From 4d8f9e2e4461de92bd1e0c92ed433480d761670f Mon Sep 17 00:00:00 2001
From: Ned Deily <nad@python.org>
Date: Mon, 19 Oct 2020 22:36:27 -0400
Subject: [PATCH] bpo-42051: Reject XML entity declarations in plist files
 (GH-22760) (GH-22801)

Co-authored-by: Ronald Oussoren <ronaldoussoren@mac.com>
(cherry picked from commit e512bc799e3864fe3b1351757261762d63471efc)

Co-authored-by: Ned Deily <nad@python.org>
---
 Lib/plistlib.py                                                    |   10 +++++
 Lib/test/test_plistlib.py                                          |   19 ++++++++++
 Misc/NEWS.d/next/Security/2020-10-19-10-56-27.bpo-42051.EU_B7u.rst |    3 +
 3 files changed, 32 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Security/2020-10-19-10-56-27.bpo-42051.EU_B7u.rst

--- a/Lib/plistlib.py
+++ b/Lib/plistlib.py
@@ -403,9 +403,19 @@ class PlistParser:
         parser.StartElementHandler = self.handleBeginElement
         parser.EndElementHandler = self.handleEndElement
         parser.CharacterDataHandler = self.handleData
+        parser.EntityDeclHandler = self.handle_entity_decl
         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 ValueError(
+            "XML entity declarations are not supported in plist files")
+
     def handleBeginElement(self, element, attrs):
         self.data = []
         handler = getattr(self, "begin_" + element, None)
--- a/Lib/test/test_plistlib.py
+++ b/Lib/test/test_plistlib.py
@@ -86,6 +86,19 @@ TESTDATA = """<?xml version="1.0" encodi
 </plist>
 """.replace(" " * 8, "\t")  # Apple as well as plistlib.py output hard tabs
 
+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):
 
@@ -195,6 +208,12 @@ class TestPlistlib(unittest.TestCase):
         self.assertEqual(test1, result1)
         self.assertEqual(test2, result2)
 
+    def test_xml_plist_with_entity_decl(self):
+        with self.assertRaisesRegexp(ValueError,
+                                     "XML entity declarations are not supported"):
+            plistlib.readPlistFromString(XML_PLIST_WITH_ENTITY)
+
+
 
 def test_main():
     test_support.run_unittest(TestPlistlib)
--- /dev/null
+++ b/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.