Fixed CVE-2025-13473 -- Standardized timing of check_password() in mod_wsgi auth handler.

Refs CVE-2024-39329, #20760.

Thanks Stackered for the report, and Jacob Walls and Markus Holtermann
for the reviews.

Co-authored-by: Natalia <124304+nessita@users.noreply.github.com>

tests/auth_tests/test_handlers.py

@@ -1,4 +1,7 @@
+from unittest import mock
+
 from django.contrib.auth.handlers.modwsgi import check_password, groups_for_user
+from django.contrib.auth.hashers import get_hasher
 from django.contrib.auth.models import Group, User
 from django.test import TransactionTestCase, override_settings
 
@@ -73,3 +76,28 @@ class ModWsgiHandlerTestCase(TransactionTestCase):
 
         self.assertEqual(groups_for_user({}, "test"), [b"test_group"])
         self.assertEqual(groups_for_user({}, "test1"), [])
+
+    def test_check_password_fake_runtime(self):
+        """
+        Hasher is run once regardless of whether the user exists. Refs #20760.
+        """
+        User.objects.create_user("test", "test@example.com", "test")
+        User.objects.create_user("inactive", "test@nono.com", "test", is_active=False)
+        User.objects.create_user("unusable", "test@nono.com")
+
+        hasher = get_hasher()
+
+        for username, password in [
+            ("test", "test"),
+            ("test", "wrong"),
+            ("inactive", "test"),
+            ("inactive", "wrong"),
+            ("unusable", "test"),
+            ("doesnotexist", "test"),
+        ]:
+            with (
+                self.subTest(username=username, password=password),
+                mock.patch.object(hasher, "encode") as mock_make_password,
+            ):
+                check_password({}, username, password)
+                mock_make_password.assert_called_once()