Check custom smtp ip at usage too (#6073)

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/helpers/monkeypatching.py

@@ -19,12 +19,26 @@
 # 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 socket
+import sys
 import types
 from datetime import datetime
 from http import cookies
 
+from django.conf import settings
 from PIL import Image
 from requests.adapters import HTTPAdapter
+from urllib3.connection import HTTPConnection, HTTPSConnection
+from urllib3.connectionpool import HTTPConnectionPool, HTTPSConnectionPool
+from urllib3.exceptions import (
+    ConnectTimeoutError, HTTPError, LocationParseError, NameResolutionError,
+    NewConnectionError,
+)
+from urllib3.util.connection import (
+    _TYPE_SOCKET_OPTIONS, _set_socket_options, allowed_gai_family,
+)
+from urllib3.util.timeout import _DEFAULT_TIMEOUT
 
 
 def monkeypatch_vobject_performance():
@@ -89,6 +103,123 @@ def monkeypatch_requests_timeout():
     HTTPAdapter.send = httpadapter_send
 
 
+def monkeypatch_urllib3_ssrf_protection():
+    """
+    pretix allows HTTP requests to untrusted URLs, e.g. through webhooks or external API URLs. This is dangerous since
+    it can allow access to private networks that should not be reachable by users ("server-side request forgery", SSRF).
+    Validating URLs at submission is not sufficient, since with DNS rebinding an attacker can make a domain name pass
+    validation and then resolve to a private IP address on actual execution. Unfortunately, there seems no clean solution
+    to this in Python land, so we monkeypatch urllib3's connection management to check the IP address to be external
+    *after* the DNS resolution.
+
+    This does not work when a global http(s) proxy is used, but in that scenario the proxy can perform the validation.
+    """
+    if getattr(settings, "ALLOW_HTTP_TO_PRIVATE_NETWORKS", False):
+        # Settings are not supposed to change during runtime, so we can optimize performance and complexity by skipping
+        # this if not needed.
+        return
+
+    def create_connection(
+        address: tuple[str, int],
+        timeout=_DEFAULT_TIMEOUT,
+        source_address: tuple[str, int] | None = None,
+        socket_options: _TYPE_SOCKET_OPTIONS | None = None,
+    ) -> socket.socket:
+        # This is copied from urllib3.util.connection v2.3.0
+        host, port = address
+        if host.startswith("["):
+            host = host.strip("[]")
+        err = None
+
+        # Using the value from allowed_gai_family() in the context of getaddrinfo lets
+        # us select whether to work with IPv4 DNS records, IPv6 records, or both.
+        # The original create_connection function always returns all records.
+        family = allowed_gai_family()
+
+        try:
+            host.encode("idna")
+        except UnicodeError:
+            raise LocationParseError(f"'{host}', label empty or too long") from None
+
+        for res in socket.getaddrinfo(host, port, family, socket.SOCK_STREAM):
+            af, socktype, proto, canonname, sa = res
+
+            if not getattr(settings, "ALLOW_HTTP_TO_PRIVATE_NETWORKS", False):
+                ip_addr = ipaddress.ip_address(sa[0])
+                if ip_addr.is_multicast:
+                    raise HTTPError(f"Request to multicast address {sa[0]} blocked")
+                if ip_addr.is_loopback or ip_addr.is_link_local:
+                    raise HTTPError(f"Request to local address {sa[0]} blocked")
+                if ip_addr.is_private:
+                    raise HTTPError(f"Request to private address {sa[0]} blocked")
+
+            sock = None
+            try:
+                sock = socket.socket(af, socktype, proto)
+
+                # If provided, set socket level options before connecting.
+                _set_socket_options(sock, socket_options)
+
+                if timeout is not _DEFAULT_TIMEOUT:
+                    sock.settimeout(timeout)
+                if source_address:
+                    sock.bind(source_address)
+                sock.connect(sa)
+                # Break explicitly a reference cycle
+                err = None
+                return sock
+
+            except OSError as _:
+                err = _
+                if sock is not None:
+                    sock.close()
+
+        if err is not None:
+            try:
+                raise err
+            finally:
+                # Break explicitly a reference cycle
+                err = None
+        else:
+            raise OSError("getaddrinfo returns an empty list")
+
+    class ProtectionMixin:
+        def _new_conn(self) -> socket.socket:
+            # This is 1:1 the version from urllib3.connection.HTTPConnection._new_conn v2.3.0
+            # just with a call to our own create_connection
+            try:
+                sock = create_connection(
+                    (self._dns_host, self.port),
+                    self.timeout,
+                    source_address=self.source_address,
+                    socket_options=self.socket_options,
+                )
+            except socket.gaierror as e:
+                raise NameResolutionError(self.host, self, e) from e
+            except socket.timeout as e:
+                raise ConnectTimeoutError(
+                    self,
+                    f"Connection to {self.host} timed out. (connect timeout={self.timeout})",
+                ) from e
+
+            except OSError as e:
+                raise NewConnectionError(
+                    self, f"Failed to establish a new connection: {e}"
+                ) from e
+
+            sys.audit("http.client.connect", self, self.host, self.port)
+            return sock
+
+    class ProtectedHTTPConnection(ProtectionMixin, HTTPConnection):
+        pass
+
+    class ProtectedHTTPSConnection(ProtectionMixin, HTTPSConnection):
+        pass
+
+    HTTPConnectionPool.ConnectionCls = ProtectedHTTPConnection
+    HTTPSConnectionPool.ConnectionCls = ProtectedHTTPSConnection
+
+
 def monkeypatch_cookie_morsel():
     # See https://code.djangoproject.com/ticket/34613
     cookies.Morsel._flags.add("partitioned")
@@ -99,4 +230,5 @@ def monkeypatch_all_at_ready():
     monkeypatch_vobject_performance()
     monkeypatch_pillow_safer()
     monkeypatch_requests_timeout()
+    monkeypatch_urllib3_ssrf_protection()
     monkeypatch_cookie_morsel()

src/pretix/settings.py

@@ -208,6 +208,7 @@ CSRF_TRUSTED_ORIGINS = [urlparse(SITE_URL).scheme + '://' + urlparse(SITE_URL).h
 
