bpo-17239: Disable external entities in SAX parser (GH-9217)

The SAX parser no longer processes general external entities by default
to increase security. Before, the parser created network connections
to fetch remote files or loaded local files from the file system for DTD
and entities.

Signed-off-by: Christian Heimes <christian@python.org>



https://bugs.python.org/issue17239

Lib/test/test_pulldom.py

@@ -3,6 +3,7 @@ import unittest
 import xml.sax
 
 from xml.sax.xmlreader import AttributesImpl
+from xml.sax.handler import feature_external_ges
 from xml.dom import pulldom
 
 from test.support import findfile
@@ -166,6 +167,12 @@ class PullDOMTestCase(unittest.TestCase):
             # This should have returned 'END_ELEMENT'.
             self.assertEqual(parser[-1][0], pulldom.START_DOCUMENT)
 
+    def test_external_ges_default(self):
+        parser = pulldom.parseString(SMALL_SAMPLE)
+        saxparser = parser.parser
+        ges = saxparser.getFeature(feature_external_ges)
+        self.assertEqual(ges, False)
+
 
 class ThoroughTestCase(unittest.TestCase):
     """Test the hard-to-reach parts of pulldom."""

Lib/test/test_sax.py

@@ -13,13 +13,14 @@ except SAXReaderNotAvailable:
 from xml.sax.saxutils import XMLGenerator, escape, unescape, quoteattr, \
                              XMLFilterBase, prepare_input_source
 from xml.sax.expatreader import create_parser
-from xml.sax.handler import feature_namespaces
+from xml.sax.handler import feature_namespaces, feature_external_ges
 from xml.sax.xmlreader import InputSource, AttributesImpl, AttributesNSImpl
 from io import BytesIO, StringIO
 import codecs
 import gc
 import os.path
 import shutil
+from urllib.error import URLError
 from test import support
 from test.support import findfile, run_unittest, TESTFN
 
@@ -911,6 +912,18 @@ class ExpatReaderTest(XmlTestBase):
         def unparsedEntityDecl(self, name, publicId, systemId, ndata):
             self._entities.append((name, publicId, systemId, ndata))
 
+
+    class TestEntityRecorder:
+        def __init__(self):
+            self.entities = []
+
+        def resolveEntity(self, publicId, systemId):
+            self.entities.append((publicId, systemId))
+            source = InputSource()
+            source.setPublicId(publicId)
+            source.setSystemId(systemId)
+            return source
+
     def test_expat_dtdhandler(self):
         parser = create_parser()
         handler = self.TestDTDHandler()
@@ -927,6 +940,32 @@ class ExpatReaderTest(XmlTestBase):
             [("GIF", "-//CompuServe//NOTATION Graphics Interchange Format 89a//EN", None)])
         self.assertEqual(handler._entities, [("img", None, "expat.gif", "GIF")])
 
+    def test_expat_external_dtd_enabled(self):
+        parser = create_parser()
+        parser.setFeature(feature_external_ges, True)
+        resolver = self.TestEntityRecorder()
+        parser.setEntityResolver(resolver)
+
+        with self.assertRaises(URLError):
+            parser.feed(
+                '<!DOCTYPE external SYSTEM "unsupported://non-existing">\n'
+            )
+        self.assertEqual(
+            resolver.entities, [(None, 'unsupported://non-existing')]
+        )
+
+    def test_expat_external_dtd_default(self):
+        parser = create_parser()
+        resolver = self.TestEntityRecorder()
+        parser.setEntityResolver(resolver)
+
+        parser.feed(
+            '<!DOCTYPE external SYSTEM "unsupported://non-existing">\n'
+        )
+        parser.feed('<doc />')
+        parser.close()
+        self.assertEqual(resolver.entities, [])
+
     # ===== EntityResolver support
 
     class TestEntityResolver:
@@ -936,8 +975,9 @@ class ExpatReaderTest(XmlTestBase):
             inpsrc.setByteStream(BytesIO(b"<entity/>"))
             return inpsrc
 
-    def test_expat_entityresolver(self):
+    def test_expat_entityresolver_enabled(self):
         parser = create_parser()
+        parser.setFeature(feature_external_ges, True)
         parser.setEntityResolver(self.TestEntityResolver())
         result = BytesIO()
         parser.setContentHandler(XMLGenerator(result))
@@ -951,6 +991,22 @@ class ExpatReaderTest(XmlTestBase):
         self.assertEqual(result.getvalue(), start +
                          b"<doc><entity></entity></doc>")
 
+    def test_expat_entityresolver_default(self):
+        parser = create_parser()
+        self.assertEqual(parser.getFeature(feature_external_ges), False)
+        parser.setEntityResolver(self.TestEntityResolver())
+        result = BytesIO()
+        parser.setContentHandler(XMLGenerator(result))
+
+        parser.feed('<!DOCTYPE doc [\n')
+        parser.feed('  <!ENTITY test SYSTEM "whatever">\n')
+        parser.feed(']>\n')
+        parser.feed('<doc>&test;</doc>')
+        parser.close()
+
+        self.assertEqual(result.getvalue(), start +
+                         b"<doc></doc>")
+
     # ===== Attributes support
 
     class AttrGatherer(ContentHandler):

Lib/test/test_xml_etree.py

@@ -91,6 +91,12 @@ ENTITY_XML = """\
 <document>&entity;</document>
 """
 
+EXTERNAL_ENTITY_XML = """\
+<!DOCTYPE points [
+<!ENTITY entity SYSTEM "file:///non-existing-file.xml">
+]>
+<document>&entity;</document>
+"""
 
 def checkwarnings(*filters, quiet=False):
     def decorator(test):
@@ -861,6 +867,13 @@ class ElementTreeTest(unittest.TestCase):
         root = parser.close()
         self.serialize_check(root, '<document>text</document>')
 
+        # 4) external (SYSTEM) entity
+
+        with self.assertRaises(ET.ParseError) as cm:
+            ET.XML(EXTERNAL_ENTITY_XML)
+        self.assertEqual(str(cm.exception),
+                'undefined entity &entity;: line 4, column 10')
+
     def test_namespace(self):
         # Test namespace issues.