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Insecure exceptions - NullPointerException

Need​

Ensure secure exception handling to prevent unexpected system behavior

Context​

  • Usage of C# for building robust and scalable applications
  • Usage of Microsoft.AspNetCore.Mvc for building web applications using the ASP.NET Core MVC framework

Description​

Non compliant code​

public class VulnerableController : Controller
{
    public IActionResult Index()
    {
        try
        {
            // some code here
            string str = null;
            var length = str.Length;
        }
        catch (NullReferenceException)
        {
            // Ignore or improperly handle the exception
        }

        return View();
    }
}

The code above is an example of a vulnerable code snippet in an ASP.NET Core backend application. In the Index method of VulnerableController, a string variable str is declared and initialized with null.

The next line attempts to access the Length property of str. However, since str is null, this operation throws a NullReferenceException.

The issue here is that the NullReferenceException is caught in a catch block but is not properly handled. The catch block is empty, meaning that the exception is simply ignored, and the program continues to execute as if nothing happened.

This is a security vulnerability known as "Insecure Exception Handling". By swallowing the exception and not taking any corrective action or logging the error, this code can lead to unexpected behavior in the system. It also makes it harder to debug issues as there is no trace of this exception happening.

Insecure exception handling can lead to program instability and potentially exploitable conditions. It's a best practice to handle exceptions properly by either logging them, returning a suitable error message, or redirecting the flow of execution to a safe location in the program.

Steps​

  • Remove the try-catch block that catches the NullReferenceException.
  • Instead of catching the exception, use proper null checking to prevent the exception from occurring.
  • Before accessing the 'str' variable, check if it is null using an if statement.
  • If 'str' is null, handle the situation appropriately, such as returning an error message or redirecting to an error page.
  • Ensure that all variables are properly initialized and assigned before using them to avoid null reference exceptions.

Compliant code​

public class FixedController : Controller
{
    public IActionResult Index()
    {
        // some code here
        string str = null;

        if (str == null)
        {
            // Handle the situation appropriately
            return RedirectToAction("Error", "Home");
        }

        var length = str.Length;

        return View();
    }
}

The fixed code now includes a null check before attempting to access the str variable. This prevents a NullReferenceException from being thrown, which could cause unexpected behavior in the system.

The try-catch block that was previously catching the NullReferenceException has been removed. This is because it's generally a bad practice to catch and ignore or improperly handle exceptions. Instead, we should aim to prevent exceptions from occurring in the first place.

In the fixed code, before accessing the str variable, we check if it is null using an if statement. If str is null, we handle the situation appropriately by redirecting to an error page. This is just one way to handle this situation - depending on the specific requirements of your application, you might choose to handle it differently (for example, by returning an error message to the user).

Finally, it's important to ensure that all variables are properly initialized and assigned before using them. This helps to avoid null reference exceptions and other similar issues. In this case, the str variable is explicitly assigned a null value for demonstration purposes, but in a real-world application, you would want to ensure that variables are assigned meaningful values before they are used.

References​

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