[5.1.x] Fixed CVE-2024-45231 -- Avoided server error on password reset when email sending fails.

On successful submission of a password reset request, an email is sent
to the accounts known to the system. If sending this email fails (due to
email backend misconfiguration, service provider outage, network issues,
etc.), an attacker might exploit this by detecting which password reset
requests succeed and which ones generate a 500 error response.

Thanks to Thibaut Spriet for the report, and to Mariusz Felisiak, Adam
Johnson, and Sarah Boyce for the reviews.

8 files changed, 83 insertions, 2 deletions

diff --git a/django/contrib/auth/forms.py b/django/contrib/auth/forms.py
index a11668944a..edf672a6e5 100644
--- a/django/contrib/auth/forms.py
+++ b/django/contrib/auth/forms.py
@@ -1,3 +1,4 @@
+import logging
 import unicodedata
 
 from django import forms
@@ -16,6 +17,7 @@ from django.utils.translation import gettext
 from django.utils.translation import gettext_lazy as _
 
 UserModel = get_user_model()
+logger = logging.getLogger("django.contrib.auth")
 
 
 def _unicode_ci_compare(s1, s2):
@@ -418,7 +420,12 @@ class PasswordResetForm(forms.Form):
             html_email = loader.render_to_string(html_email_template_name, context)
             email_message.attach_alternative(html_email, "text/html")
 
-        email_message.send()
+        try:
+            email_message.send()
+        except Exception:
+            logger.exception(
+                "Failed to send password reset email to %s", context["user"].pk
+            )
 
     def get_users(self, email):
         """Given an email, return matching user(s) who should receive a reset.
diff --git a/docs/ref/logging.txt b/docs/ref/logging.txt
index 8a7e58997e..24ab5d97f3 100644
--- a/docs/ref/logging.txt
+++ b/docs/ref/logging.txt
@@ -209,6 +209,18 @@ Django development server. This logger generates an ``INFO`` message upon
 detecting a modification in a source code file and may produce ``WARNING``
 messages during filesystem inspection and event subscription processes.
 
+.. _django-contrib-auth-logger:
+
+``django.contrib.auth``
+~~~~~~~~~~~~~~~~~~~~~~~
+
+.. versionadded:: 4.2.16
+
+Log messages related to :doc:`contrib/auth`, particularly ``ERROR`` messages
+are generated when a :class:`~django.contrib.auth.forms.PasswordResetForm` is
+successfully submitted but the password reset email cannot be delivered due to
+a mail sending exception.
+
 .. _django-contrib-gis-logger:
 
 ``django.contrib.gis``
diff --git a/docs/releases/4.2.16.txt b/docs/releases/4.2.16.txt
index 2a84186867..963036345c 100644
--- a/docs/releases/4.2.16.txt
+++ b/docs/releases/4.2.16.txt
@@ -13,3 +13,14 @@ CVE-2024-45230: Potential denial-of-service vulnerability in ``django.utils.html
 :tfilter:`urlize` and :tfilter:`urlizetrunc` were subject to a potential
 denial-of-service attack via very large inputs with a specific sequence of
 characters.
+
+CVE-2024-45231: Potential user email enumeration via response status on password reset
+======================================================================================
+
+Due to unhandled email sending failures, the
+:class:`~django.contrib.auth.forms.PasswordResetForm` class allowed remote
+attackers to enumerate user emails by issuing password reset requests and
+observing the outcomes.
+
+To mitigate this risk, exceptions occurring during password reset email sending
+are now handled and logged using the :ref:`django-contrib-auth-logger` logger.
diff --git a/docs/releases/5.0.9.txt b/docs/releases/5.0.9.txt
index 50e94ea3f2..52595ae4ff 100644
--- a/docs/releases/5.0.9.txt
+++ b/docs/releases/5.0.9.txt
@@ -13,3 +13,14 @@ CVE-2024-45230: Potential denial-of-service vulnerability in ``django.utils.html
 :tfilter:`urlize` and :tfilter:`urlizetrunc` were subject to a potential
 denial-of-service attack via very large inputs with a specific sequence of
 characters.
+
+CVE-2024-45231: Potential user email enumeration via response status on password reset
+======================================================================================
+
+Due to unhandled email sending failures, the
+:class:`~django.contrib.auth.forms.PasswordResetForm` class allowed remote
+attackers to enumerate user emails by issuing password reset requests and
+observing the outcomes.
+
+To mitigate this risk, exceptions occurring during password reset email sending
+are now handled and logged using the :ref:`django-contrib-auth-logger` logger.
diff --git a/docs/releases/5.1.1.txt b/docs/releases/5.1.1.txt
index dd24929100..8c0468d727 100644
--- a/docs/releases/5.1.1.txt
+++ b/docs/releases/5.1.1.txt
@@ -14,6 +14,17 @@ CVE-2024-45230: Potential denial-of-service vulnerability in ``django.utils.html
 denial-of-service attack via very large inputs with a specific sequence of
 characters.
 
