Insecure functionality - File Creation - Python

Insecure functionality - File Creation - Python

Need

Enforce server-side validation and authorization for restricted functionality

Context

  • Usage of Python 3 for scripting and application development
  • Usage of Django for building web applications

Description

Non compliant code

        from django.http import HttpResponse
from django.views import View
from django.core.files.storage import FileSystemStorage

class FileUploadView(View):
    def post(self, request):
        uploaded_file = request.FILES['document']
        fs = FileSystemStorage()
        fs.save(uploaded_file.name, uploaded_file)
        return HttpResponse('File uploaded successfully')

In the above code, we have a Django View FileUploadView that handles file uploads. This view takes a POST request and gets the file from the request. It then saves the file using Django's FileSystemStorage which is used for storing uploaded files.

The vulnerability here lies in the fact that there are no checks in place to ensure that the user is authorized to upload files. The restriction is only done on the frontend by disabling the upload button for certain users. However, a malicious user can bypass this frontend restriction and send a direct POST request to the server to upload a file.

This can lead to various security risks like unauthorized data upload, data tampering, and even potential code execution if the uploaded file is a script. The server should always validate and check the permissions of a user on the backend before processing their request, regardless of any frontend restrictions.

Steps

  • Implement server-side validation to ensure that only authorized users can upload files.
  • Check the user's permissions before processing the file upload request.
  • Do not rely solely on front-end controls for security, as they can be easily bypassed.
  • Use Django's built-in authentication and authorization features to restrict access to certain views based on user permissions.
  • Ensure that the user is authenticated before processing the file upload request.

Compliant code

        from django.http import HttpResponse
from django.views import View
from django.core.files.storage import FileSystemStorage
from django.contrib.auth.decorators import login_required, permission_required
from django.utils.decorators import method_decorator

@method_decorator(login_required, name='dispatch')
@method_decorator(permission_required('app_name.can_upload_file'), name='dispatch')
class FileUploadView(View):
    def post(self, request):
        uploaded_file = request.FILES['document']
        fs = FileSystemStorage()
        fs.save(uploaded_file.name, uploaded_file)
        return HttpResponse('File uploaded successfully')

The updated code now includes Django's built-in decorators for authentication and authorization.

The @login_required decorator ensures that the user is authenticated before they can access the view. If the user is not authenticated, they will be redirected to the login page.

The @permission_required('app_name.can_upload_file') decorator checks if the logged-in user has the specific permission to upload files. If the user does not have the required permission, a 403 Forbidden response is returned.

The @method_decorator is used to apply these decorators to class-based views. The name='dispatch' argument is used to apply the decorators to all the methods in the view (GET, POST, etc.).

This way, we ensure that only authenticated users with the correct permissions can upload files, providing a server-side security control that cannot be bypassed like client-side controls.

Remember to replace 'app_name.can_upload_file' with the actual name of the permission required to upload files in your application. You can define custom permissions in your Django models.

References