CWE-733: Compiler Optimization Removal or Modification of Security-critical Code

What is Compiler Optimization Removal or Modification of Security-critical Code?

• Overview: Compiler optimization can unintentionally remove or change security-critical code, compromising the intended security mechanisms built into the application by developers.

• Exploitation Methods:

• Attackers can exploit this vulnerability by relying on the absence or alteration of expected security checks, potentially bypassing authentication or access controls.
• Common attack patterns include observing the behavior of compiled code to find missing security mechanisms that were expected to be present.

• Security Impact:

• Direct consequences of successful exploitation include unauthorized access, data breaches, or failure of critical security features.
• Potential cascading effects could involve lateral movement within a network or escalation of privileges.
• Business impact might involve loss of data integrity, regulatory non-compliance, reputational damage, and financial losses.

• Prevention Guidelines:

• Specific code-level fixes include using volatile keywords or inline assembly to prevent optimization on critical code paths.
• Security best practices involve thorough testing and code reviews, particularly with an emphasis on how optimizations affect security mechanisms.
• Recommended tools and frameworks include static analysis tools to detect optimization-related issues and compilers with flags that restrict certain optimizations on critical sections of code.

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Technical Details

Likelihood of Exploit: Not specified

Affected Languages: C, C++, Compiled

Affected Technologies: Not specified

Vulnerable Code Example

// Vulnerable code where a security-critical memory zeroing operation is optimized out by the compiler

#include <string.h>
#include <stdio.h>

// This function attempts to zero out memory containing sensitive data.
// However, due to compiler optimizations, this operation might be removed 
// if the compiler determines that the data is not used afterward.
void clear_sensitive_data(char *data, size_t length) {
    for (size_t i = 0; i < length; i++) {
        data[i] = 0;
    }
}

int main() {
    char sensitive_info[] = "secret_password";
    printf("Sensitive info before clearing: %s\n", sensitive_info);

    clear_sensitive_data(sensitive_info, strlen(sensitive_info));

    // Due to compiler optimization, the above clearing operation might get removed.
    // This could leave sensitive information in memory, potentially exposing it.
    printf("Sensitive info after clearing: %s\n", sensitive_info);
    return 0;
}

How to fix Compiler Optimization Removal or Modification of Security-critical Code?

The issue here is that compilers may optimize away the loop that zeros out the memory containing sensitive data, especially if the compiler determines that the data is not used afterward. This is problematic because it leaves sensitive information in memory, potentially exposing it to unauthorized access.

To fix this, use functions that are specifically designed to prevent such optimizations. In C11, the memset_s function is introduced, which is guaranteed not to be optimized away by the compiler. If memset_s is not available, a common alternative is to use a volatile pointer to trick the compiler into thinking the data is still in use.

Fixed Code Example

// Fixed code using memset_s to ensure that the memory zeroing operation is not optimized away

#include <string.h>
#include <stdio.h>

// This function uses memset_s to securely clear sensitive data from memory.
// memset_s is guaranteed not to be optimized away by the compiler.
void clear_sensitive_data(char *data, size_t length) {
    // Use memset_s if available, which is guaranteed not to be optimized away
    memset_s(data, length, 0, length);
}

int main() {
    char sensitive_info[] = "secret_password";
    printf("Sensitive info before clearing: %s\n", sensitive_info);

    clear_sensitive_data(sensitive_info, strlen(sensitive_info));

    // The sensitive data is now securely cleared from memory.
    printf("Sensitive info after clearing: %s\n", sensitive_info);
    return 0;
}

If memset_s is not available in your environment, you can use the following alternative:

// Alternative fixed code using volatile to prevent optimization

#include <string.h>
#include <stdio.h>

// This function uses a volatile pointer to ensure that the memory zeroing operation
// is not optimized away by the compiler, even if the data appears unused afterward.
void clear_sensitive_data(char *data, size_t length) {
    volatile char *volatile_data = data;
    for (size_t i = 0; i < length; i++) {
        volatile_data[i] = 0;
    }
}

int main() {
    char sensitive_info[] = "secret_password";
    printf("Sensitive info before clearing: %s\n", sensitive_info);

    clear_sensitive_data(sensitive_info, strlen(sensitive_info));

    // The sensitive data is now securely cleared from memory.
    printf("Sensitive info after clearing: %s\n", sensitive_info);
    return 0;
}

In this alternative approach, we use a volatile pointer to ensure that the write operations to clear the memory are not optimized away by the compiler. This is a common technique when memset_s is not available.