 TRUST_X_FORWARDED_FOR = config.getboolean('pretix', 'trust_x_forwarded_for', fallback=False)
 USE_X_FORWARDED_HOST = config.getboolean('pretix', 'trust_x_forwarded_host', fallback=False)
+ALLOW_HTTP_TO_PRIVATE_NETWORKS = config.getboolean('pretix', 'allow_http_to_private_networks', fallback=False)
 
 
 REQUEST_ID_HEADER = config.get('pretix', 'request_id_header', fallback=False)
@@ -248,7 +249,8 @@ 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

src/tests/helpers/test_urllib.py

@@ -0,0 +1,93 @@
+#
+# This file is part of pretix (Community Edition).
+#
+# Copyright (C) 2014-2020  Raphael Michel and contributors
+# Copyright (C) 2020-today pretix GmbH and contributors
+#
+# This program is free software: you can redistribute it and/or modify it under the terms of the GNU Affero General
+# Public License as published by the Free Software Foundation in version 3 of the License.
+#
+# ADDITIONAL TERMS APPLY: Pursuant to Section 7 of the GNU Affero General Public License, additional terms are
+# applicable granting you additional permissions and placing additional restrictions on your usage of this software.
+# Please refer to the pretix LICENSE file to obtain the full terms applicable to this work. If you did not receive
+# this file, see <https://pretix.eu/about/en/license>.
+#
+# This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied
+# warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Affero General Public License for more
+# details.
+#
+# 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/>.
+#
+from socket import AF_INET, SOCK_STREAM
+from unittest import mock
+
+import pytest
+import requests
+from django.test import override_settings
+from dns.inet import AF_INET6
+from urllib3.exceptions import HTTPError
+
+
+def test_local_blocked():
+    with pytest.raises(HTTPError, match="Request to local address.*"):
+        requests.get("http://localhost", timeout=0.1)
+    with pytest.raises(HTTPError, match="Request to local address.*"):
+        requests.get("https://localhost", timeout=0.1)
+
+
+def test_private_ip_blocked():
+    with pytest.raises(HTTPError, match="Request to private address.*"):
+        requests.get("http://10.0.0.1", timeout=0.1)
+    with pytest.raises(HTTPError, match="Request to private address.*"):
+        requests.get("https://10.0.0.1", timeout=0.1)
+
+
+@pytest.mark.django_db
+@pytest.mark.parametrize("res", [
+    [(AF_INET, SOCK_STREAM, 6, '', ('10.0.0.3', 443))],
+    [(AF_INET, SOCK_STREAM, 6, '', ('0.0.0.0', 443))],
+    [(AF_INET, SOCK_STREAM, 6, '', ('127.1.1.1', 443))],
+    [(AF_INET, SOCK_STREAM, 6, '', ('192.168.5.3', 443))],
+    [(AF_INET, SOCK_STREAM, 6, '', ('224.0.0.1', 443))],
+    [(AF_INET6, SOCK_STREAM, 6, '', ('::1', 443, 0, 0))],
+    [(AF_INET6, SOCK_STREAM, 6, '', ('fe80::1', 443, 0, 0))],
+    [(AF_INET6, SOCK_STREAM, 6, '', ('ff00::1', 443, 0, 0))],
+    [(AF_INET6, SOCK_STREAM, 6, '', ('fc00::1', 443, 0, 0))],
+])
+def test_dns_resolving_to_local_blocked(res):
+    with mock.patch('socket.getaddrinfo') as mock_addr:
+        mock_addr.return_value = res
+        with pytest.raises(HTTPError, match="Request to (multicast|private|local) address.*"):
+            requests.get("https://example.org", timeout=0.1)
+        with pytest.raises(HTTPError, match="Request to (multicast|private|local) address.*"):
+            requests.get("http://example.org", timeout=0.1)
+
+
+def test_dns_remote_allowed():
+    class SocketOk(Exception):
+        pass
+
+    def side_effect(*args, **kwargs):
+        raise SocketOk
+
+    with mock.patch('socket.getaddrinfo') as mock_addr, mock.patch('socket.socket') as mock_socket:
+        mock_addr.return_value = [(AF_INET, SOCK_STREAM, 6, '', ('8.8.8.8', 443))]
+        mock_socket.side_effect = side_effect
+        with pytest.raises(SocketOk):
+            requests.get("https://example.org", timeout=0.1)
+
+
+@override_settings(ALLOW_HTTP_TO_PRIVATE_NETWORKS=True)
+def test_local_is_allowed():
+    class SocketOk(Exception):
+        pass
+
+    def side_effect(*args, **kwargs):
+        raise SocketOk
+
+    with mock.patch('socket.getaddrinfo') as mock_addr, mock.patch('socket.socket') as mock_socket:
+        mock_addr.return_value = [(AF_INET, SOCK_STREAM, 6, '', ('10.0.0.1', 443))]
+        mock_socket.side_effect = side_effect
+        with pytest.raises(SocketOk):
+            requests.get("https://example.org", timeout=0.1)