[3.2.x] Fixed CVE-2023-46695 -- Fixed potential DoS in UsernameField on Windows.

Thanks MProgrammer (https://hackerone.com/mprogrammer) for the report.

django/contrib/auth/forms.py

@@ -62,7 +62,15 @@ class ReadOnlyPasswordHashField(forms.Field):
 
 class UsernameField(forms.CharField):
     def to_python(self, value):
-        return unicodedata.normalize('NFKC', super().to_python(value))
+        value = super().to_python(value)
+        if self.max_length is not None and len(value) > self.max_length:
+            # Normalization can increase the string length (e.g.
+            # "ff" -> "ff", "½" -> "1⁄2") but cannot reduce it, so there is no
+            # point in normalizing invalid data. Moreover, Unicode
+            # normalization is very slow on Windows and can be a DoS attack
+            # vector.
+            return value
+        return unicodedata.normalize("NFKC", value)
 
     def widget_attrs(self, widget):
         return {

docs/releases/3.2.23.txt

@@ -6,4 +6,14 @@ Django 3.2.23 release notes
 
 Django 3.2.23 fixes a security issue with severity "moderate" in 3.2.22.
 
-...
+CVE-2023-46695: Potential denial of service vulnerability in ``UsernameField`` on Windows
+=========================================================================================
+
+The :func:`NFKC normalization <python:unicodedata.normalize>` is slow on
+Windows. As a consequence, ``django.contrib.auth.forms.UsernameField`` was
+subject to a potential denial of service attack via certain inputs with a very
+large number of Unicode characters.
+
+In order to avoid the vulnerability, invalid values longer than
+``UsernameField.max_length`` are no longer normalized, since they cannot pass
+validation anyway.

tests/auth_tests/test_forms.py

@@ -5,7 +5,7 @@ from unittest import mock
 from django.contrib.auth.forms import (
     AdminPasswordChangeForm, AuthenticationForm, PasswordChangeForm,
     PasswordResetForm, ReadOnlyPasswordHashField, ReadOnlyPasswordHashWidget,
-    SetPasswordForm, UserChangeForm, UserCreationForm,
+    SetPasswordForm, UserChangeForm, UserCreationForm, UsernameField,
 )
 from django.contrib.auth.models import User
 from django.contrib.auth.signals import user_login_failed
@@ -132,6 +132,12 @@ class UserCreationFormTest(TestDataMixin, TestCase):
         self.assertNotEqual(user.username, ohm_username)
         self.assertEqual(user.username, 'testΩ')  # U+03A9 GREEK CAPITAL LETTER OMEGA
 
+    def test_invalid_username_no_normalize(self):
+        field = UsernameField(max_length=254)
+        # Usernames are not normalized if they are too long.
+        self.assertEqual(field.to_python("½" * 255), "½" * 255)
+        self.assertEqual(field.to_python("ff" * 254), "ff" * 254)
+
     def test_duplicate_normalized_unicode(self):
         """
         To prevent almost identical usernames, visually identical but differing