Doc/library/xml.rst - platform/external/python/cpython3 - Gitiles

 .. _xml:

 XML Processing Modules
 ======================

 .. module:: xml
    :synopsis: Package containing XML processing modules

 .. sectionauthor:: Christian Heimes <christian@python.org>
 .. sectionauthor:: Georg Brandl <georg@python.org>

 **Source code:** :source:`Lib/xml/`

 --------------

 Python's interfaces for processing XML are grouped in the ``xml`` package.

 .. warning::

    The XML modules are not secure against erroneous or maliciously
    constructed data.  If you need to parse untrusted or
    unauthenticated data see the :ref:`xml-vulnerabilities` and
    :ref:`defused-packages` sections.

 It is important to note that modules in the :mod:`xml` package require that
 there be at least one SAX-compliant XML parser available. The Expat parser is
 included with Python, so the :mod:`xml.parsers.expat` module will always be
 available.

 The documentation for the :mod:`xml.dom` and :mod:`xml.sax` packages are the
 definition of the Python bindings for the DOM and SAX interfaces.

 The XML handling submodules are:

 * :mod:`xml.etree.ElementTree`: the ElementTree API, a simple and lightweight
   XML processor

 ..

 * :mod:`xml.dom`: the DOM API definition
 * :mod:`xml.dom.minidom`: a minimal DOM implementation
 * :mod:`xml.dom.pulldom`: support for building partial DOM trees

 ..

 * :mod:`xml.sax`: SAX2 base classes and convenience functions
 * :mod:`xml.parsers.expat`: the Expat parser binding


 .. _xml-vulnerabilities:

 XML vulnerabilities
 -------------------

 The XML processing modules are not secure against maliciously constructed data.
 An attacker can abuse XML features to carry out denial of service attacks,
 access local files, generate network connections to other machines, or
 circumvent firewalls.

 The following table gives an overview of the known attacks and whether
 the various modules are vulnerable to them.

 =========================  ========  =========  =========  ========  =========
 kind                       sax       etree      minidom    pulldom   xmlrpc
 =========================  ========  =========  =========  ========  =========
 billion laughs             **Yes**   **Yes**    **Yes**    **Yes**   **Yes**
 quadratic blowup           **Yes**   **Yes**    **Yes**    **Yes**   **Yes**
 external entity expansion  **Yes**   No    (1)  No    (2)  **Yes**   No    (3)
 `DTD`_ retrieval           **Yes**   No         No         **Yes**   No
 decompression bomb         No        No         No         No        **Yes**
 =========================  ========  =========  =========  ========  =========

 1. :mod:`xml.etree.ElementTree` doesn't expand external entities and raises a
    :exc:`ParserError` when an entity occurs.
 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.


 billion laughs / exponential entity expansion
   The `Billion Laughs`_ attack -- also known as exponential entity expansion --
   uses multiple levels of nested entities. Each entity refers to another entity
   several times, and the final entity definition contains a small string.
   The exponential expansion results in several gigabytes of text and
   consumes lots of memory and CPU time.

 quadratic blowup entity expansion
   A quadratic blowup attack is similar to a `Billion Laughs`_ attack; it abuses
   entity expansion, too. Instead of nested entities it repeats one large entity
   with a couple of thousand chars over and over again. The attack isn't as
   efficient as the exponential case but it avoids triggering parser countermeasures
   that forbid deeply-nested entities.

 external entity expansion
   Entity declarations can contain more than just text for replacement. They can
   also point to external resources or local files. The XML
   parser accesses the resource and embeds the content into the XML document.

 `DTD`_ retrieval
   Some XML libraries like Python's :mod:`xml.dom.pulldom` retrieve document type
   definitions from remote or local locations. The feature has similar
   implications as the external entity expansion issue.

 decompression bomb
   Decompression bombs (aka `ZIP bomb`_) apply to all XML libraries
   that can parse compressed XML streams such as gzipped HTTP streams or
   LZMA-compressed
   files. For an attacker it can reduce the amount of transmitted data by three
   magnitudes or more.

 The documentation for `defusedxml`_ on PyPI has further information about
 all known attack vectors with examples and references.

 .. _defused-packages:

 The :mod:`defusedxml` and :mod:`defusedexpat` Packages
 ------------------------------------------------------

 `defusedxml`_ is a pure Python package with modified subclasses of all stdlib
 XML parsers that prevent any potentially malicious operation. Use of this
 package is recommended for any server code that parses untrusted XML data. The
 package also ships with example exploits and extended documentation on more
 XML exploits such as XPath injection.

 `defusedexpat`_ provides a modified libexpat and a patched
 :mod:`pyexpat` module that have countermeasures against entity expansion
 DoS attacks. The :mod:`defusedexpat` module still allows a sane and configurable amount of entity
 expansions. The modifications may be included in some future release of Python,
 but will not be included in any bugfix releases of
 Python because they break backward compatibility.


