CWE-14: Compiler Removal of Code to Clear Buffers

What is Compiler Removal of Code to Clear Buffers?

• Overview: Compiler Removal of Code to Clear Buffers (CWE-14) occurs when compilers optimize code by removing instructions intended to clear sensitive data from memory. This happens because the compiler sees these instructions as unnecessary since the memory isn't used afterward.

• Exploitation Methods:

• Attackers can exploit this vulnerability by accessing memory that should have been cleared, potentially retrieving sensitive information like passwords or cryptographic keys.
• Common attack patterns include memory scraping and exploiting other vulnerabilities to read residual data from memory.

• Security Impact:

• Direct consequences include unauthorized access to sensitive information that was believed to be erased.
• Potential cascading effects involve compromised data integrity, privacy breaches, and increased vulnerability to further attacks.
• Business impact could involve data breaches, loss of client trust, regulatory penalties, and financial loss.

• Prevention Guidelines:

• Specific code-level fixes include using functions or libraries specifically designed to securely clear memory, such as using volatile pointers or assembly code to prevent optimization.
• Security best practices involve regularly reviewing and testing code for optimization issues and ensuring sensitive data is properly handled and cleared.
• Recommended tools and frameworks include static analysis tools that detect dead store vulnerabilities and libraries that provide secure memory handling functions.

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Technical Details

Likelihood of Exploit: Not specified

Affected Languages: C, C++

Affected Technologies: Not specified

Vulnerable Code Example

#include <string.h>

void handleSensitiveData() {
    char sensitiveData[100];
    // Simulate filling sensitiveData with sensitive information
    strcpy(sensitiveData, "Sensitive Information");

    // Attempt to clear the sensitive buffer
    memset(sensitiveData, 0, sizeof(sensitiveData));
    // Vulnerability: The compiler may optimize away this memset call
    // due to dead store elimination, as sensitiveData is not used afterwards
}

How to fix Compiler Removal of Code to Clear Buffers?

The vulnerability arises from compiler optimizations that can remove calls to memset when the buffer is not utilized subsequently, potentially leaving sensitive data in memory. To ensure the buffer is cleared, even after compiler optimizations, use methods that prevent the optimization of the clear operation. The C11 standard introduced memset_s, which is guaranteed not to be optimized away. If memset_s is unavailable, use a volatile function pointer to memset or a custom clearing function.

Fixed Code Example

#include <string.h>

// Custom secure memset function using volatile pointer to prevent optimization
void secure_memset(void *v, int c, size_t n) {
    volatile unsigned char *p = v;
    while (n--) {
        *p++ = c;
    }
}

void handleSensitiveData() {
    char sensitiveData[100];
    // Simulate filling sensitiveData with sensitive information
    strcpy(sensitiveData, "Sensitive Information");

    // Securely clear the buffer to prevent sensitive data leakage
    secure_memset(sensitiveData, 0, sizeof(sensitiveData));
    // Using a secure memset ensures that the buffer is cleared even after compiler optimizations
}

In the fixed code, secure_memset uses a volatile pointer to ensure the compiler does not optimize away the clearing operation. This method is effective because the volatile keyword instructs the compiler that the memory content may change at any time, thus preventing the removal of this operation during optimization. This approach ensures sensitive data is reliably cleared from memory.