bpo-42051: Reject XML entity declarations in plist files (python#22760) (pythonGH-22801)

Co-authored-by: Ronald Oussoren <ronaldoussoren@mac.com>

Author: ned-deily

Lib/plistlib.py

@@ -257,9 +257,16 @@ def parse(self, fileobj):
  self.parser.StartElementHandler = self.handle_begin_element
  self.parser.EndElementHandler = self.handle_end_element
  self.parser.CharacterDataHandler = self.handle_data
+ self.parser.EntityDeclHandler = self.handle_entity_decl
  self.parser.ParseFile(fileobj)
  return self.root
 
+ def handle_entity_decl(self, entity_name, is_parameter_entity, value, base, system_id, public_id, notation_name):
+ # Reject plist files with entity declarations to avoid XML vulnerabilies in expat.
+ # Regular plist files don't contain those declerations, and Apple's plutil tool does not
+ # accept them either.
+ raise InvalidFileException("XML entity declarations are not supported in plist files")
+
  def handle_begin_element(self, element, attrs):
  self.data = []
  handler = getattr(self, "begin_" + element, None)

Lib/test/test_plistlib.py

@@ -90,6 +90,19 @@
  xQHHAsQC0gAAAAAAAAIBAAAAAAAAADkAAAAAAAAAAAAAAAAAAALs'''),
 }
 
+XML_PLIST_WITH_ENTITY=b'''\
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd" [
+ <!ENTITY entity "replacement text">
+ ]>
+<plist version="1.0">
+ <dict>
+ <key>A</key>
+ <string>&entity;</string>
+ </dict>
+</plist>
+'''
+
 
 class TestPlistlib(unittest.TestCase):
 
@@ -443,6 +456,11 @@ def test_xml_encodings(self):
  pl2 = plistlib.loads(data)
  self.assertEqual(dict(pl), dict(pl2))
 
+ def test_xml_plist_with_entity_decl(self):
+ with self.assertRaisesRegex(plistlib.InvalidFileException,
+ "XML entity declarations are not supported"):
+ plistlib.loads(XML_PLIST_WITH_ENTITY, fmt=plistlib.FMT_XML)
+
 
 class TestBinaryPlistlib(unittest.TestCase):
 

Misc/NEWS.d/next/Security/2020-10-19-10-56-27.bpo-42051.EU_B7u.rst

@@ -0,0 +1,3 @@
+The :mod:`plistlib` module no longer accepts entity declarations in XML
+plist files to avoid XML vulnerabilities. This should not affect users as
+entity declarations are not used in regular plist files.