From f9a7fb8466a7ba4857eaf930099b5258f3eafb2b Mon Sep 17 00:00:00 2001
From: Mariusz Felisiak <felisiak.mariusz@gmail.com>
Date: Tue, 17 Oct 2023 11:48:32 +0200
Subject: [PATCH] [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   | 10 +++++++++-
 tests/auth_tests/test_forms.py |  8 +++++++-
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/django/contrib/auth/forms.py b/django/contrib/auth/forms.py
index e6f73fe..26d3ca7 100644
--- a/django/contrib/auth/forms.py
+++ b/django/contrib/auth/forms.py
@@ -68,7 +68,15 @@ 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)
 
 
 class UserCreationForm(forms.ModelForm):
diff --git a/tests/auth_tests/test_forms.py b/tests/auth_tests/test_forms.py
index bed23af..e73d4b8 100644
--- a/tests/auth_tests/test_forms.py
+++ b/tests/auth_tests/test_forms.py
@@ -6,7 +6,7 @@ from django import forms
 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
-- 
2.30.0

