Fixed #28699 -- Fixed CSRF validation with remote user middleware.

Ensured process_view() always accesses the CSRF token from the session
or cookie, rather than the request, as rotate_token() may have been called
by an authentication middleware during the process_request() phase.

tests/auth_tests/test_remote_user.py

@@ -5,7 +5,8 @@ from django.contrib.auth import authenticate
 from django.contrib.auth.backends import RemoteUserBackend
 from django.contrib.auth.middleware import RemoteUserMiddleware
 from django.contrib.auth.models import User
-from django.test import TestCase, modify_settings, override_settings
+from django.middleware.csrf import _get_new_csrf_string, _mask_cipher_secret
+from django.test import Client, TestCase, modify_settings, override_settings
 from django.utils import timezone
 
 
@@ -50,6 +51,35 @@ class RemoteUserTest(TestCase):
         self.assertTrue(response.context['user'].is_anonymous)
         self.assertEqual(User.objects.count(), num_users)
 
+    def test_csrf_validation_passes_after_process_request_login(self):
+        """
+        CSRF check must access the CSRF token from the session or cookie,
+        rather than the request, as rotate_token() may have been called by an
+        authentication middleware during the process_request() phase.
+        """
+        csrf_client = Client(enforce_csrf_checks=True)
+        csrf_secret = _get_new_csrf_string()
+        csrf_token = _mask_cipher_secret(csrf_secret)
+        csrf_token_form = _mask_cipher_secret(csrf_secret)
+        headers = {self.header: 'fakeuser'}
+        data = {'csrfmiddlewaretoken': csrf_token_form}
+
+        # Verify that CSRF is configured for the view
+        csrf_client.cookies.load({settings.CSRF_COOKIE_NAME: csrf_token})
+        response = csrf_client.post('/remote_user/', **headers)
+        self.assertEqual(response.status_code, 403)
+        self.assertIn(b'CSRF verification failed.', response.content)
+
+        # This request will call django.contrib.auth.login() which will call
+        # django.middleware.csrf.rotate_token() thus changing the value of
+        # request.META['CSRF_COOKIE'] from the user submitted value set by
+        # CsrfViewMiddleware.process_request() to the new csrftoken value set
+        # by rotate_token(). Csrf validation should still pass when the view is
+        # later processed by CsrfViewMiddleware.process_view()
+        csrf_client.cookies.load({settings.CSRF_COOKIE_NAME: csrf_token})
+        response = csrf_client.post('/remote_user/', data, **headers)
+        self.assertEqual(response.status_code, 200)
+
     def test_unknown_user(self):
         """
         Tests the case where the username passed in the header does not exist