CWE-196: Unsigned to Signed Conversion Error

What is Unsigned to Signed Conversion Error?

• Overview: Unsigned to signed conversion error occurs when an unsigned integer is cast to a signed integer, which can lead to unexpected negative values if the original unsigned value exceeds the maximum representable value of the signed type. This can result in incorrect program logic or behavior, particularly in languages like C and C++.

• Exploitation Methods:

• Attackers can exploit this vulnerability by providing large unsigned input values that, when cast to signed types, produce negative numbers.
• Common attack patterns involve using these negative values to manipulate buffer indexes, leading to buffer underwrites or overwrites, and potentially allowing arbitrary code execution.

• Security Impact:

• Direct consequences of successful exploitation include data corruption, application crashes, or unauthorized access to memory.
• Potential cascading effects involve further exploitation of memory corruption to escalate privileges, execute arbitrary code, or exfiltrate sensitive information.
• Business impact can include system downtime, data breaches, loss of customer trust, and potential legal ramifications.

• Prevention Guidelines:

• Specific code-level fixes include avoiding unnecessary casting between signed and unsigned data types and using explicit checks for value ranges during conversions.
• Security best practices involve favoring safe library functions that handle type conversions securely and conducting regular code reviews to identify risky casting operations.
• Recommended tools and frameworks include static analysis tools that can detect improper type conversions and runtime analysis tools to observe unintended behavior during execution.

Corgea can automatically detect and fix Unsigned to Signed Conversion Error in your codebase. [Try Corgea free today](https://corgea.app).

Technical Details

Likelihood of Exploit: Medium

Affected Languages: C, C++

Affected Technologies: Not specified

Vulnerable Code Example

// Example of Unsigned to Signed Conversion Error

#include <stdio.h>
#include <limits.h>

void processValue(unsigned int value) {
    // Vulnerable: Casting unsigned int to signed int
    // If `value` is greater than INT_MAX, it will wrap around to a negative number
    int signedValue = (int)value;
    
    // This can lead to unexpected behavior, e.g., incorrect logic due to negative value
    if (signedValue < 0) {
        printf("Error: Negative value detected!\n");
    } else {
        printf("Processed value: %d\n", signedValue);
    }
}

int main() {
    unsigned int largeValue = 4294967295U; // Maximum value for unsigned int
    processValue(largeValue); // This will demonstrate the issue
    return 0;
}

How to fix Unsigned to Signed Conversion Error?

To fix the unsigned to signed conversion error, ensure that the conversion does not result in an unexpected negative value. One approach is to check if the unsigned value exceeds the maximum limit of the signed type before performing the conversion. If the value exceeds this limit, handle the scenario appropriately, such as by triggering an error or adjusting the logic to accommodate large values.

Fixed Code Example

// Fixed version with proper checks before conversion

#include <stdio.h>
#include <limits.h>

void processValue(unsigned int value) {
    // Check if the value can be safely converted to a signed int
    if (value > INT_MAX) {
        printf("Error: Value too large to convert to signed int!\n");
        return;
    }
    
    int signedValue = (int)value;
    
    // Safe to use signedValue without unexpected negative behavior
    printf("Processed value: %d\n", signedValue);
}

int main() {
    unsigned int largeValue = 4294967295U; // Maximum value for unsigned int
    processValue(largeValue); // This will now handle the large value safely
    return 0;
}

Explanation

In the vulnerable example, converting a large unsigned integer to a signed integer can lead to unexpected negative values due to integer overflow. This is demonstrated by passing a value larger than INT_MAX to the function, which results in incorrect logic execution.

In the fixed example, a check is added to ensure that the unsigned integer does not exceed INT_MAX before conversion. This prevents the conversion from causing the value to wrap around to a negative number, ensuring the program logic behaves as expected.