From 1f6b2bc233d4bd3d73d5bfa6722f9a499067479d Mon Sep 17 00:00:00 2001
From: Ronald Oussoren <ronaldoussoren@mac.com>
Date: Mon, 19 Oct 2020 20:13:49 +0200
Subject: [PATCH] bpo-42051: Reject XML entity declarations in plist files
 (#22760)

---
 Lib/plistlib.py                                |  7 +++++++
 Lib/test/test_plistlib.py                      | 18 ++++++++++++++++++
 .../2020-10-19-10-56-27.bpo-42051.EU_B7u.rst   |  3 +++
 3 files changed, 28 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Security/2020-10-19-10-56-27.bpo-42051.EU_B7u.rst

diff --git a/Lib/plistlib.py b/Lib/plistlib.py
index 33b79a133b0a84..b273a15645d949 100644
--- a/Lib/plistlib.py
+++ b/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)
diff --git a/Lib/test/test_plistlib.py b/Lib/test/test_plistlib.py
index 8d8e0a750a224b..bfe06fd35831d9 100644
--- a/Lib/test/test_plistlib.py
+++ b/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):
 
diff --git a/Misc/NEWS.d/next/Security/2020-10-19-10-56-27.bpo-42051.EU_B7u.rst b/Misc/NEWS.d/next/Security/2020-10-19-10-56-27.bpo-42051.EU_B7u.rst
new file mode 100644
index 00000000000000..e865ed12a03878
--- /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.