 .. _defusedxml: https://pypi.python.org/pypi/defusedxml/
 .. _defusedexpat: https://pypi.python.org/pypi/defusedexpat/
 .. _Billion Laughs: https://en.wikipedia.org/wiki/Billion_laughs
 .. _ZIP bomb: https://en.wikipedia.org/wiki/Zip_bomb
 .. _DTD: https://en.wikipedia.org/wiki/Document_type_definition
	.. _xml:

	XML Processing Modules
	======================

	.. module:: xml
	:synopsis: Package containing XML processing modules

	.. sectionauthor:: Christian Heimes <christian@python.org>
	.. sectionauthor:: Georg Brandl <georg@python.org>

	Source code: :source:`Lib/xml/`

	--------------

	Python's interfaces for processing XML are grouped in the ``xml`` package.

	.. warning::

	The XML modules are not secure against erroneous or maliciously
	constructed data. If you need to parse untrusted or
	unauthenticated data see the :ref:`xml-vulnerabilities` and
	:ref:`defused-packages` sections.

	It is important to note that modules in the :mod:`xml` package require that
	there be at least one SAX-compliant XML parser available. The Expat parser is
	included with Python, so the :mod:`xml.parsers.expat` module will always be
	available.

	The documentation for the :mod:`xml.dom` and :mod:`xml.sax` packages are the
	definition of the Python bindings for the DOM and SAX interfaces.

	The XML handling submodules are:

	* :mod:`xml.etree.ElementTree`: the ElementTree API, a simple and lightweight
	XML processor

	..

	* :mod:`xml.dom`: the DOM API definition
	* :mod:`xml.dom.minidom`: a minimal DOM implementation
	* :mod:`xml.dom.pulldom`: support for building partial DOM trees

	..

	* :mod:`xml.sax`: SAX2 base classes and convenience functions
	* :mod:`xml.parsers.expat`: the Expat parser binding


	.. _xml-vulnerabilities:

	XML vulnerabilities
	-------------------

	The XML processing modules are not secure against maliciously constructed data.
	An attacker can abuse XML features to carry out denial of service attacks,
	access local files, generate network connections to other machines, or
	circumvent firewalls.

	The following table gives an overview of the known attacks and whether
	the various modules are vulnerable to them.

	========================= ======== ========= ========= ======== =========
	kind sax etree minidom pulldom xmlrpc
	========================= ======== ========= ========= ======== =========
	billion laughs Yes Yes Yes Yes Yes
	quadratic blowup Yes Yes Yes Yes Yes
	external entity expansion Yes No (1) No (2) Yes No (3)
	`DTD`_ retrieval Yes No No Yes No
	decompression bomb No No No No Yes
	========================= ======== ========= ========= ======== =========

	1. :mod:`xml.etree.ElementTree` doesn't expand external entities and raises a
	:exc:`ParserError` when an entity occurs.
	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.


	billion laughs / exponential entity expansion
	The `Billion Laughs`_ attack -- also known as exponential entity expansion --
	uses multiple levels of nested entities. Each entity refers to another entity
	several times, and the final entity definition contains a small string.
	The exponential expansion results in several gigabytes of text and
	consumes lots of memory and CPU time.

	quadratic blowup entity expansion
	A quadratic blowup attack is similar to a `Billion Laughs`_ attack; it abuses
	entity expansion, too. Instead of nested entities it repeats one large entity
	with a couple of thousand chars over and over again. The attack isn't as
	efficient as the exponential case but it avoids triggering parser countermeasures
	that forbid deeply-nested entities.

	external entity expansion
	Entity declarations can contain more than just text for replacement. They can
	also point to external resources or local files. The XML
	parser accesses the resource and embeds the content into the XML document.

	`DTD`_ retrieval
	Some XML libraries like Python's :mod:`xml.dom.pulldom` retrieve document type
	definitions from remote or local locations. The feature has similar
	implications as the external entity expansion issue.

	decompression bomb
	Decompression bombs (aka `ZIP bomb`_) apply to all XML libraries
	that can parse compressed XML streams such as gzipped HTTP streams or
	LZMA-compressed
	files. For an attacker it can reduce the amount of transmitted data by three
	magnitudes or more.

	The documentation for `defusedxml`_ on PyPI has further information about
	all known attack vectors with examples and references.

	.. _defused-packages:

	The :mod:`defusedxml` and :mod:`defusedexpat` Packages
	------------------------------------------------------

	`defusedxml`_ is a pure Python package with modified subclasses of all stdlib
	XML parsers that prevent any potentially malicious operation. Use of this
	package is recommended for any server code that parses untrusted XML data. The
	package also ships with example exploits and extended documentation on more
	XML exploits such as XPath injection.

	`defusedexpat`_ provides a modified libexpat and a patched
	:mod:`pyexpat` module that have countermeasures against entity expansion
	DoS attacks. The :mod:`defusedexpat` module still allows a sane and configurable amount of entity
	expansions. The modifications may be included in some future release of Python,
	but will not be included in any bugfix releases of
	Python because they break backward compatibility.


	.. _defusedxml: https://pypi.python.org/pypi/defusedxml/
	.. _defusedexpat: https://pypi.python.org/pypi/defusedexpat/
	.. _Billion Laughs: https://en.wikipedia.org/wiki/Billion_laughs
	.. _ZIP bomb: https://en.wikipedia.org/wiki/Zip_bomb
	.. _DTD: https://en.wikipedia.org/wiki/Document_type_definition