating zeroed addresses.

   • Avoid Large Static Arrays on the Stack: Declaring very large static arrays within functions allocates them on the stack. This can lead to predictable address patterns.

     // Bad (potentially creates a large, predictable stack allocation)

     void myFunction() {
       char buffer[1024 * 1024]; // Large array
       ...}
     

     Instead, allocate memory dynamically using malloc or similar functions.

     // Good (allocates memory on the heap)
     #include <stdlib.h>
     void myFunction() {
       char *buffer = (char *)malloc(1024 * 1024);
       if (buffer == NULL) { /* Handle allocation error */ }
       ...
       free(buffer); // Remember to free the memory!
     }

   • Be Careful with Stack-Based Structures: Similar to arrays, large structures allocated on the stack can create predictable patterns. Consider using heap allocation for these as well.
4. Stack Protector (GCC/Clang)

   The -fstack-protector flag adds a ‘canary’ value to the stack before function returns. If this canary is modified during execution, it indicates a potential stack buffer overflow and terminates the program.

   • -fstack-protector: Enables basic stack protection.
   • -fstack-protector-all: Enables more aggressive stack protection for all functions (recommended).
   • -fstack-protector-strong: Enables stack protection for functions with character arrays larger than a certain size.
5. Testing and Validation

   Thoroughly test your application to ensure that the changes have not introduced any new issues.

   • Fuzzing: Use fuzz testing tools to automatically generate a large number of inputs and check for crashes or unexpected behavior.
   • Static Analysis: Employ static analysis tools to identify potential vulnerabilities in your code.
   • Dynamic Analysis: Use debuggers and memory profilers to monitor stack usage during runtime.