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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
This commit is contained in:
Christian Heimes 2018-09-23 09:50:25 +02:00 committed by Miss Islington (bot)
parent 9fb051f032
commit 17b1d5d4e3
9 changed files with 120 additions and 5 deletions
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14
Doc/library/xml.dom.pulldom.rst
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@ -25,6 +25,20 @@ events until either processing is finished or an error condition occurs.
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::

6
Doc/library/xml.rst
View File

@ -65,8 +65,8 @@ kind sax etree minidom p
========================= ============== =============== ============== ============== ==============
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 @@ decompression bomb Safe Safe Safe S
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

8
Doc/library/xml.sax.rst
View File

@ -24,6 +24,14 @@ the SAX API.
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:

12
Doc/whatsnew/3.8.rst
View File

@ -155,6 +155,15 @@ venv
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 @@ Changes in the Python API
* :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
------------------------

7
Lib/test/test_pulldom.py
View File

@ -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."""

60
Lib/test/test_sax.py
View File

@ -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):

13
Lib/test/test_xml_etree.py
View File

@ -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.

2
Lib/xml/sax/expatreader.py
View File

@ -95,7 +95,7 @@ class ExpatParser(xmlreader.IncrementalParser, xmlreader.Locator):
self._lex_handler_prop = None
self._parsing = 0
self._entity_stack = []
self._external_ges = 1
self._external_ges = 0
self._interning = None
# XMLReader methods

3
Misc/NEWS.d/next/Security/2018-09-11-18-30-55.bpo-17239.kOpwK2.rst Normal file
View File

@ -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.