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Double Free Vulnerability: Code Execution Risk

G5 Cyber Security

TL;DR

Deleting an object twice (a “double free”) can corrupt memory and potentially allow attackers to execute arbitrary code. This happens because the second deletion can overwrite important data structures, leading to unpredictable behaviour when other objects are allocated or freed.

Understanding Double Free Vulnerabilities

When you delete an object in a programming language like C++, Java, or Python, memory is returned to the system. A double free occurs when you try to delete the same memory location twice. This isn’t usually caught by the language itself (though some debuggers and tools can help).

How Double Free Exploitation Works

  1. Initial Allocation: An object is created in memory.
  2. First Deallocation: The object’s memory is freed. The system marks this memory as available for reuse, but doesn’t immediately wipe it clean.
  3. Second Deallocation: This is where the problem starts. Attempting to free the same memory again corrupts the heap metadata (the data structures that manage memory).
  4. Heap Corruption: The heap’s internal lists and pointers get overwritten, leading to inconsistencies.
  5. Controlled Allocation: An attacker can then allocate new objects in a way that exploits this corruption. They might be able to control where the new object is placed in memory.
  6. Code Execution: By carefully crafting the contents of these allocated objects (e.g., overwriting function pointers), an attacker can redirect program execution to their own malicious code.

Example Scenario (Simplified C++)

This example demonstrates the concept, but real-world exploitation is much more complex and depends on the specific memory allocator used by the system.

#include <iostream>

int main() {
  int *ptr = new int(10);
  std::cout << "Initial allocation: " << ptr << std::endl;

  delete ptr;
  std::cout << "First deallocation: " << ptr << std::endl;

  delete ptr; // Double free!
  std::cout << "Second deallocation: " << ptr << std::endl;

  // Further memory operations after the double free are likely to crash or be exploited.
  return 0;
}

In this simplified example, the second `delete ptr` will cause a crash or unpredictable behaviour. In a more sophisticated attack, an attacker would try to allocate new objects immediately after the double free and manipulate them.

Steps to Prevent Double Free Vulnerabilities

  1. Zeroing Pointers After Deletion: Immediately set pointers to `nullptr` (or equivalent) after freeing the memory they point to. This prevents accidental re-use of dangling pointers. 
    delete ptr;
ptr = nullptr; // Important!
  2. Smart Pointers: Use smart pointers (e.g., `std::unique_ptr`, `std::shared_ptr` in C++) to automatically manage memory and prevent manual deallocation errors. 
    #include <memory>

std::unique_ptr<int> ptr = std::make_unique<int>(10);
// Memory is automatically freed when ptr goes out of scope.
  3. Object Ownership: Clearly define object ownership. Who is responsible for deleting an object? Avoid multiple parts of your code trying to delete the same object.
  4. Heap Protections: Enable heap protections provided by your operating system and compiler (e.g., AddressSanitizer, Heap Integrity Checks). These can detect double frees at runtime.
  5. Code Reviews & Static Analysis: Thoroughly review your code for potential memory management errors. Use static analysis tools to identify possible vulnerabilities before deployment.
  6. Fuzzing: Employ fuzz testing techniques to automatically generate inputs that might trigger double free conditions and other memory corruption bugs.

cyber security Implications

Double-free vulnerabilities are a serious cyber security risk. Successful exploitation can lead to:

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