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Improper type assignation

Need​

Enforce proper type assignment in the source code

Context​

  • Usage of Python 3.0 for building applications and scripts
  • Usage of Django for building web applications in Python

Description​

Non compliant code​

def process_data(request):
    data = request.POST.get('data')
    processed_data = data + " is processed"
    return HttpResponse(processed_data)

In the above code, the 'data' variable is a variant variable as it has no specific type declared for it. This is a case of Improper type assignation vulnerability.

Here, request.POST.get('data') can return any type of data. It could be a string, a number, a list etc. When we try to concatenate it with a string in the next line, it might raise a TypeError if the data is not a string.

Moreover, if the 'data' variable is used in several other places in the code, it becomes necessary to identify its data type and cast it every time it is used. This could lead to inefficiencies and potential bugs in the code.

This type of vulnerability can be exploited by an attacker to cause unexpected behavior in the application. For example, if an attacker manages to pass a dictionary or a list instead of a string, it can lead to unexpected errors and potential security risks.

Steps​

  • Identify the expected data type of the 'data' variable. If it's supposed to be a string, ensure it is so before processing.
  • Use explicit type casting to convert the 'data' variable to the expected data type. This can be done using Python's built-in functions like str(), int(), float(), etc.
  • Add error handling to catch any exceptions that may occur during type casting. This will prevent the application from crashing if the 'data' variable is not of the expected type.
  • Consider using Python's type hinting feature to indicate the expected data type of the 'data' variable. This can help prevent improper type assignation.

Compliant code​

from django.http import HttpResponse, JsonResponse

def process_data(request):
    data = request.POST.get('data')
    if data is not None:
        try:
            data = str(data)
        except ValueError:
            return JsonResponse({'error': 'Invalid data type. String expected.'}, status=400)
        processed_data = data + " is processed"
        return HttpResponse(processed_data)
    else:
        return JsonResponse({'error': 'No data provided.'}, status=400)

The updated code now includes explicit type casting and error handling to ensure that the 'data' variable is always a string before it is processed.

Firstly, the 'data' variable is retrieved from the POST request. If 'data' is not None, the code attempts to convert it to a string using Python's built-in str() function. If this conversion is successful, the 'data' variable is then processed and returned in an HttpResponse.

If the conversion fails (i.e., if 'data' is not a string), a ValueError is raised. This exception is caught and handled by returning a JsonResponse with an error message and a 400 status code. This prevents the application from crashing and provides a helpful error message to the user.

If 'data' is None (i.e., if no 'data' was provided in the POST request), a JsonResponse with an appropriate error message and a 400 status code is returned.

This approach ensures that the 'data' variable is always of the correct type before it is processed, thereby fixing the improper type assignation vulnerability.

References​

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