CWE-469: Use of Pointer Subtraction to Determine Size

What is Use of Pointer Subtraction to Determine Size?

• Overview: The CWE-469 vulnerability arises when a program incorrectly attempts to calculate the size of a data structure by subtracting two pointers, which may not belong to the same memory block or array, leading to incorrect size calculations and potential undefined behavior.

• Exploitation Methods:

• Attackers can exploit this vulnerability by manipulating memory layouts, causing the program to calculate incorrect sizes and potentially access out-of-bounds memory.
• Common attack patterns include buffer overflows and memory corruption by diverting pointer arithmetic to unauthorized memory regions.

• Security Impact:

• Direct consequences of successful exploitation include incorrect program behavior, crashes, and potential disclosure of sensitive information.
• Potential cascading effects can involve memory corruption, leading to more severe vulnerabilities such as code execution or denial of service.
• Business impact can include compromised data integrity, loss of customer trust, and increased operational costs due to breach recovery efforts.

• Prevention Guidelines:

• Specific code-level fixes involve ensuring that pointer arithmetic is only performed within the bounds of the same array or allocated memory block.
• Security best practices include rigorous bounds checking and using safer alternatives for memory management, like std::vector in C++.
• Recommended tools and frameworks include static analysis tools to detect potential pointer arithmetic issues and the use of memory-safe languages or libraries wherever possible.

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Technical Details

Likelihood of Exploit: Medium

Affected Languages: C, C++

Affected Technologies: Not specified

Vulnerable Code Example

// Vulnerable code demonstrating CWE-469 (Use of Pointer Subtraction to Determine Size)
#include <stdio.h>
#include <string.h>
#include <stdlib.h>

void process_data(char *buffer1, char *buffer2) {
    // Assume buffer1 and buffer2 are supposed to be part of the same array
    size_t size = buffer2 - buffer1; // Incorrect pointer arithmetic
    char *data = (char *)malloc(size * sizeof(char)); // Allocate memory based on incorrect size

    if (data) {
        strncpy(data, buffer1, size); // Copy data based on incorrect size
        printf("Data: %s\n", data);   // Potentially undefined behavior
        free(data);
    }
}

int main() {
    char buffer1[] = "Hello, World!";
    char buffer2[] = "Welcome!";
    process_data(buffer1, buffer2); // Pass two unrelated buffers
    return 0;
}

Explanation:

• Vulnerability: The code calculates the size between buffer1 and buffer2, which are not part of the same array. This results in undefined behavior because pointer arithmetic is only valid within the same array.
• Impact: Using pointer subtraction across different arrays can lead to incorrect memory operations, potentially causing memory corruption or access violations.

How to fix Use of Pointer Subtraction to Determine Size?

To fix this vulnerability:

1. Ensure pointers belong to the same array: Pointer arithmetic should only be performed within a single array.
2. Use valid bounds checking: Use functions like strlen to safely determine the length of strings.
3. Avoid assumptions: Do not assume different arrays are contiguous in memory.

Fixed Code Example

// Fixed code demonstrating proper handling of array sizes
#include <stdio.h>
#include <string.h>
#include <stdlib.h>

void process_data(char *buffer) {
    size_t size = strlen(buffer); // Correctly determine the length of the string
    char *data = (char *)malloc((size + 1) * sizeof(char)); // Allocate space for null-terminator

    if (data) {
        strncpy(data, buffer, size + 1); // Use size + 1 to include null-terminator
        printf("Data: %s\n", data);      // Safe memory operations
        free(data);
    }
}

int main() {
    char buffer[] = "Hello, World!";
    process_data(buffer); // Pass a single buffer to process_data
    return 0;
}

Explanation:

• Fix: The code now uses strlen to determine the length of a single buffer, avoiding unsafe pointer arithmetic.
• Improvement: This ensures memory operations are safe and within bounds, preventing undefined behavior.
• Best Practice: Always verify array boundaries when performing pointer arithmetic and use standard library functions for size determination when possible.