1 .. _xml: 2 3 XML Processing Modules 4 ====================== 5 6 .. module:: xml 7 :synopsis: Package containing XML processing modules 8 9 .. sectionauthor:: Christian Heimes <christian (a] python.org> 10 .. sectionauthor:: Georg Brandl <georg (a] python.org> 11 12 **Source code:** :source:`Lib/xml/` 13 14 -------------- 15 16 Python'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 25 It is important to note that modules in the :mod:`xml` package require that 26 there be at least one SAX-compliant XML parser available. The Expat parser is 27 included with Python, so the :mod:`xml.parsers.expat` module will always be 28 available. 29 30 The documentation for the :mod:`xml.dom` and :mod:`xml.sax` packages are the 31 definition of the Python bindings for the DOM and SAX interfaces. 32 33 The 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 52 XML vulnerabilities 53 ------------------- 54 55 The XML processing modules are not secure against maliciously constructed data. 56 An attacker can abuse XML features to carry out denial of service attacks, 57 access local files, generate network connections to other machines, or 58 circumvent firewalls. 59 60 The following table gives an overview of the known attacks and whether 61 the various modules are vulnerable to them. 62 63 ========================= ============== =============== ============== ============== ============== 64 kind sax etree minidom pulldom xmlrpc 65 ========================= ============== =============== ============== ============== ============== 66 billion laughs **Vulnerable** **Vulnerable** **Vulnerable** **Vulnerable** **Vulnerable** 67 quadratic blowup **Vulnerable** **Vulnerable** **Vulnerable** **Vulnerable** **Vulnerable** 68 external entity expansion **Vulnerable** Safe (1) Safe (2) **Vulnerable** Safe (3) 69 `DTD`_ retrieval **Vulnerable** Safe Safe **Vulnerable** Safe 70 decompression bomb Safe Safe Safe Safe **Vulnerable** 71 ========================= ============== =============== ============== ============== ============== 72 73 1. :mod:`xml.etree.ElementTree` doesn't expand external entities and raises a 74 :exc:`ParserError` when an entity occurs. 75 2. :mod:`xml.dom.minidom` doesn't expand external entities and simply returns 76 the unexpanded entity verbatim. 77 3. :mod:`xmlrpclib` doesn't expand external entities and omits them. 78 79 80 billion laughs / exponential entity expansion 81 The `Billion Laughs`_ attack -- also known as exponential entity expansion -- 82 uses multiple levels of nested entities. Each entity refers to another entity 83 several times, and the final entity definition contains a small string. 84 The exponential expansion results in several gigabytes of text and 85 consumes lots of memory and CPU time. 86 87 quadratic blowup entity expansion 88 A quadratic blowup attack is similar to a `Billion Laughs`_ attack; it abuses 89 entity expansion, too. Instead of nested entities it repeats one large entity 90 with a couple of thousand chars over and over again. The attack isn't as 91 efficient as the exponential case but it avoids triggering parser countermeasures 92 that forbid deeply-nested entities. 93 94 external entity expansion 95 Entity declarations can contain more than just text for replacement. They can 96 also point to external resources or local files. The XML 97 parser accesses the resource and embeds the content into the XML document. 98 99 `DTD`_ retrieval 100 Some XML libraries like Python's :mod:`xml.dom.pulldom` retrieve document type 101 definitions from remote or local locations. The feature has similar 102 implications as the external entity expansion issue. 103 104 decompression bomb 105 Decompression bombs (aka `ZIP bomb`_) apply to all XML libraries 106 that can parse compressed XML streams such as gzipped HTTP streams or 107 LZMA-compressed 108 files. For an attacker it can reduce the amount of transmitted data by three 109 magnitudes or more. 110 111 The documentation for `defusedxml`_ on PyPI has further information about 112 all known attack vectors with examples and references. 113 114 .. _defused-packages: 115 116 The :mod:`defusedxml` and :mod:`defusedexpat` Packages 117 ------------------------------------------------------ 118 119 `defusedxml`_ is a pure Python package with modified subclasses of all stdlib 120 XML parsers that prevent any potentially malicious operation. Use of this 121 package is recommended for any server code that parses untrusted XML data. The 122 package also ships with example exploits and extended documentation on more 123 XML exploits such as XPath injection. 124 125 `defusedexpat`_ provides a modified libexpat and a patched 126 :mod:`pyexpat` module that have countermeasures against entity expansion 127 DoS attacks. The :mod:`defusedexpat` module still allows a sane and configurable amount of entity 128 expansions. The modifications may be included in some future release of Python, 129 but will not be included in any bugfix releases of 130 Python because they break backward compatibility. 131 132 133 .. _defusedxml: https://pypi.python.org/pypi/defusedxml/ 134 .. _defusedexpat: https://pypi.python.org/pypi/defusedexpat/ 135 .. _Billion Laughs: https://en.wikipedia.org/wiki/Billion_laughs 136 .. _ZIP bomb: https://en.wikipedia.org/wiki/Zip_bomb 137 .. _DTD: https://en.wikipedia.org/wiki/Document_type_definition 138
