CWE 416 - Use After Free

Disclaimer

  • The code provided is for **educational purposes** only and should not be used in a **production environment** without proper review and testing.

  • The provided code should be regarded as **best practice**. There are also several ways to remediate the Vulnerabilities.

  • The code provided **may contain vulnerabilities** that could be exploited by attackers, and is intended to be used as a learning tool to help improve security awareness and best practices.

  • The mitigations provided are intended to address **specific vulnerabilities** in the code, but may not be effective against all potential attack vectors or scenarios.

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About CWE ID 416

Use After Free

The Vulnerability occurs when the program continues to reference memory after it has been freed or deallocated, which can result in unpredictable behavior or a crash.

Impact

  • Arbitrary code Execution

  • Access to Unauthorized Sensitive Data

  • Privilege Escalation

  • RCE/Sandbox Escape

Example with Code Explanation

C

  • Let us consider an example case and understand the CWE 416 with context of Vulnerable code and Mitigated code.

Vulnerable Code

#include <stdlib.h>

int main() {
  int* ptr = (int*) malloc(sizeof(int)); // allocate memory
  *ptr = 42; // set the value of the memory

  free(ptr); // free the memory

  // use the pointer after the memory has been freed
  int result = *ptr; // This is a use after free vulnerability!

  return 0;
}

In this code, the malloc() function is used to allocate memory for an integer, which is then assigned the value of 42. The free() function is then called to deallocate the memory. However, the code then attempts to access the memory pointed to by ptr after it has been freed, which can result in undefined behavior or a crash. This is the Use After Free vulnerability.

  • Some of the ways the Vulnerable code can be mitigated is:

    • Nullifying the pointer after freeing the chunk. After freeing a chunk, set the pointer to NULL to avoid accidental dereferencing of the pointer. This can prevent the Use After Free vulnerability in some cases, but not in all cases.

    • Using a memory management library like jemalloc, tcmalloc, or others that have built-in protections against Use After Free vulnerabilities.

    • Using dynamic memory allocation may help mitigate CWE-416 in some cases, but it is not a guarantee. Dynamic memory allocation still requires proper management of pointers and memory allocation/deallocation functions such as malloc(), calloc(), free() and realloc(). If these functions are used incorrectly or inconsistently, they can still cause use-after-free errors or other memory-related problems.

Mitigated Code

#include <stdlib.h>

int main() {
  int* ptr = NULL; // initialize the pointer to NULL
  ptr = (int*) malloc(sizeof(int)); // allocate memory
  if (ptr == NULL) {
    // handle error, such as by exiting the program
    return 1;
  }
  *ptr = 42; // set the value of the memory

  free(ptr); // free the memory
  ptr = NULL; // set the pointer to NULL

  // avoid using the pointer after it has been freed

  return 0;
}

The Mitigated Code does the following:

  • The pointer ptr is set to NULL after freeing the memory, which prevents the possibility of a use after free vulnerability.

  • The free function is called immediately after using the allocated memory, which reduces the potential window of vulnerability.

C++

Vulnerable Code

#include <iostream>

int main()
{
    int* ptr = new int;  // allocate memory
    *ptr = 42;           // set value
    std::cout << *ptr << std::endl;

    delete ptr;          // free memory

    // use-after-free vulnerability:
    // attempting to access freed memory
    std::cout << *ptr << std::endl;

    return 0;
}

In this program, a new integer is allocated and assigned a value of 42. The value is then printed to the console. Next, the memory allocated to the integer is freed using the delete operator. The program then attempts to access the freed memory by printing the value of the integer again. This can result in undefined behavior or a crash.

  • Some of the ways the Vulnerable code can be mitigated is:

    • Avoid using raw pointers. Instead of using raw pointers, consider using smart pointers, such as std::unique_ptr or std::shared_ptr, which automatically manage memory and help prevent use-after-free vulnerabilities.

    • Nullify pointers after deallocation. Always nullify pointers after freeing the memory they point to. This helps prevent accidental use of the freed memory and can help catch use-after-free vulnerabilities.

    • Use RAII (Resource Acquisition Is Initialization). RAII is a programming technique in which resource allocation and deallocation are tied to object lifetimes. By using RAII, you can ensure that resources are properly cleaned up when objects go out of scope, which helps prevent use-after-free vulnerabilities.

Mitigated Code

#include <iostream>
#include <memory>

int main()
{
    std::unique_ptr<int> ptr(new int); // allocate memory with unique_ptr
    *ptr = 42;                         // set value
    std::cout << *ptr << std::endl;

    // no need to free memory, unique_ptr does it automatically

    // attempting to access freed memory is not possible because unique_ptr
    // automatically sets the pointer to null
    std::cout << ptr.get() << std::endl;

    return 0;
}

  • The Mitigated Code does the following:

    • •Smart pointers, such as std::unique_ptr and std::shared_ptr, are C++ language features that help manage memory by automatically deallocating memory when it is no longer needed.

    • Smart pointers mitigate use-after-free vulnerabilities by ensuring that memory is properly deallocated before it is accessed again.

    • Smart pointers prevent use-after-free vulnerabilities by nullifying the pointer when the memory is deallocated, which makes it impossible to access the freed memory using the pointer.

💡 RAII Implementation can also be used to mitigate CWE-416.

JAVA

Vulnerable Code

public class UseAfterFreeExample {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        list.add("foo");
        list.add("bar");

        // Free memory by setting list to null
        list = null;

        // Accessing freed memory
        System.out.println(list.get(0));
    }
}

In this code, a list of strings is created and populated with two elements using the add() method. The memory allocated for the list is freed by setting it to null. However, the code then attempts to access the freed memory by calling the get() method on the null reference. This can result in undefined behavior, including a runtime exception or unexpected behavior.

  • Some of the ways the Vulnerable code can be mitigated is:

    • Avoid manual memory management. Instead of manually allocating and deallocating memory, use Java's garbage collector to manage memory automatically.

    • Before dereferencing a pointer, check if it is null to prevent accessing freed memory.

    • Use safe memory management techniques: Use Java's WeakReference, SoftReference, or PhantomReference classes to manage memory safely. These classes provide a way to create references to objects that can be garbage collected when they are no longer needed.

Mitigated Code

import java.util.ArrayList;
import java.util.List;

public class MitigatedExample {

    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        list.add("Hello");
        list.add("World");
        
        // use try-with-resources to ensure that the list is closed after use
        try (ResourceWrapper wrapper = new ResourceWrapper(list)) {
            System.out.println(wrapper.getList().get(0));
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

class ResourceWrapper implements AutoCloseable {
    private List<String> list;
    
    public ResourceWrapper(List<String> list) {
        this.list = list;
    }
    
    public List<String> getList() {
        return list;
    }
    
    @Override
    public void close() throws Exception {
        // close any resources that need to be released
        list = null; // set list to null to prevent further use
    }
}

  • The Mitigated code does the following:

    • The code uses a ResourceWrapper class that implements the AutoCloseable interface to wrap the List object and ensure that it is properly released after use.

    • The ResourceWrapper class exposes the List object through a public getter method, but sets the reference to null in its close() method to prevent further use of the object after it has been released.

    • The main() method uses the try-with-resources statement to automatically close the ResourceWrapper object after use and ensure that the List object is properly released.

References

LogoCWE - CWE-416: Use After Free (4.18)cwe.mitre.org

Using freed memory | OWASP Foundation

MEM30-C. Do not access freed memory - SEI CERT C Coding Standard - Confluence

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