gh-90949: add Expat API to prevent XML deadly allocations

Doc/library/pyexpat.rst

@@ -231,7 +231,55 @@
    .. versionadded:: 3.13
 
 
+:class:`xmlparser` objects have the following methods to mitigate some
+common XML vulnerabilities.
+
+.. method:: xmlparser.SetAllocTrackerActivationThreshold(threshold, /)
+
+   Sets the number of allocated bytes of dynamic memory needed to activate
+   protection against disproportionate use of RAM.
+
+   By default, parser objects have an allocation activation threshold of 64 MiB,
+   or equivalently 67,108,864 bytes.
+
+   An :exc:`ExpatError` is raised if this method is called on a non-root parser.
+   The corresponding :attr:`~ExpatError.lineno` and :attr:`~ExpatError.offset`
+   should not be used as they may have no special meaning.
+
+   .. versionadded:: next
+
+.. method:: xmlparser.SetAllocTrackerMaximumAmplification(max_factor, /)
+
+   Sets the maximum amplification factor between direct input and bytes
+   of dynamic memory allocated.
+
+   The amplification factor is calculated as ``allocated / direct``
+   while parsing, where ``direct`` is the number of bytes read from
+   the primary document in parsing and ``allocated`` is the number
+   of bytes of dynamic memory allocated in the parser hierarchy.
+
+   The *max_factor* value must be a non-NaN :class:`float` value greater than
+   or equal to 1.0. Amplification factors greater than 100.0 can be observed
+   near the start of parsing even with benign files in practice. In particular,
+   the activation threshold should be carefully chosen to avoid false positives.
+
+   By default, parser objects have a maximum amplification factor of 100.0.
+
+   An :exc:`ExpatError` is raised if this method is called on a non-root
+   parser or if *max_factor* is outside the valid range.
+   The corresponding :attr:`~ExpatError.lineno` and :attr:`~ExpatError.offset`
+   should not be used as they may have no special meaning.
+
+   .. note::
+
+      The maximum amplification factor is only considered if the threshold
+      that can be adjusted :meth:`.SetAllocTrackerActivationThreshold` is
+      exceeded.
+
+   .. versionadded:: next
+
+
 :class:`xmlparser` objects have the following attributes:
 
 
 .. attribute:: xmlparser.buffer_size

Doc/whatsnew/3.15.rst

@@ -540,6 +540,16 @@ unittest
   (Contributed by Garry Cairns in :gh:`134567`.)
 
 
+xml.parsers.expat
+-----------------
+
+* Add :func:`~xml.parsers.expat.xmlparser.SetAllocTrackerActivationThreshold`
+  and :func:`~xml.parsers.expat.xmlparser.SetAllocTrackerMaximumAmplification`
+  to :ref:`xmlparser <xmlparser-objects>` objects to prevent use of
+  disproportional amounts of dynamic memory from within an Expat parser.
+  (Contributed by Bénédikt Tran in :gh:`90949`.)
+
+
 zlib
 ----
 

Lib/test/test_pyexpat.py

@@ -1,14 +1,17 @@
 # XXX TypeErrors on calling handlers, or on bad return values from a
 # handler, are obscure and unhelpful.
 
+import abc
+import functools
 import os
+import re
 import sys
 import sysconfig
 import unittest
 import traceback
 from io import BytesIO
 from test import support
-from test.support import os_helper
+from test.support import import_helper, os_helper
 from test.support import sortdict
 from unittest import mock
 from xml.parsers import expat
@@ -821,5 +824,181 @@ def start_element(name, _):
         self.assertEqual(started, ['doc'])
 
