Doc/library/xml.rst

.. _xml:

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

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

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

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

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

.. note::

   If you need to parse untrusted or unauthenticated data, see
   :ref:`xml-security`.

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

This module also defines utility functions.

.. function:: is_valid_name(name)

   Return ``True`` if the string is a valid element or attribute name,
   ``False`` otherwise.

   Almost all characters are permitted in names, except control characters and
   those which either are or reasonably could be used as delimiters.
   Characters like ":", "-", ".", "_", and "·" are permitted, but "<", "/",
   "!", "?", and "=" are forbidden.
   The name cannot start with a digit or a character like "-", ".", and "·".

   .. versionadded:: 3.15


.. function:: is_valid_text(data)

   Return ``True`` if the string is a sequence of legal XML 1.0 characters,
   ``False`` otherwise.

   Almost all characters are permitted in XML 1.0 documents, except C0 control
   characters (excluding TAB, CR and LF), surrogate characters and special
   Unicode characters U+FFFE and U+FFFF.

   .. versionadded:: 3.15


.. _xml-security:
.. _xml-vulnerabilities:

XML security
------------

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 when attacker-controlled XML is being parsed,
in Python or elsewhere.

The built-in XML parsers of Python rely on the library `libexpat`_, commonly
called Expat, for parsing XML.

By default, Expat itself does not access local files or create network
connections.

Expat versions lower than 2.7.2 may be vulnerable to the "billion laughs",
"quadratic blowup" and "large tokens" vulnerabilities, or to disproportional
use of dynamic memory.
Python bundles a copy of Expat, and whether Python uses the bundled or a
system-wide Expat, depends on how the Python interpreter
:option:`has been configured <--with-system-expat>` in your environment.
Python may be vulnerable if it uses such older versions of Expat.
Check :const:`!pyexpat.EXPAT_VERSION`.

:mod:`xmlrpc` is **vulnerable** to the "decompression bomb" attack.


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.

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.

large tokens
  Expat needs to re-parse unfinished tokens; without the protection
  introduced in Expat 2.6.0, this can lead to quadratic runtime that can
  be used to cause denial of service in the application parsing XML.
  The issue is known as :cve:`2023-52425`.

.. _libexpat: https://github.com/libexpat/libexpat
.. _Billion Laughs: https://en.wikipedia.org/wiki/Billion_laughs
.. _ZIP bomb: https://en.wikipedia.org/wiki/Zip_bomb