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Lack of data validation - Type confusion

Need​

Implementation of proper data validation and type checking

Context​

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

Description​

Non compliant code​

public class DataController : Controller
{
    [HttpGet]
    public IActionResult Get(int id)
    {
        // Some code to fetch data using id
    }
}

In the above code, the Get method of DataController is vulnerable to type confusion due to lack of data validation. It expects an integer id as a parameter. However, it does not validate the input and directly uses it to fetch data.

This means that although it indicates that it only accepts numbers, it allows values in the form 0xff. In C#, 0xff is a hexadecimal number that will be interpreted as an integer (255 in decimal). This can lead to unexpected behavior as the server can interpret the input as a different type than intended.

The lack of proper input validation and type checking makes this code susceptible to type confusion attacks. An attacker could potentially craft a request with a hexadecimal value, which could cause the server to behave in unexpected ways.

Steps​

  • Implement input validation to ensure that the 'id' parameter only accepts valid numeric values.
  • Use appropriate data validation techniques such as regular expressions or built-in validation attributes.
  • Consider using the 'int.TryParse' method to parse the 'id' parameter and handle invalid input gracefully.
  • Avoid directly interpreting the 'id' parameter without proper validation to prevent type confusion vulnerabilities.
  • Ensure that the server-side code only processes the 'id' parameter as an integer and rejects any non-numeric input.

Compliant code​

public class DataController : Controller
{
    [HttpGet]
    public IActionResult Get(string id)
    {
        int parsedId;
        if (int.TryParse(id, out parsedId))
        {
            // Some code to fetch data using parsedId
        }
        else
        {
            return BadRequest("Invalid ID format.");
        }
    }
}

The updated code now takes the 'id' parameter as a string. This allows us to perform validation on the input before attempting to use it as an integer.

The 'int.TryParse' method is used to attempt to parse the 'id' string as an integer. If the parsing is successful, the method returns true and the parsed integer is stored in the 'parsedId' variable. The data fetching code can then use this 'parsedId' variable.

If the parsing is unsuccessful, the 'int.TryParse' method returns false. In this case, the code returns a BadRequest response with a message indicating that the ID format is invalid. This prevents the server from attempting to process an invalid 'id' value, thereby avoiding the type confusion vulnerability.

This approach ensures that the server-side code only processes the 'id' parameter as an integer and rejects any non-numeric input. It also provides a clear error message to the client when the 'id' format is invalid, allowing the client to correct the input and try again.

References​

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