 
+class AttackProtectionTestBase(abc.ABC):
+    """
+    Base class for testing protections against XML payloads with
+    disproportionate amplification.
+
+    The protections being tested should detect and prevent attacks
+    that leverage disproportionate amplification from small inputs.
+    """
+
+    @staticmethod
+    def exponential_expansion_payload(ncols, nrows, text='.'):
+        """Create a billion laughs attack payload.
+
+        Be careful: the number of total items is pow(n, k), thereby
+        requiring at least pow(ncols, nrows) * sizeof(text) memory!
+        """
+        body = '\n'.join(
+            f'<!ENTITY row{i + 1} "{f"&row{i};" * ncols}">'
+            for i in range(nrows)
+        )
+        return f"""\
+<?xml version="1.0"?>
+<!DOCTYPE doc [
+<!ENTITY row0 "{text}">
+<!ELEMENT doc (#PCDATA)>
+{body}
+]>
+<doc>&row{nrows};</doc>"""
+
+    def assert_root_parser_failure(self, func, /, *args, **kwargs):
+        """Check that func(*args, **kwargs) is invalid for a sub-parser."""
+        msg = "parser must be a root parser"
+        self.assertRaisesRegex(expat.ExpatError, msg, func, *args, **kwargs)
+
+    @abc.abstractmethod
+    def assert_rejected(self, func, /, *args, **kwargs):
+        """Assert that func(*args, **kwargs) triggers the attack protection.
+
+        Note: this method must ensure that the attack protection being tested
+        is the one that is actually triggered at runtime, e.g., by matching
+        the exact error message.
+        """
+
+    @abc.abstractmethod
+    def set_activation_threshold(self, parser, threshold):
+        """Set the activation threshold for the tested protection."""
+
+    @abc.abstractmethod
+    def set_maximum_amplification(self, parser, max_factor):
+        """Set the maximum amplification factor for the tested protection."""
+
+    @abc.abstractmethod
+    def test_set_activation_threshold__threshold_reached(self):
+        """Test when the activation threshold is exceeded."""
+
+    @abc.abstractmethod
+    def test_set_activation_threshold__threshold_not_reached(self):
+        """Test when the activation threshold is not exceeded."""
+
+    def test_set_activation_threshold__invalid_threshold_type(self):
+        parser = expat.ParserCreate()
+        setter = functools.partial(self.set_activation_threshold, parser)
+
+        self.assertRaises(TypeError, setter, 1.0)
+        self.assertRaises(TypeError, setter, -1.5)
+        self.assertRaises(ValueError, setter, -5)
+
+    def test_set_activation_threshold__invalid_threshold_range(self):
+        _testcapi = import_helper.import_module("_testcapi")
+        parser = expat.ParserCreate()
+        setter = functools.partial(self.set_activation_threshold, parser)
+
+        self.assertRaises(OverflowError, setter, _testcapi.ULLONG_MAX + 1)
+
+    def test_set_activation_threshold__fail_for_subparser(self):
+        parser = expat.ParserCreate()
+        subparser = parser.ExternalEntityParserCreate(None)
+        setter = functools.partial(self.set_activation_threshold, subparser)
+        self.assert_root_parser_failure(setter, 12345)
+
+    @abc.abstractmethod
+    def test_set_maximum_amplification__amplification_exceeded(self):
+        """Test when the amplification factor is exceeded."""
+
+    @abc.abstractmethod
+    def test_set_maximum_amplification__amplification_not_exceeded(self):
+        """Test when the amplification factor is not exceeded."""
+
+    def test_set_maximum_amplification__infinity(self):
+        inf = float('inf')  # an 'inf' threshold is allowed by Expat
+        parser = expat.ParserCreate()
+        self.assertIsNone(self.set_maximum_amplification(parser, inf))
+
+    def test_set_maximum_amplification__invalid_max_factor_type(self):
+        parser = expat.ParserCreate()
+        setter = functools.partial(self.set_maximum_amplification, parser)
+
+        self.assertRaises(TypeError, setter, None)
+        self.assertRaises(TypeError, setter, 'abc')
+
+    def test_set_maximum_amplification__invalid_max_factor_range(self):
+        parser = expat.ParserCreate()
+        setter = functools.partial(self.set_maximum_amplification, parser)
+
+        msg = re.escape("'max_factor' must be at least 1.0")
