[4.2.x] Fixed CVE-2024-39329 -- Standarized timing of verify_password() when checking unusuable passwords.

Refs #20760.

Thanks Michael Manfre for the fix and to Adam Johnson for the review.

3 files changed, 47 insertions, 2 deletions

diff --git a/django/contrib/auth/hashers.py b/django/contrib/auth/hashers.py
index 9db5a12e13..f11bc8a0f3 100644
--- a/django/contrib/auth/hashers.py
+++ b/django/contrib/auth/hashers.py
@@ -43,14 +43,20 @@ def check_password(password, encoded, setter=None, preferred="default"):
     If setter is specified, it'll be called when you need to
     regenerate the password.
     """
-    if password is None or not is_password_usable(encoded):
-        return False
+    fake_runtime = password is None or not is_password_usable(encoded)
 
     preferred = get_hasher(preferred)
     try:
         hasher = identify_hasher(encoded)
     except ValueError:
         # encoded is gibberish or uses a hasher that's no longer installed.
+        fake_runtime = True
+
+    if fake_runtime:
+        # Run the default password hasher once to reduce the timing difference
+        # between an existing user with an unusable password and a nonexistent
+        # user or missing hasher (similar to #20760).
+        make_password(get_random_string(UNUSABLE_PASSWORD_SUFFIX_LENGTH))
         return False
 
     hasher_changed = hasher.algorithm != preferred.algorithm
diff --git a/docs/releases/4.2.14.txt b/docs/releases/4.2.14.txt
index f32c0cf8d4..556cff4437 100644
--- a/docs/releases/4.2.14.txt
+++ b/docs/releases/4.2.14.txt
@@ -13,3 +13,10 @@ CVE-2024-38875: Potential denial-of-service vulnerability in ``django.utils.html
 :tfilter:`urlize` and :tfilter:`urlizetrunc` were subject to a potential
 denial-of-service attack via certain inputs with a very large number of
 brackets.
+
+CVE-2024-39329: Username enumeration through timing difference for users with unusable passwords
+================================================================================================
+
+The :meth:`~django.contrib.auth.backends.ModelBackend.authenticate()` method
+allowed remote attackers to enumerate users via a timing attack involving login
+requests for users with unusable passwords.
diff --git a/tests/auth_tests/test_hashers.py b/tests/auth_tests/test_hashers.py
index 36f22d5f09..3da495f2be 100644
--- a/tests/auth_tests/test_hashers.py
+++ b/tests/auth_tests/test_hashers.py
@@ -613,6 +613,38 @@ class TestUtilsHashPass(SimpleTestCase):
             check_password("wrong_password", encoded)
             self.assertEqual(hasher.harden_runtime.call_count, 1)
 
+    def test_check_password_calls_make_password_to_fake_runtime(self):
+        hasher = get_hasher("default")
+        cases = [
+            (None, None, None),  # no plain text password provided
+            ("foo", make_password(password=None), None),  # unusable encoded
+            ("letmein", make_password(password="letmein"), ValueError),  # valid encoded
+        ]
+        for password, encoded, hasher_side_effect in cases:
+            with (
+                self.subTest(encoded=encoded),
+                mock.patch(
+                    "django.contrib.auth.hashers.identify_hasher",
+                    side_effect=hasher_side_effect,
+                ) as mock_identify_hasher,
+                mock.patch(
+                    "django.contrib.auth.hashers.make_password"
+                ) as mock_make_password,
+                mock.patch(
+                    "django.contrib.auth.hashers.get_random_string",
+                    side_effect=lambda size: "x" * size,
+                ),
+                mock.patch.object(hasher, "verify"),
+            ):
+                # Ensure make_password is called to standardize timing.
+                check_password(password, encoded)
+                self.assertEqual(hasher.verify.call_count, 0)
+                self.assertEqual(mock_identify_hasher.mock_calls, [mock.call(encoded)])
+                self.assertEqual(
+                    mock_make_password.mock_calls,
+                    [mock.call("x" * UNUSABLE_PASSWORD_SUFFIX_LENGTH)],
+                )
+
     def test_encode_invalid_salt(self):
         hasher_classes = [
             MD5PasswordHasher,