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

Doc/library/xml.dom.pulldom.rst

@@ -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::
 

Doc/library/xml.rst

@@ -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

Doc/library/xml.sax.rst

@@ -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:
 

Doc/whatsnew/3.8.rst

@@ -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
 ------------------------