+        self.assertRaisesRegex(expat.ExpatError, msg, setter, float('nan'))
+        self.assertRaisesRegex(expat.ExpatError, msg, setter, 0.99)
+
+    def test_set_maximum_amplification__fail_for_subparser(self):
+        parser = expat.ParserCreate()
+        subparser = parser.ExternalEntityParserCreate(None)
+        setter = functools.partial(self.set_maximum_amplification, subparser)
+        self.assert_root_parser_failure(setter, 123.45)
+
+
+@unittest.skipIf(expat.version_info < (2, 7, 2), "requires Expat >= 2.7.2")
+class MemoryProtectionTest(AttackProtectionTestBase, unittest.TestCase):
+
+    # With the default Expat configuration, the billion laughs protection may
+    # hit before the allocation limiter if exponential_expansion_payload() is
+    # not carefully parametrized. As such, the payloads should be chosen so
+    # that either the allocation limiter is hit before other protections are
+    # triggered or no protection at all is triggered.
+
+    def assert_rejected(self, func, /, *args, **kwargs):
+        """Check that func(*args, **kwargs) hits the allocation limit."""
+        msg = r"out of memory: line \d+, column \d+"
+        self.assertRaisesRegex(expat.ExpatError, msg, func, *args, **kwargs)
+
+    def set_activation_threshold(self, parser, threshold):
+        return parser.SetAllocTrackerActivationThreshold(threshold)
+
+    def set_maximum_amplification(self, parser, max_factor):
+        return parser.SetAllocTrackerMaximumAmplification(max_factor)
+
+    def test_set_activation_threshold__threshold_reached(self):
+        parser = expat.ParserCreate()
+        # Choose a threshold expected to be always reached.
+        self.set_activation_threshold(parser, 3)
+        # Check that the threshold is reached by choosing a small factor
+        # and a payload whose peak amplification factor exceeds it.
+        self.assertIsNone(self.set_maximum_amplification(parser, 1.0))
+        payload = self.exponential_expansion_payload(10, 4)
+        self.assert_rejected(parser.Parse, payload, True)
+
+    def test_set_activation_threshold__threshold_not_reached(self):
+        parser = expat.ParserCreate()
+        # Choose a threshold expected to be never reached.
+        self.set_activation_threshold(parser, pow(10, 5))
+        # Check that the threshold is reached by choosing a small factor
+        # and a payload whose peak amplification factor exceeds it.
+        self.assertIsNone(self.set_maximum_amplification(parser, 1.0))
+        payload = self.exponential_expansion_payload(10, 4)
+        self.assertIsNotNone(parser.Parse(payload, True))
+
+    def test_set_maximum_amplification__amplification_exceeded(self):
+        parser = expat.ParserCreate()
+        # Unconditionally enable maximum activation factor.
+        self.set_activation_threshold(parser, 0)
+        # Choose a max amplification factor expected to always be exceeded.
+        self.assertIsNone(self.set_maximum_amplification(parser, 1.0))
+        # Craft a payload for which the peak amplification factor is > 1.0.
+        payload = self.exponential_expansion_payload(1, 2)
+        self.assert_rejected(parser.Parse, payload, True)
+
+    def test_set_maximum_amplification__amplification_not_exceeded(self):
+        parser = expat.ParserCreate()
+        # Unconditionally enable maximum activation factor.
+        self.set_activation_threshold(parser, 0)
+        # Choose a max amplification factor expected to never be exceeded.
+        self.assertIsNone(self.set_maximum_amplification(parser, 1e4))
+        # Craft a payload for which the peak amplification factor is < 1e4.
+        payload = self.exponential_expansion_payload(1, 2)
+        self.assertIsNotNone(parser.Parse(payload, True))
+
+
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Library/2025-09-22-14-40-11.gh-issue-90949.UM35nb.rst

@@ -0,0 +1,5 @@
+Add :func:`~xml.parsers.expat.xmlparser.SetAllocTrackerActivationThreshold`
+and :func:`~xml.parsers.expat.xmlparser.SetAllocTrackerMaximumAmplification`
+to :ref:`xmlparser <xmlparser-objects>` objects to prevent use of
+disproportional amounts of dynamic memory from within an Expat parser. Patch
+by Bénédikt Tran.