Email: Check custom SMTP IP at usage time

src/pretix/base/email.py

@@ -19,7 +19,10 @@
 # You should have received a copy of the GNU Affero General Public License along with this program.  If not, see
 # <https://www.gnu.org/licenses/>.
 #
+import ipaddress
 import logging
+import smtplib
+import socket
 from itertools import groupby
 from smtplib import SMTPResponseException
 from typing import TypeVar
@@ -237,3 +240,80 @@ def base_renderers(sender, **kwargs):
 
 def get_email_context(**kwargs):
     return PlaceholderContext(**kwargs).render_all()
+
+
+def create_connection(address, timeout=socket.getdefaulttimeout(),
+                      source_address=None, *, all_errors=False):
+    # Taken from the python stdlib, extended with a check for local ips
+
+    host, port = address
+    exceptions = []
+    for res in socket.getaddrinfo(host, port, 0, socket.SOCK_STREAM):
+        af, socktype, proto, canonname, sa = res
+
+        if not settings.get("MAIL_CUSTOM_SMTP_ALLOW_PRIVATE_NETWORKS", False):
+            ip_addr = ipaddress.ip_address(sa[0])
+            if ip_addr.is_multicast:
+                raise socket.error(f"Request to multicast address {sa[0]} blocked")
+            if ip_addr.is_loopback or ip_addr.is_link_local:
+                raise socket.error(f"Request to local address {sa[0]} blocked")
+            if ip_addr.is_private:
+                raise socket.error(f"Request to private address {sa[0]} blocked")
+
+        sock = None
+        try:
+            sock = socket.socket(af, socktype, proto)
+            if timeout is not socket.getdefaulttimeout():
+                sock.settimeout(timeout)
+            if source_address:
+                sock.bind(source_address)
+            sock.connect(sa)
+            # Break explicitly a reference cycle
+            exceptions.clear()
+            return sock
+
+        except socket.error as exc:
+            if not all_errors:
+                exceptions.clear()  # raise only the last error
+            exceptions.append(exc)
+            if sock is not None:
+                sock.close()
+
+    if len(exceptions):
+        try:
+            if not all_errors:
+                raise exceptions[0]
+            raise ExceptionGroup("create_connection failed", exceptions)
+        finally:
+            # Break explicitly a reference cycle
+            exceptions.clear()
+    else:
+        raise socket.error("getaddrinfo returns an empty list")
+
+
+class CheckPrivateNetworkMixin:
+    # _get_socket taken 1:1 from smtplib, just with a call to our own create_connection
+    def _get_socket(self, host, port, timeout):
+        # This makes it simpler for SMTP_SSL to use the SMTP connect code
+        # and just alter the socket connection bit.
+        if timeout is not None and not timeout:
+            raise ValueError('Non-blocking socket (timeout=0) is not supported')
+        if self.debuglevel > 0:
+            self._print_debug('connect: to', (host, port), self.source_address)
+        return create_connection((host, port), timeout, self.source_address)
+
+
+class SMTP(CheckPrivateNetworkMixin, smtplib.SMTP):
+    pass
+
+
+# SMTP used here instead of mixin, because smtp.SMTP_SSL._get_socket calls super()._get_socket and then wraps this socket
+# super()._get_socket needs to be our version from the mixin
+class SMTP_SSL(smtplib.SMTP_SSL, SMTP):  # noqa: N801
+    pass
+
+
+class CheckPrivateNetworkSmtpBackend(EmailBackend):
+    @property
+    def connection_class(self):
+        return SMTP_SSL if self.use_ssl else SMTP

src/pretix/settings.py

@@ -264,7 +264,8 @@ EMAIL_HOST_PASSWORD = config.get('mail', 'password', fallback='')
 EMAIL_USE_TLS = config.getboolean('mail', 'tls', fallback=False)
 EMAIL_USE_SSL = config.getboolean('mail', 'ssl', fallback=False)
 EMAIL_SUBJECT_PREFIX = '[pretix] '
-EMAIL_BACKEND = EMAIL_CUSTOM_SMTP_BACKEND = 'django.core.mail.backends.smtp.EmailBackend'
+EMAIL_BACKEND = 'django.core.mail.backends.smtp.EmailBackend'
+EMAIL_CUSTOM_SMTP_BACKEND = 'pretixbase.email.CheckPrivateNetworkSmtpBackend'
 EMAIL_TIMEOUT = 60
 
 ADMINS = [('Admin', n) for n in config.get('mail', 'admins', fallback='').split(",") if n]

src/tests/base/test_mail.py

@@ -35,8 +35,11 @@
 import datetime
 import os
 import re
+import socket
+from contextlib import contextmanager
 from decimal import Decimal
 from email.mime.text import MIMEText
+from unittest import mock
 
