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Insecure object reference - Files

Need​

Secure access control for files and validate permissions before granting access

Context​

  • Usage of Python 3 for writing and executing Python code
  • Usage of Django for building web applications

Description​

Non compliant code​

from django.shortcuts import get_object_or_404
from django.http import HttpResponse
from .models import UserFile

def download_file(request, file_id):
    file = get_object_or_404(UserFile, pk=file_id)
    response = HttpResponse(file.content, content_type='application/force-download')
    response['Content-Disposition'] = 'attachment; filename=%s' % file.filename
    return response

The code above is a simple Django view that allows a user to download a file. It uses the get_object_or_404 function to retrieve a file object from the database using its ID, which is passed in the URL.

The vulnerability here is that there is no access control in place to check if the user requesting the file is the same user who uploaded it or if they have the necessary permissions to access it. This means that any user who knows the ID of a file can download it, even if they are not the owner of the file. This is a clear example of Insecure Direct Object Reference (IDOR) vulnerability.

The download_file function does not perform any authentication or authorization checks. It simply takes the file_id parameter from the request, uses it to fetch a file from the database, and then sends the file to the user. If an attacker were to guess or brute-force the file_id of a file that they should not have access to, they would be able to download it.

Steps​

  • Add an authentication layer to the application to ensure that only authorized users can access the files.
  • Implement access control checks to validate that the authenticated user has the necessary permissions to access the requested file.
  • Use session management to track the user's session and ensure that the user is authorized to access the requested file.
  • Ensure that the file identifiers are not predictable. Use random, unique identifiers for each file to prevent unauthorized access.
  • Implement rate limiting to prevent brute force attacks where an attacker might try to guess the file identifiers.
  • Log and monitor all file access attempts. This will help in identifying any unauthorized access attempts and take necessary actions.

Compliant code​

from django.shortcuts import get_object_or_404
from django.http import HttpResponse
from .models import UserFile
from django.contrib.auth.decorators import login_required

@login_required
def download_file(request, file_id):
    file = get_object_or_404(UserFile, pk=file_id)

    # Check if the user has permission to access the file
    if request.user != file.user:
        return HttpResponse('Unauthorized', status=401)

    response = HttpResponse(file.content, content_type='application/force-download')
    response['Content-Disposition'] = 'attachment; filename=%s' % file.filename
    return response

The updated code now includes an authentication layer to ensure that only authorized users can access the files. This is done using the @login_required decorator from Django's authentication framework. This decorator ensures that the user is authenticated before they can access the download_file view.

In addition to this, an access control check has been added to validate that the authenticated user has the necessary permissions to access the requested file. This is done by comparing the request.user (the currently authenticated user) with the file.user (the user who owns the file). If these two users are not the same, the server responds with an 'Unauthorized' message and a 401 status code, indicating that the user does not have permission to access the file.

This solution assumes that the UserFile model has a user field that references the user who owns the file. If this is not the case, you will need to modify the access control check to suit your application's needs.

Please note that this code does not include the implementation of session management, random unique identifiers for each file, rate limiting, and logging and monitoring of file access attempts. These are additional security measures that should be implemented as part of your overall security strategy, but are beyond the scope of this specific code fix.

References​

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