1.. _xml:
2
3XML Processing Modules
4======================
5
6.. module:: xml
7   :synopsis: Package containing XML processing modules
8
9.. sectionauthor:: Christian Heimes <christian@python.org>
10.. sectionauthor:: Georg Brandl <georg@python.org>
11
12**Source code:** :source:`Lib/xml/`
13
14--------------
15
16Python's interfaces for processing XML are grouped in the ``xml`` package.
17
18.. warning::
19
20   The XML modules are not secure against erroneous or maliciously
21   constructed data.  If you need to parse untrusted or
22   unauthenticated data see the :ref:`xml-vulnerabilities` and
23   :ref:`defusedxml-package` sections.
24
25It is important to note that modules in the :mod:`xml` package require that
26there be at least one SAX-compliant XML parser available. The Expat parser is
27included with Python, so the :mod:`xml.parsers.expat` module will always be
28available.
29
30The documentation for the :mod:`xml.dom` and :mod:`xml.sax` packages are the
31definition of the Python bindings for the DOM and SAX interfaces.
32
33The XML handling submodules are:
34
35* :mod:`xml.etree.ElementTree`: the ElementTree API, a simple and lightweight
36  XML processor
37
38..
39
40* :mod:`xml.dom`: the DOM API definition
41* :mod:`xml.dom.minidom`: a minimal DOM implementation
42* :mod:`xml.dom.pulldom`: support for building partial DOM trees
43
44..
45
46* :mod:`xml.sax`: SAX2 base classes and convenience functions
47* :mod:`xml.parsers.expat`: the Expat parser binding
48
49
50.. _xml-vulnerabilities:
51
52XML vulnerabilities
53-------------------
54
55The XML processing modules are not secure against maliciously constructed data.
56An attacker can abuse XML features to carry out denial of service attacks,
57access local files, generate network connections to other machines, or
58circumvent firewalls.
59
60The following table gives an overview of the known attacks and whether
61the various modules are vulnerable to them.
62
63=========================  ==============   ===============   ==============   ==============   ==============
64kind                       sax              etree             minidom          pulldom          xmlrpc
65=========================  ==============   ===============   ==============   ==============   ==============
66billion laughs             **Vulnerable**   **Vulnerable**    **Vulnerable**   **Vulnerable**   **Vulnerable**
67quadratic blowup           **Vulnerable**   **Vulnerable**    **Vulnerable**   **Vulnerable**   **Vulnerable**
68external entity expansion  Safe (4)         Safe    (1)       Safe    (2)      Safe (4)         Safe    (3)
69`DTD`_ retrieval           Safe (4)         Safe              Safe             Safe (4)         Safe
70decompression bomb         Safe             Safe              Safe             Safe             **Vulnerable**
71=========================  ==============   ===============   ==============   ==============   ==============
72
731. :mod:`xml.etree.ElementTree` doesn't expand external entities and raises a
74   :exc:`ParserError` when an entity occurs.
752. :mod:`xml.dom.minidom` doesn't expand external entities and simply returns
76   the unexpanded entity verbatim.
773. :mod:`xmlrpclib` doesn't expand external entities and omits them.
784. Since Python 3.7.1, external general entities are no longer processed by
79   default.
80
81
82billion laughs / exponential entity expansion
83  The `Billion Laughs`_ attack -- also known as exponential entity expansion --
84  uses multiple levels of nested entities. Each entity refers to another entity
85  several times, and the final entity definition contains a small string.
86  The exponential expansion results in several gigabytes of text and
87  consumes lots of memory and CPU time.
88
89quadratic blowup entity expansion
90  A quadratic blowup attack is similar to a `Billion Laughs`_ attack; it abuses
91  entity expansion, too. Instead of nested entities it repeats one large entity
92  with a couple of thousand chars over and over again. The attack isn't as
93  efficient as the exponential case but it avoids triggering parser countermeasures
94  that forbid deeply-nested entities.
95
96external entity expansion
97  Entity declarations can contain more than just text for replacement. They can
98  also point to external resources or local files. The XML
99  parser accesses the resource and embeds the content into the XML document.
100
101`DTD`_ retrieval
102  Some XML libraries like Python's :mod:`xml.dom.pulldom` retrieve document type
103  definitions from remote or local locations. The feature has similar
104  implications as the external entity expansion issue.
105
106decompression bomb
107  Decompression bombs (aka `ZIP bomb`_) apply to all XML libraries
108  that can parse compressed XML streams such as gzipped HTTP streams or
109  LZMA-compressed
110  files. For an attacker it can reduce the amount of transmitted data by three
111  magnitudes or more.
112
113The documentation for `defusedxml`_ on PyPI has further information about
114all known attack vectors with examples and references.
115
116.. _defusedxml-package:
117
118The :mod:`defusedxml` Package
119------------------------------------------------------
120
121`defusedxml`_ is a pure Python package with modified subclasses of all stdlib
122XML parsers that prevent any potentially malicious operation. Use of this
123package is recommended for any server code that parses untrusted XML data. The
124package also ships with example exploits and extended documentation on more
125XML exploits such as XPath injection.
126
127
128.. _defusedxml: https://pypi.org/project/defusedxml/
129.. _Billion Laughs: https://en.wikipedia.org/wiki/Billion_laughs
130.. _ZIP bomb: https://en.wikipedia.org/wiki/Zip_bomb
131.. _DTD: https://en.wikipedia.org/wiki/Document_type_definition
132