 import pytest
 from django.conf import settings
@@ -591,3 +594,117 @@ def test_attached_ical_localization(env, order):
         assert len(djmail.outbox) == 1
         assert len(djmail.outbox[0].attachments) == 1
         assert description in djmail.outbox[0].attachments[0][1]
+
+
+PRIVATE_IPS_RES = [
+    [(socket.AF_INET, socket.SOCK_STREAM, 6, '', ('10.0.0.3', 443))],
+    [(socket.AF_INET, socket.SOCK_STREAM, 6, '', ('0.0.0.0', 443))],
+    [(socket.AF_INET, socket.SOCK_STREAM, 6, '', ('127.1.1.1', 443))],
+    [(socket.AF_INET, socket.SOCK_STREAM, 6, '', ('192.168.5.3', 443))],
+    [(socket.AF_INET, socket.SOCK_STREAM, 6, '', ('224.0.0.1', 443))],
+    [(socket.AF_INET6, socket.SOCK_STREAM, 6, '', ('::1', 443, 0, 0))],
+    [(socket.AF_INET6, socket.SOCK_STREAM, 6, '', ('fe80::1', 443, 0, 0))],
+    [(socket.AF_INET6, socket.SOCK_STREAM, 6, '', ('ff00::1', 443, 0, 0))],
+    [(socket.AF_INET6, socket.SOCK_STREAM, 6, '', ('fc00::1', 443, 0, 0))],
+]
+
+
+@contextmanager
+def test_mail_connection(res, should_connect, use_ssl):
+    with (
+        mock.patch('socket.socket') as mock_socket,
+        mock.patch('socket.getaddrinfo', return_value=res),
+        mock.patch('smtplib.SMTP.getreply', return_value=(220, "")),
+        mock.patch('smtplib.SMTP.sendmail'),
+        mock.patch('ssl.SSLContext.wrap_socket') as mock_ssl
+    ):
+        yield
+
+        if should_connect:
+            mock_socket.assert_called_once()
+            mock_socket.return_value.connect.assert_called_once_with(res[0][-1])
+            if use_ssl:
+                mock_ssl.assert_called_once()
+        else:
+            mock_socket.assert_not_called()
+            mock_socket.return_value.connect.assert_not_called()
+            mock_ssl.assert_not_called()
+
+
+@pytest.mark.parametrize("res", PRIVATE_IPS_RES)
+@pytest.mark.parametrize("use_ssl", [
+    True, False
+])
+def test_private_smtp_ip(res, use_ssl, settings):
+    settings.EMAIL_CUSTOM_SMTP_BACKEND = 'pretix.base.email.CheckPrivateNetworkSmtpBackend'
+    settings.MAIL_CUSTOM_SMTP_ALLOW_PRIVATE_NETWORKS = False
+    with test_mail_connection(res=res, should_connect=False, use_ssl=use_ssl), pytest.raises(match="Request to .* blocked"):
+        connection = djmail.get_connection(backend=settings.EMAIL_CUSTOM_SMTP_BACKEND,
+                                           host="localhost",
+                                           use_ssl=use_ssl)
+        connection.open()
+
+    settings.MAIL_CUSTOM_SMTP_ALLOW_PRIVATE_NETWORKS = True
+    with test_mail_connection(res=res, should_connect=True, use_ssl=use_ssl):
+        connection = djmail.get_connection(backend=settings.EMAIL_CUSTOM_SMTP_BACKEND,
+                                           host="localhost",
+                                           use_ssl=use_ssl)
+        connection.open()
+
+
+@pytest.mark.parametrize("use_ssl", [
+    True, False
+])
+@pytest.mark.parametrize("allow_private", [
+    True, False
+])
+def test_public_smtp_ip(use_ssl, allow_private, settings):
+    settings.EMAIL_CUSTOM_SMTP_BACKEND = 'pretix.base.email.CheckPrivateNetworkSmtpBackend'
+    settings.MAIL_CUSTOM_SMTP_ALLOW_PRIVATE_NETWORKS = allow_private
+
+    with test_mail_connection(res=[(socket.AF_INET, socket.SOCK_STREAM, 6, '', ('8.8.8.8', 443))], should_connect=True, use_ssl=use_ssl):
+        connection = djmail.get_connection(backend=settings.EMAIL_CUSTOM_SMTP_BACKEND,
+                                           host="localhost",
+                                           use_ssl=use_ssl)
+        connection.open()
+
+
+@pytest.mark.django_db
+@pytest.mark.parametrize("use_ssl", [
+    True, False
+])
+@pytest.mark.parametrize("allow_private_networks", [
+    True, False
+])
+@pytest.mark.parametrize("res", PRIVATE_IPS_RES)
+def test_send_mail_private_ip(res, use_ssl, allow_private_networks, env):
+    settings.EMAIL_CUSTOM_SMTP_BACKEND = 'pretix.base.email.CheckPrivateNetworkSmtpBackend'
+    settings.MAIL_CUSTOM_SMTP_ALLOW_PRIVATE_NETWORKS = allow_private_networks
+
+    event, user, organizer = env
+    event.settings.smtp_use_custom = True
+    event.settings.smtp_host = "example.com"
+    event.settings.smtp_use_ssl = use_ssl
+    event.settings.smtp_use_tls = False
+
+    def send_mail():
+        m = OutgoingMail.objects.create(
+            to=['recipient@example.com'],
+            subject='Test',
+            body_plain='Test',
+            sender='sender@example.com',
+            event=event
+        )
+        assert m.status == OutgoingMail.STATUS_QUEUED
+        mail_send_task.apply(kwargs={
+            'outgoing_mail': m.pk,
+        }, max_retries=0)
+        m.refresh_from_db()
+        return m
+
+    with test_mail_connection(res=res, should_connect=allow_private_networks, use_ssl=use_ssl):
+        m = send_mail()
+        if allow_private_networks:
+            assert m.status == OutgoingMail.STATUS_SENT
+        else:
+            assert m.status == OutgoingMail.STATUS_FAILED