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
diff --git a/Doc/library/xml.dom.pulldom.rst b/Doc/library/xml.dom.pulldom.rst
index 56f545c..eb2b16b 100644
--- a/Doc/library/xml.dom.pulldom.rst
+++ b/Doc/library/xml.dom.pulldom.rst
@@ -25,6 +25,20 @@
maliciously constructed data. If you need to parse untrusted or
unauthenticated data see :ref:`xml-vulnerabilities`.
+.. versionchanged:: 3.8
+
+ The SAX parser no longer processes general external entities by default to
+ increase security by default. To enable processing of external entities,
+ pass a custom parser instance in::
+
+ from xml.dom.pulldom import parse
+ from xml.sax import make_parser
+ from xml.sax.handler import feature_external_ges
+
+ parser = make_parser()
+ parser.setFeature(feature_external_ges, True)
+ parse(filename, parser=parser)
+
Example::
diff --git a/Doc/library/xml.rst b/Doc/library/xml.rst
index 63c24f8..9b8ba6b 100644
--- a/Doc/library/xml.rst
+++ b/Doc/library/xml.rst
@@ -65,8 +65,8 @@
========================= ============== =============== ============== ============== ==============
billion laughs **Vulnerable** **Vulnerable** **Vulnerable** **Vulnerable** **Vulnerable**
quadratic blowup **Vulnerable** **Vulnerable** **Vulnerable** **Vulnerable** **Vulnerable**
-external entity expansion **Vulnerable** Safe (1) Safe (2) **Vulnerable** Safe (3)
-`DTD`_ retrieval **Vulnerable** Safe Safe **Vulnerable** Safe
+external entity expansion Safe (4) Safe (1) Safe (2) Safe (4) Safe (3)
+`DTD`_ retrieval Safe (4) Safe Safe Safe (4) Safe
decompression bomb Safe Safe Safe Safe **Vulnerable**
========================= ============== =============== ============== ============== ==============
@@ -75,6 +75,8 @@
2. :mod:`xml.dom.minidom` doesn't expand external entities and simply returns
the unexpanded entity verbatim.
3. :mod:`xmlrpclib` doesn't expand external entities and omits them.
+4. Since Python 3.8.0, external general entities are no longer processed by
+ default since Python.
billion laughs / exponential entity expansion
diff --git a/Doc/library/xml.sax.rst b/Doc/library/xml.sax.rst
index 78d6633..aa3ea9b 100644
--- a/Doc/library/xml.sax.rst
+++ b/Doc/library/xml.sax.rst
@@ -24,6 +24,14 @@
constructed data. If you need to parse untrusted or unauthenticated data see
:ref:`xml-vulnerabilities`.
+.. versionchanged:: 3.8
+
+ 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. The feature can be enabled again with method
+ :meth:`~xml.sax.xmlreader.XMLReader.setFeature` on the parser object
+ and argument :data:`~xml.sax.handler.feature_external_ges`.
The convenience functions are:
diff --git a/Doc/whatsnew/3.8.rst b/Doc/whatsnew/3.8.rst
index 9aaaa76..e37a70f 100644
--- a/Doc/whatsnew/3.8.rst
+++ b/Doc/whatsnew/3.8.rst
@@ -155,6 +155,15 @@
activating virtual environments under PowerShell Core 6.1.
(Contributed by Brett Cannon in :issue:`32718`.)
+xml
+---
+
+* As mitigation against DTD and external entity retrieval, the
+ :mod:`xml.dom.minidom` and mod:`xml.sax` modules no longer process
+ external entities by default.
+ (Contributed by Christian Heimes in :issue:`17239`.)
+
+
Optimizations
=============
@@ -333,6 +342,9 @@
* :class:`uuid.UUID` now uses ``__slots__``, therefore instances can no longer
be weak-referenced and attributes can no longer be added.
+* :mod:`xml.dom.minidom` and mod:`xml.sax` modules no longer process
+ external entities by default.
+ (Contributed by Christian Heimes in :issue:`17239`.)
CPython bytecode changes
------------------------
diff --git a/Lib/test/test_pulldom.py b/Lib/test/test_pulldom.py
index f454098..4a1bad3 100644
--- a/Lib/test/test_pulldom.py
+++ b/Lib/test/test_pulldom.py
@@ -3,6 +3,7 @@
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 @@
# 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."""
diff --git a/Lib/test/test_sax.py b/Lib/test/test_sax.py
index 2eb6290..3044960 100644
--- a/Lib/test/test_sax.py
+++ b/Lib/test/test_sax.py
@@ -13,13 +13,14 @@
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 @@
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 @@
[("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 @@
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 @@
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):
diff --git a/Lib/test/test_xml_etree.py b/Lib/test/test_xml_etree.py
index a525290..ecb910f 100644
--- a/Lib/test/test_xml_etree.py
+++ b/Lib/test/test_xml_etree.py
@@ -91,6 +91,12 @@
<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 @@
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.
diff --git a/Lib/xml/sax/expatreader.py b/Lib/xml/sax/expatreader.py
index 421358f..5066ffc 100644
--- a/Lib/xml/sax/expatreader.py
+++ b/Lib/xml/sax/expatreader.py
@@ -95,7 +95,7 @@
self._lex_handler_prop = None
self._parsing = 0
self._entity_stack = []
- self._external_ges = 1
+ self._external_ges = 0
self._interning = None
# XMLReader methods
diff --git a/Misc/NEWS.d/next/Security/2018-09-11-18-30-55.bpo-17239.kOpwK2.rst b/Misc/NEWS.d/next/Security/2018-09-11-18-30-55.bpo-17239.kOpwK2.rst
new file mode 100644
index 0000000..8dd0fe8
--- /dev/null
+++ b/Misc/NEWS.d/next/Security/2018-09-11-18-30-55.bpo-17239.kOpwK2.rst
@@ -0,0 +1,3 @@
+The xml.sax and xml.dom.minidom parsers no longer processes external
+entities by default. External DTD and ENTITY declarations no longer
+load files or create network connections.