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:`defused-packages` 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** (1)  **Vulnerable** (1)  **Vulnerable** (1)  **Vulnerable** (1)  **Vulnerable** (1)
67quadratic blowup           **Vulnerable** (1)  **Vulnerable** (1)  **Vulnerable** (1)  **Vulnerable** (1)  **Vulnerable** (1)
68external entity expansion  Safe (5)            Safe (2)            Safe (3)            Safe (5)            Safe (4)
69`DTD`_ retrieval           Safe (5)            Safe                Safe                Safe (5)            Safe
70decompression bomb         Safe                Safe                Safe                Safe                **Vulnerable**
71=========================  ==================  ==================  ==================  ==================  ==================
72
731. Expat 2.4.1 and newer is not vulnerable to the "billion laughs" and
74   "quadratic blowup" vulnerabilities. Items still listed as vulnerable due to
75   potential reliance on system-provided libraries. Check
76   :data:`pyexpat.EXPAT_VERSION`.
772. :mod:`xml.etree.ElementTree` doesn't expand external entities and raises a
78   :exc:`ParserError` when an entity occurs.
793. :mod:`xml.dom.minidom` doesn't expand external entities and simply returns
80   the unexpanded entity verbatim.
814. :mod:`xmlrpclib` doesn't expand external entities and omits them.
825. Since Python 3.7.1, external general entities are no longer processed by
83   default.
84
85
86billion laughs / exponential entity expansion
87  The `Billion Laughs`_ attack -- also known as exponential entity expansion --
88  uses multiple levels of nested entities. Each entity refers to another entity
89  several times, and the final entity definition contains a small string.
90  The exponential expansion results in several gigabytes of text and
91  consumes lots of memory and CPU time.
92
93quadratic blowup entity expansion
94  A quadratic blowup attack is similar to a `Billion Laughs`_ attack; it abuses
95  entity expansion, too. Instead of nested entities it repeats one large entity
96  with a couple of thousand chars over and over again. The attack isn't as
97  efficient as the exponential case but it avoids triggering parser countermeasures
98  that forbid deeply-nested entities.
99
100external entity expansion
101  Entity declarations can contain more than just text for replacement. They can
102  also point to external resources or local files. The XML
103  parser accesses the resource and embeds the content into the XML document.
104
105`DTD`_ retrieval
106  Some XML libraries like Python's :mod:`xml.dom.pulldom` retrieve document type
107  definitions from remote or local locations. The feature has similar
108  implications as the external entity expansion issue.
109
110decompression bomb
111  Decompression bombs (aka `ZIP bomb`_) apply to all XML libraries
112  that can parse compressed XML streams such as gzipped HTTP streams or
113  LZMA-compressed
114  files. For an attacker it can reduce the amount of transmitted data by three
115  magnitudes or more.
116
117The documentation for `defusedxml`_ on PyPI has further information about
118all known attack vectors with examples and references.
119
120.. _defused-packages:
121
122The :mod:`defusedxml` and :mod:`defusedexpat` Packages
123------------------------------------------------------
124
125`defusedxml`_ is a pure Python package with modified subclasses of all stdlib
126XML parsers that prevent any potentially malicious operation. Use of this
127package is recommended for any server code that parses untrusted XML data. The
128package also ships with example exploits and extended documentation on more
129XML exploits such as XPath injection.
130
131`defusedexpat`_ provides a modified libexpat and a patched
132:mod:`pyexpat` module that have countermeasures against entity expansion
133DoS attacks. The :mod:`defusedexpat` module still allows a sane and configurable amount of entity
134expansions. The modifications may be included in some future release of Python,
135but will not be included in any bugfix releases of
136Python because they break backward compatibility.
137
138
139.. _defusedxml: https://pypi.org/project/defusedxml/
140.. _defusedexpat: https://pypi.org/project/defusedexpat/
141.. _Billion Laughs: https://en.wikipedia.org/wiki/Billion_laughs
142.. _ZIP bomb: https://en.wikipedia.org/wiki/Zip_bomb
143.. _DTD: https://en.wikipedia.org/wiki/Document_type_definition
144