Need

Implementation of input length validation

Context

• Usage of Python 3.6+ for developing Python applications
• Usage of Django for building web applications in Python

Description

Non compliant code

        from django.http import JsonResponse
from django.views.decorators.csrf import csrf_exempt

@csrf_exempt
def vulnerable_endpoint(request):
    data = request.body
    # process data
    return JsonResponse({"status": "ok"})
        
        

In this Django view function named vulnerable_endpoint, the input data is directly obtained from the request body using request.body without any validation or limitation on its length.

This makes the endpoint vulnerable to attacks where an attacker can send a large amount of data in the request body, potentially causing a Denial of Service (DoS) attack by exhausting server resources.

Moreover, if this data is used in any operations without proper validation, it can lead to other vulnerabilities such as Buffer Overflow, Injection attacks, etc.

In this case, the server might try to process a large amount of data, consuming a lot of memory and CPU resources, and eventually causing the server to crash or become unresponsive.

In addition, if the received data is stored in a database without proper length checks, it could lead to storage space exhaustion or database errors.

Therefore, it's crucial to validate the length of the incoming data before processing it.

Steps

• Add a middleware that checks the length of the request body before it reaches the endpoint.
• In the middleware, if the length of the request body exceeds a certain limit, return an error response.
• The limit should be set according to the maximum expected size of the request body for the endpoint.
• Ensure that the limit is not too high to prevent potential attacks, but also not too low to avoid false positives.

Compliant code

        from django.http import JsonResponse, HttpResponseBadRequest
from django.views.decorators.csrf import csrf_exempt

class LengthLimitMiddleware:
    def __init__(self, get_response):
        self.get_response = get_response
        self.max_length = 10000  # Set your maximum length here

    def __call__(self, request):
        if len(request.body) > self.max_length:
            return HttpResponseBadRequest("Request body too large")
        return self.get_response(request)

@csrf_exempt
def vulnerable_endpoint(request):
    data = request.body
    # process data
    return JsonResponse({"status": "ok"})
        
        

The above code introduces a middleware, LengthLimitMiddleware, that checks the length of the request body before it reaches the endpoint. This middleware is initialized with a get_response function and a max_length attribute, which is the maximum allowed length of the request body.

In the __call__ method of the middleware, it checks if the length of the request body exceeds the max_length. If it does, it returns an HttpResponseBadRequest with a message indicating that the request body is too large. If the length of the request body is within the limit, it calls the get_response function to continue processing the request.

The vulnerable_endpoint function is decorated with csrf_exempt to disable CSRF checks, and it processes the request body as before.

This solution ensures that the length of the request body is checked before it reaches the endpoint, thus preventing potential attacks that involve sending massive amounts of characters. The max_length should be set according to the maximum expected size of the request body for the endpoint. It should not be too high to prevent potential attacks, but also not too low to avoid false positives.

References

• 194. Lack of data validation - Input Length