CWE-478: Missing Default Case in Multiple Condition Expression

What is Missing Default Case in Multiple Condition Expression?

• Overview: Missing Default Case in Multiple Condition Expression occurs when a switch statement or similar control structure does not include a default case, which can lead to unhandled conditions and logical errors if an unexpected value is encountered.

• Exploitation Methods:

• Attackers may exploit this vulnerability by inputting unexpected values that are not handled by existing cases, causing the program to behave unpredictably.
• Common attack patterns include crafting input data that falls through the switch statement without triggering any case, leading to unexpected program flow.

• Security Impact:

• Direct consequences of successful exploitation can include application crashes or malfunctioning features.
• Potential cascading effects may involve incorrect decision-making processes, leading to further vulnerabilities being exposed or exploited.
• Business impact could range from reduced software reliability to increased maintenance costs and potential data breaches.

• Prevention Guidelines:

• Specific code-level fixes include always implementing a default case in switch statements to handle unexpected or unhandled values safely.
• Security best practices suggest regularly reviewing control structures for completeness and ensuring all possible inputs are considered.
• Recommended tools and frameworks include static analysis tools that can detect missing default cases in switch statements, such as SonarQube or Coverity.

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Technical Details

Likelihood of Exploit: Not specified

Affected Languages: C, C++, Java, C#, Python, JavaScript

Affected Technologies: Not specified

Vulnerable Code Example

#include <stdio.h>

void handleInput(int input) {
    switch (input) {
        case 1:
            printf("Option 1 selected.\n");
            break;
        case 2:
            printf("Option 2 selected.\n");
            break;
        // Missing default case to handle unexpected input values.
    }
}

int main() {
    int userInput = 3; // Simulating user input
    handleInput(userInput); // No output, as there's no default case for input 3
    return 0;
}

Explanation:

• Lines 11-15: The switch statement lacks a default case. Without a default case, the program does not handle unexpected or undefined inputs, leading to no feedback or action for input values not explicitly covered by the case statements. This can result in undefined behavior or a poor user experience when unexpected values are encountered.

How to fix Missing Default Case in Multiple Condition Expression?

A default case should be integrated into the switch statement to manage any unexpected or undefined inputs. This default case serves as a safety net for any values not explicitly handled by the other cases, ensuring the program can gracefully handle unexpected scenarios. It often provides a mechanism to log errors, return default values, or activate error-handling processes. Including a default case enhances the reliability and maintainability of the code.

Fixed Code Example

#include <stdio.h>

void handleInput(int input) {
    switch (input) {
        case 1:
            printf("Option 1 selected.\n");
            break;
        case 2:
            printf("Option 2 selected.\n");
            break;
        default: // Added default case to handle unexpected inputs
            printf("Invalid option selected.\n");
            break;
    }
}

int main() {
    int userInput = 3; // Simulating user input
    handleInput(userInput); // Now outputs "Invalid option selected."
    return 0;
}

Explanation:

• Lines 15-18: The default case has been added to the switch statement. It provides feedback by printing a message indicating that an invalid option was selected. This ensures that any input not matching the specified cases is handled properly, thus avoiding undefined behavior and improving user experience by providing clear feedback for unsupported inputs.