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** (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.. _defusedxml-package: 121 122The :mod:`defusedxml` Package 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. 130 131 132.. _defusedxml: https://pypi.org/project/defusedxml/ 133.. _Billion Laughs: https://en.wikipedia.org/wiki/Billion_laughs 134.. _ZIP bomb: https://en.wikipedia.org/wiki/Zip_bomb 135.. _DTD: https://en.wikipedia.org/wiki/Document_type_definition 136