cpython/Doc/library/xml.rst
Victor Stinner cb99d99277
gh-127502: Remove XML vulnerability table (GH-135294)
* Remove the table
* Replace warnings with notes


Latest releases of Python 3.9-3.15 include expat 2.7.1 which is not vulnerable.

expat 2.6.0 was released in February 2024.
2025-07-07 11:03:07 +02:00

94 lines
3.2 KiB
ReStructuredText

.. _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.
.. 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
.. _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.
Expat versions lower that 2.6.0 may be vulnerable to "billion laughs",
"quadratic blowup" and "large tokens". Python may be vulnerable if it uses such
older versions of Expat as a system-provided library.
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`.
.. _Billion Laughs: https://en.wikipedia.org/wiki/Billion_laughs
.. _ZIP bomb: https://en.wikipedia.org/wiki/Zip_bomb