CWE-194: Unexpected Sign Extension

What is Unexpected Sign Extension?

• Overview: Unexpected Sign Extension occurs when a number, typically a smaller signed integer, is promoted to a larger data type in a way that incorrectly extends the sign bit. This conversion can lead to incorrect calculations or unexpected behavior, particularly if the original number was negative.

• Exploitation Methods:

• Attackers can exploit this vulnerability by providing negative input values that are incorrectly sign-extended, leading to logic errors.
• Common attack patterns include manipulating index calculations or loop counters to access out-of-bounds memory.

• Security Impact:

• Direct consequences of successful exploitation include potential data corruption and erratic program behavior.
• Potential cascading effects involve access violations, arbitrary code execution, or denial of service.
• Business impact could involve data breaches, system downtime, and loss of customer trust.

• Prevention Guidelines:

• Specific code-level fixes include using unsigned data types where sign extension is not desired and carefully managing type conversions.
• Security best practices include performing thorough input validation and employing static analysis tools to detect improper type conversions.
• Recommended tools and frameworks include GCC's -Wconversion flag, Clang's -fsanitize=implicit-conversion, and using language features like C++'s safe integer classes.

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Technical Details

Likelihood of Exploit: High

Affected Languages: C, C++

Affected Technologies: Not specified

Vulnerable Code Example

C Example

#include <stdio.h>

// Vulnerable function demonstrating CWE-194: Unexpected Sign Extension
void processInput(short userInput) {
    unsigned int size;
    // Vulnerable line: Sign extension occurs when negative short is converted to unsigned int
    size = userInput; // {5}

    // Size is used for memory allocation or array indexing
    printf("Allocating buffer of size: %u\n", size);
    char buffer[size]; // {7}
    // Potentially dangerous operation due to unexpected large size from sign extension
    // which can lead to buffer overflow or other undefined behavior
}

How to fix Unexpected Sign Extension?

The issue in the code arises from the conversion of a signed short to an unsigned int, which can cause unexpected sign extension if userInput is negative. In C, when a negative number is stored in an unsigned type, it can result in a large positive integer due to how binary representations work, leading to potential buffer overflow or other memory issues.

Fix Approach:

1. Validate Input Range: Ensure that the input is within a valid range before using it for operations like memory allocation.
2. Use Appropriate Data Types: Consider using unsigned types if negative values are not expected.
3. Explicit Casting and Checks: If casting is necessary, perform explicit checks and handle potential negative values properly.

Fixed Code Example

#include <stdio.h>

// Fixed function to prevent unexpected sign extension
void processInput(short userInput) {
    unsigned int size;

    // Ensure userInput is non-negative before casting
    if (userInput < 0) { // {8}
        printf("Error: Negative input is not allowed.\n");
        return;
    }

    // Safe conversion as the input is validated
    size = (unsigned int)userInput; // {12}

    // Use size safely knowing it is within expected bounds
    printf("Allocating buffer of size: %u\n", size);
    char buffer[size]; // {13}
    // Buffer is safely allocated as size is within valid range
}

Explanation of Fix:

• Input Validation (Line 8): Before casting userInput to an unsigned integer, the code checks if userInput is negative. If it is, the function prints an error message and returns early, preventing any further operations with an invalid size.
• Safe Casting (Line 12): The input is cast to unsigned int only after validation ensures it is non-negative, preventing any unexpected sign extension.
• Ensured Buffer Safety (Line 13): The buffer allocation uses the validated size, ensuring the memory allocation is safe and within expected limits.

Best Practices:

• Always validate input when dealing with memory allocation to prevent undefined behavior.
• Use appropriate data types that match the expected range of values to avoid unnecessary type conversions.
• Clearly document the intent and safety checks in the code comments for maintainability and clarity.