CWE-416: Use After Free

What is Use After Free?

• Overview: Use After Free (CWE-416) occurs when a program continues to use memory after it has been freed, leading to undefined behavior. In languages like C and C++, once memory is freed, the program should not attempt to access it, as it may have been reallocated for other purposes.

• Exploitation Methods:

Attackers can exploit this vulnerability by manipulating the program to access the freed memory space, potentially leading to arbitrary code execution.
Common attack patterns include overwriting sensitive data in the freed memory or causing the program to crash by accessing invalid memory.

• Security Impact:

Direct consequences of successful exploitation include unauthorized access to sensitive information, program crashes, or execution of malicious code.
Potential cascading effects involve system instability or the compromise of other software components.
Business impact can range from data breaches to financial loss and damage to the organization's reputation.

• Prevention Guidelines:

Specific code-level fixes include setting pointers to NULL immediately after freeing memory to avoid accidental reuse.
Security best practices involve rigorous code reviews, static analysis, and dynamic testing to identify and mitigate use-after-free vulnerabilities.
Recommended tools and frameworks include AddressSanitizer for detecting memory misuse and adopting modern C++ practices that utilize smart pointers to manage memory safely.

Corgea can automatically detect and fix Use After Free in your codebase. [Try Corgea free today](https://corgea.app).

Technical Details

Likelihood of Exploit: High

Affected Languages: C, C++

Affected Technologies: Not specified

Vulnerable Code Example

// This code demonstrates a Use After Free vulnerability.
// The memory allocated for `data` is freed, but the pointer is still used afterward,
// potentially leading to undefined behavior or security vulnerabilities.

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

void process_data(int *data) {
    // Process the data
    printf("Processing data: %d\n", *data);
}

int main() {
    int *data = (int *)malloc(sizeof(int)); // Allocate memory for an integer
    if (data == NULL) {
        perror("Failed to allocate memory");
        return EXIT_FAILURE;
    }

    *data = 42; // Assign a value to the allocated memory
    free(data); // Free the allocated memory

    process_data(data); // Use after free: accessing freed memory

    return 0;
}

How to fix Use After Free?

To fix the Use After Free vulnerability, ensure that pointers are not used after the memory they point to has been freed. This can be achieved by setting the pointer to NULL immediately after freeing it. This way, any subsequent access attempts will result in a null pointer dereference, which is easier to detect and debug. Additionally, ensure that any code that might use the pointer checks for NULL before accessing it.

Set the pointer to NULL after freeing: This prevents accidental use of the pointer, as accessing NULL is typically caught by the operating system.
Check for NULL before using the pointer: This ensures that you do not attempt to access memory that has been freed.

Fixed Code Example

// This code fixes the Use After Free vulnerability by setting the pointer to `NULL`
// after freeing it and checking for `NULL` before using the pointer.

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

void process_data(int *data) {
    if (data != NULL) { // Check for NULL before accessing data
        printf("Processing data: %d\n", *data);
    } else {
        printf("Data is NULL, cannot process.\n");
    }
}

int main() {
    int *data = (int *)malloc(sizeof(int)); // Allocate memory for an integer
    if (data == NULL) {
        perror("Failed to allocate memory");
        return EXIT_FAILURE;
    }

    *data = 42; // Assign a value to the allocated memory
    free(data); // Free the allocated memory
    data = NULL; // Set the pointer to NULL after freeing

    process_data(data); // Safe: process_data checks for NULL

    return 0;
}

This fixed code ensures that accessing freed memory is avoided by setting the pointer to NULL after freeing it and checking for NULL before any further use. This approach helps prevent undefined behavior and potential security vulnerabilities.