CWE-562: Return of Stack Variable Address

What is Return of Stack Variable Address?

• Overview: This vulnerability occurs when a function returns the address of a local stack variable. This is problematic because local variables are stored on the stack, and their memory is reclaimed after the function exits. Using such an address leads to undefined behavior, often resulting in crashes when the memory is reused by other parts of the program.

• Exploitation Methods:

• Attackers can exploit this vulnerability by tricking the program into operating on invalid memory, potentially causing it to crash or behave unpredictably.
• Common attack patterns include crafting inputs that force the program to return stack addresses and then leveraging subsequent function calls to overwrite those addresses.

• Security Impact:

• Direct consequences include program crashes and unpredictable behavior.
• Potential cascading effects involve data corruption and security breaches if sensitive information is mishandled.
• Business impact may include service downtime, data loss, and loss of customer trust.

• Prevention Guidelines:

• Avoid returning pointers to local stack variables. Instead, use dynamic memory allocation for variables that need to persist beyond the function scope.
• Adopt security best practices like input validation and memory management discipline.
• Recommended tools and frameworks include static analysis tools to detect potential vulnerabilities and adopting modern C++ features that manage resources more safely, such as smart pointers.

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Technical Details

Likelihood of Exploit: Not specified

Affected Languages: C, C++

Affected Technologies: Not specified

Vulnerable Code Example

C Example

// Vulnerable function that returns the address of a local stack variable
char* getStackBuffer() {
    char buffer[256];  // Local buffer allocated on the stack
    strcpy(buffer, "This is a string stored in a stack-allocated buffer."); // Copying a string into buffer
    return buffer;  // Returning the address of the stack-allocated buffer
}

int main() {
    char* string = getStackBuffer();  // Attempt to use the returned buffer
    printf("String: %s\n", string);   // Undefined behavior: buffer may be overwritten
    return 0;
}

Explanation:

• Stack Allocation: The buffer buffer is allocated on the stack within the function getStackBuffer. Once the function returns, the stack frame is destroyed, and the buffer becomes invalid.
• Undefined Behavior: Accessing this buffer after the function has returned leads to undefined behavior, as the memory may be overwritten by subsequent function calls or operations.

How to fix Return of Stack Variable Address?

The issue arises because the function getStackBuffer returns a pointer to a local stack-allocated buffer, which becomes invalid once the function exits. The solution is to return a pointer to a buffer that is allocated on the heap or use a static/global buffer that remains valid after the function returns.

Best Practices:

1. Heap Allocation: Use malloc to allocate memory on the heap. This memory persists until explicitly freed.
2. Static or Global Buffers: If the buffer size is fixed and the function is not reentrant, a static or global buffer can be used.

Fixed Code Example

// Fixed function that returns the address to a heap-allocated buffer
#include <stdlib.h>
#include <string.h>

char* getHeapBuffer() {
    char* buffer = (char*)malloc(256);  // Allocate buffer on the heap
    if (buffer == NULL) {
        return NULL;  // Handle allocation failure
    }
    strcpy(buffer, "This is a string stored in a heap-allocated buffer."); // Copying string into buffer
    return buffer;  // Returning the address of the heap-allocated buffer
}

int main() {
    char* string = getHeapBuffer();  // Properly retrieve and use the buffer
    if (string != NULL) {
        printf("String: %s\n", string);   // Safe to use, buffer is valid
        free(string);  // Free the allocated memory to prevent memory leaks
    }
    return 0;
}

Explanation:

• Heap Allocation: The buffer is allocated using malloc, ensuring its lifetime extends beyond the function call. This makes the pointer returned by getHeapBuffer valid for use in the main function.
• Memory Management: The allocated memory is freed after use with free, preventing memory leaks, which is a crucial aspect of using heap allocations responsibly.
• Error Handling: The code checks if malloc returns NULL, which indicates allocation failure, to handle errors gracefully.

These improvements ensure the code examples clearly demonstrate the vulnerability and its fix, while adhering to best practices for C programming.