+CVE-2024-45231: Potential user email enumeration via response status on password reset
+======================================================================================
+
+Due to unhandled email sending failures, the
+:class:`~django.contrib.auth.forms.PasswordResetForm` class allowed remote
+attackers to enumerate user emails by issuing password reset requests and
+observing the outcomes.
+
+To mitigate this risk, exceptions occurring during password reset email sending
+are now handled and logged using the :ref:`django-contrib-auth-logger` logger.
+
 Bugfixes
 ========
 
diff --git a/docs/topics/auth/default.txt b/docs/topics/auth/default.txt
index 15af779dfe..4b40cb16f2 100644
--- a/docs/topics/auth/default.txt
+++ b/docs/topics/auth/default.txt
@@ -1752,7 +1752,9 @@ provides several built-in forms located in :mod:`django.contrib.auth.forms`:
     .. method:: send_mail(subject_template_name, email_template_name, context, from_email, to_email, html_email_template_name=None)
 
         Uses the arguments to send an ``EmailMultiAlternatives``.
-        Can be overridden to customize how the email is sent to the user.
+        Can be overridden to customize how the email is sent to the user. If
+        you choose to override this method, be mindful of handling potential
+        exceptions raised due to email sending failures.
 
         :param subject_template_name: the template for the subject.
         :param email_template_name: the template for the email body.
diff --git a/tests/auth_tests/test_forms.py b/tests/auth_tests/test_forms.py
index 13f86b6b77..9463f07d57 100644
--- a/tests/auth_tests/test_forms.py
+++ b/tests/auth_tests/test_forms.py
@@ -1351,6 +1351,27 @@ class PasswordResetFormTest(TestDataMixin, TestCase):
             )
         )
 
+    @override_settings(EMAIL_BACKEND="mail.custombackend.FailingEmailBackend")
+    def test_save_send_email_exceptions_are_catched_and_logged(self):
+        (user, username, email) = self.create_dummy_user()
+        form = PasswordResetForm({"email": email})
+        self.assertTrue(form.is_valid())
+
+        with self.assertLogs("django.contrib.auth", level=0) as cm:
+            form.save()
+
+        self.assertEqual(len(mail.outbox), 0)
+        self.assertEqual(len(cm.output), 1)
+        errors = cm.output[0].split("\n")
+        pk = user.pk
+        self.assertEqual(
+            errors[0],
+            f"ERROR:django.contrib.auth:Failed to send password reset email to {pk}",
+        )
+        self.assertEqual(
+            errors[-1], "ValueError: FailingEmailBackend is doomed to fail."
+        )
+
     @override_settings(AUTH_USER_MODEL="auth_tests.CustomEmailField")
     def test_custom_email_field(self):
         email = "test@mail.com"
diff --git a/tests/mail/custombackend.py b/tests/mail/custombackend.py
index 14e7f077ba..c63f1c07b5 100644
--- a/tests/mail/custombackend.py
+++ b/tests/mail/custombackend.py
@@ -12,3 +12,9 @@ class EmailBackend(BaseEmailBackend):
         # Messages are stored in an instance variable for testing.
         self.test_outbox.extend(email_messages)
         return len(email_messages)
+
+
+class FailingEmailBackend(BaseEmailBackend):
+
+    def send_messages(self, email_messages):
+        raise ValueError("FailingEmailBackend is doomed to fail.")