[1.11.x] Fixed CVE-2019-19844 -- Used verified user email for password reset requests.

Backport of 5b1fbcef7a8bec991ebe7b2a18b5d5a95d72cb70 from master.

Co-Authored-By: Florian Apolloner <florian@apolloner.eu>

3 files changed, 86 insertions, 6 deletions

diff --git a/django/contrib/auth/forms.py b/django/contrib/auth/forms.py
index 02250d83da..3741fdea7c 100644
--- a/django/contrib/auth/forms.py
+++ b/django/contrib/auth/forms.py
@@ -16,12 +16,27 @@ from django.core.mail import EmailMultiAlternatives
 from django.template import loader
 from django.utils.encoding import force_bytes
 from django.utils.http import urlsafe_base64_encode
+from django.utils.six import PY3
 from django.utils.text import capfirst
 from django.utils.translation import ugettext, ugettext_lazy as _
 
 UserModel = get_user_model()
 
 
+def _unicode_ci_compare(s1, s2):
+    """
+    Perform case-insensitive comparison of two identifiers, using the
+    recommended algorithm from Unicode Technical Report 36, section
+    2.11.2(B)(2).
+    """
+    normalized1 = unicodedata.normalize('NFKC', s1)
+    normalized2 = unicodedata.normalize('NFKC', s2)
+    if PY3:
+        return normalized1.casefold() == normalized2.casefold()
+    # lower() is the best alternative available on Python 2.
+    return normalized1.lower() == normalized2.lower()
+
+
 class ReadOnlyPasswordHashWidget(forms.Widget):
     template_name = 'auth/widgets/read_only_password_hash.html'
 
@@ -249,11 +264,16 @@ class PasswordResetForm(forms.Form):
         that prevent inactive users and users with unusable passwords from
         resetting their password.
         """
+        email_field_name = UserModel.get_email_field_name()
         active_users = UserModel._default_manager.filter(**{
-            '%s__iexact' % UserModel.get_email_field_name(): email,
+            '%s__iexact' % email_field_name: email,
             'is_active': True,
         })
-        return (u for u in active_users if u.has_usable_password())
+        return (
+            u for u in active_users
+            if u.has_usable_password() and
+            _unicode_ci_compare(email, getattr(u, email_field_name))
+        )
 
     def save(self, domain_override=None,
              subject_template_name='registration/password_reset_subject.txt',
@@ -266,6 +286,7 @@ class PasswordResetForm(forms.Form):
         user.
         """
         email = self.cleaned_data["email"]
+        email_field_name = UserModel.get_email_field_name()
         for user in self.get_users(email):
             if not domain_override:
                 current_site = get_current_site(request)
@@ -273,8 +294,9 @@ class PasswordResetForm(forms.Form):
                 domain = current_site.domain
             else:
                 site_name = domain = domain_override
+            user_email = getattr(user, email_field_name)
             context = {
-                'email': email,
+                'email': user_email,
                 'domain': domain,
                 'site_name': site_name,
                 'uid': urlsafe_base64_encode(force_bytes(user.pk)),
@@ -286,7 +308,7 @@ class PasswordResetForm(forms.Form):
                 context.update(extra_email_context)
             self.send_mail(
                 subject_template_name, email_template_name, context, from_email,
-                email, html_email_template_name=html_email_template_name,
+                user_email, html_email_template_name=html_email_template_name,
             )
 
 
diff --git a/docs/releases/1.11.27.txt b/docs/releases/1.11.27.txt
index cb4329afdb..6197dee1f6 100644
--- a/docs/releases/1.11.27.txt
+++ b/docs/releases/1.11.27.txt
@@ -2,9 +2,25 @@
 Django 1.11.27 release notes
 ============================
 
-*Expected January 2, 2020*
+*December 18, 2019*
 
-Django 1.11.27 fixes a data loss bug in 1.11.26.
+Django 1.11.27 fixes a security issue and a data loss bug in 1.11.26.
+
+CVE-2019-19844: Potential account hijack via password reset form
+================================================================
+
+By submitting a suitably crafted email address making use of Unicode
+characters, that compared equal to an existing user email when lower-cased for
+comparison, an attacker could be sent a password reset token for the matched
+account.
+
+In order to avoid this vulnerability, password reset requests now compare the
+submitted email using the stricter, recommended algorithm for case-insensitive
+comparison of two identifiers from `Unicode Technical Report 36, section
+2.11.2(B)(2)`__. Upon a match, the email containing the reset token will be
+sent to the email address on record rather than the submitted address.
+
+.. __: https://www.unicode.org/reports/tr36/#Recommendations_General
 
 Bugfixes
 ========
diff --git a/tests/auth_tests/test_forms.py b/tests/auth_tests/test_forms.py
index e09285277f..213d58b773 100644
--- a/tests/auth_tests/test_forms.py
+++ b/tests/auth_tests/test_forms.py
@@ -694,6 +694,48 @@ class PasswordResetFormTest(TestDataMixin, TestCase):
         self.assertFalse(form.is_valid())
         self.assertEqual(form['email'].errors, [_('Enter a valid email address.')])
 
+    def test_user_email_unicode_collision(self):
+        User.objects.create_user('mike123', 'mike@example.org', 'test123')
+        User.objects.create_user('mike456', 'mıke@example.org', 'test123')
+        data = {'email': 'mıke@example.org'}
+        form = PasswordResetForm(data)
+        if six.PY2:
+            self.assertFalse(form.is_valid())
+        else:
+            self.assertTrue(form.is_valid())
+            form.save()
+            self.assertEqual(len(mail.outbox), 1)
+            self.assertEqual(mail.outbox[0].to, ['mıke@example.org'])
+
+    def test_user_email_domain_unicode_collision(self):
+        User.objects.create_user('mike123', 'mike@ixample.org', 'test123')
+        User.objects.create_user('mike456', 'mike@ıxample.org', 'test123')
+        data = {'email': 'mike@ıxample.org'}
+        form = PasswordResetForm(data)
+        self.assertTrue(form.is_valid())
+        form.save()
+        self.assertEqual(len(mail.outbox), 1)
+        self.assertEqual(mail.outbox[0].to, ['mike@ıxample.org'])
+
+    def test_user_email_unicode_collision_nonexistent(self):
+        User.objects.create_user('mike123', 'mike@example.org', 'test123')
+        data = {'email': 'mıke@example.org'}
+        form = PasswordResetForm(data)
+        if six.PY2:
+            self.assertFalse(form.is_valid())
+        else:
+            self.assertTrue(form.is_valid())
+            form.save()
+            self.assertEqual(len(mail.outbox), 0)
+
+    def test_user_email_domain_unicode_collision_nonexistent(self):
+        User.objects.create_user('mike123', 'mike@ixample.org', 'test123')
+        data = {'email': 'mike@ıxample.org'}
+        form = PasswordResetForm(data)
+        self.assertTrue(form.is_valid())
+        form.save()
+        self.assertEqual(len(mail.outbox), 0)
+
     def test_nonexistent_email(self):
         """
         Test nonexistent email address. This should not fail because it would