Restrict the XML deserializer to prevent network and entity-expansion DoS attacks.

This is a security fix. Disclosure and advisory coming shortly.

tests/regressiontests/serializers_regress/tests.py

@@ -10,6 +10,7 @@ from __future__ import absolute_import, unicode_literals
 
 import datetime
 import decimal
+from django.core.serializers.xml_serializer import DTDForbidden
 
 try:
     import yaml
@@ -514,3 +515,17 @@ for format in serializers.get_serializer_formats():
     if format != 'python':
         setattr(SerializerTests, 'test_' + format + '_serializer_stream', curry(streamTest, format))
 
+
+class XmlDeserializerSecurityTests(TestCase):
+
+    def test_no_dtd(self):
+        """
+        The XML deserializer shouldn't allow a DTD.
+
+        This is the most straightforward way to prevent all entity definitions
+        and avoid both external entities and entity-expansion attacks.
+
+        """
+        xml = '<?xml version="1.0" standalone="no"?><!DOCTYPE example SYSTEM "http://example.com/example.dtd">'
+        with self.assertRaises(DTDForbidden):
+            next(serializers.deserialize('xml', xml))