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Authentication mechanism absence or evasion - Redirect

Need​

Implementation of a robust and secure authentication mechanism

Context​

  • Usage of Python 3.0+ for developing Python applications
  • Usage of Django for building web applications in Python

Description​

Non compliant code​

from django.http import HttpResponseRedirect
from django.shortcuts import render

def login(request):
    if request.method == 'POST':
        username = request.POST.get('username')
        password = request.POST.get('password')
        if username == "valid_username" and password == "valid_password":
            return HttpResponseRedirect('/home/')
        else:
            return HttpResponseRedirect('/login/')
    else:
        return render(request, 'login.html')

In the above code, the login view is handling both GET and POST requests. When a POST request is made, it checks if the username and password are valid. If they are valid, it redirects the user to the home page. If they are not valid, it redirects the user back to the login page.

The vulnerability here is that an attacker can bypass the authentication process by simply changing the redirect URL from '/login/' to '/home/'. This is possible because the server response, which includes the redirect, can be manipulated by the client (i.e., the attacker). Therefore, even if the username and password are not valid, the attacker can still gain access to the home page by changing the redirect URL.

This vulnerability is due to the absence of a proper authentication mechanism. The application should not rely solely on redirects for authentication. Instead, it should use sessions or tokens to keep track of authenticated users. Also, sensitive server responses should be properly protected to prevent manipulation by the client.

Steps​

  • Implement an authentication system instead of hardcoding valid username and password in the code. Django provides built-in authentication which can be used.
  • Use Django's built-in login view which handles the authentication process.
  • Instead of redirecting to a different URL based on the success or failure of the login, render a response that indicates the success or failure. This prevents an attacker from being able to determine a valid username based on the redirect URL.
  • Use Django's messages framework to display one-time notifications to the user about the success or failure of their login attempt.
  • Ensure that the login form is protected against Cross-Site Request Forgery (CSRF) attacks. Django's CSRF middleware provides this protection.

Compliant code​

from django.contrib.auth import authenticate, login
from django.contrib import messages
from django.shortcuts import render, redirect
from django.views.decorators.csrf import csrf_protect

@csrf_protect
def login_view(request):
    if request.method == 'POST':
        username = request.POST.get('username')
        password = request.POST.get('password')
        user = authenticate(request, username=username, password=password)
        if user is not None:
            login(request, user)
            messages.success(request, 'You are now logged in.')
            return redirect('/home/')
        else:
            messages.error(request, 'Invalid username or password.')
            return redirect('/login/')
    else:
        return render(request, 'login.html')

The updated code now uses Django's built-in authenticate and login functions to handle user authentication. The authenticate function checks the provided username and password against the user data stored in the database. If the authentication is successful, the login function is used to log the user in.

The messages framework is used to provide feedback to the user about the success or failure of their login attempt. If the login is successful, a success message is added to the messages framework. If the login fails, an error message is added instead.

The redirect function is used to send the user to the appropriate page after their login attempt. Regardless of whether the login is successful or not, the user is redirected to the same URL. This prevents an attacker from being able to determine a valid username based on the redirect URL.

The csrf_protect decorator is used to protect the login form against Cross-Site Request Forgery (CSRF) attacks. This decorator ensures that a valid CSRF token is present in the form data when the form is submitted. If the token is not present or is invalid, a 403 Forbidden response is returned.

References​

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