CWE-825: Expired Pointer Dereference

What is Expired Pointer Dereference?

• Overview: Expired Pointer Dereference occurs when a program tries to access memory through a pointer that points to a location that was valid but has since been deallocated or repurposed. This typically happens in C and C++ when memory is freed, yet pointers to that memory are not updated or nullified.

• Exploitation Methods:

• Attackers can exploit this vulnerability by manipulating program execution to dereference expired pointers, potentially accessing or corrupting memory.
• Common attack patterns include reusing freed memory, which can lead to arbitrary code execution or unauthorized data access.

• Security Impact:

• Direct consequences include application crashes, unintended behavior, or memory corruption.
• Potential cascading effects might involve security breaches, data leaks, or system instability.
• Business impact could involve loss of customer trust, regulatory penalties, or financial damages due to data breaches or service outages.

• Prevention Guidelines:

• Specific code-level fixes include setting pointers to NULL after freeing memory and always checking pointer validity before dereferencing.
• Security best practices involve using smart pointers or memory management libraries that handle memory lifecycle automatically.
• Recommended tools and frameworks include static analysis tools to detect pointer issues and runtime analysis tools to monitor memory usage.

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Technical Details

Likelihood of Exploit: Not specified

Affected Languages: C, C++

Affected Technologies: Not specified

Vulnerable Code Example

// Vulnerable code demonstrating CWE-825: Expired Pointer Dereference
#include <stdio.h>
#include <stdlib.h>

void processArray() {
    int *array = (int *)malloc(5 * sizeof(int)); // Allocate memory for an array of 5 integers
    if (array == NULL) {
        fprintf(stderr, "Memory allocation failed\n");
        return;
    }

    for (int i = 0; i < 5; ++i) {
        array[i] = i * 2; // Initialize array with some values
    }
    
    free(array); // Memory is freed, pointer is now expired

    // Attempting to use the expired pointer results in undefined behavior
    printf("First element: %d\n", array[0]); // Dereferencing an expired pointer
}

int main() {
    processArray();
    return 0;
}

How to fix Expired Pointer Dereference?

To fix the expired pointer dereference issue, ensure you do not access or dereference pointers after they have been freed. When memory is freed, the pointer becomes invalid, and any access to it is undefined behavior. Here are some best practices to prevent this:

1. Set the pointer to NULL after freeing it: This ensures that any subsequent dereference attempts will result in a null pointer dereference, which is often easier to detect and debug.

2. Avoid using freed memory: Once memory is freed, do not use the pointer without re-allocating and re-initializing it.

3. Use static analysis tools: These tools can help detect and warn about potential dereferences of expired pointers.

Fixed Code Example

// Fixed code implementing security controls for expired pointer dereference
#include <stdio.h>
#include <stdlib.h>

void processArray() {
    int *array = (int *)malloc(5 * sizeof(int)); // Allocate memory for an array of 5 integers
    if (array == NULL) {
        fprintf(stderr, "Memory allocation failed\n");
        return;
    }

    for (int i = 0; i < 5; ++i) {
        array[i] = i * 2; // Initialize array with some values
    }
    
    free(array); // Free the allocated memory
    array = NULL; // Set pointer to NULL after freeing to avoid accidental dereference

    // Safely handle the pointer after freeing
    if (array != NULL) {
        printf("First element: %d\n", array[0]); // This block will not be executed
    } else {
        printf("Pointer is NULL, cannot dereference.\n"); // Safe handling of expired pointer
    }
}

int main() {
    processArray();
    return 0;
}

In the fixed code, after freeing the allocated memory, the pointer array is set to NULL. Then, before attempting to dereference the pointer, a check is performed to ensure it is not NULL. This prevents the program from dereferencing an expired pointer, which could otherwise lead to undefined behavior.