1 .. _xml: 2 3 XML Processing Modules 4 ====================== 5 6 .. module:: xml 7 :synopsis: Package containing XML processing modules 8 .. sectionauthor:: Christian Heimes <christian (a] python.org> 9 .. sectionauthor:: Georg Brandl <georg (a] python.org> 10 11 12 Python's interfaces for processing XML are grouped in the ``xml`` package. 13 14 .. warning:: 15 16 The XML modules are not secure against erroneous or maliciously 17 constructed data. If you need to parse untrusted or unauthenticated data see 18 :ref:`xml-vulnerabilities`. 19 20 It is important to note that modules in the :mod:`xml` package require that 21 there be at least one SAX-compliant XML parser available. The Expat parser is 22 included with Python, so the :mod:`xml.parsers.expat` module will always be 23 available. 24 25 The documentation for the :mod:`xml.dom` and :mod:`xml.sax` packages are the 26 definition of the Python bindings for the DOM and SAX interfaces. 27 28 The XML handling submodules are: 29 30 * :mod:`xml.etree.ElementTree`: the ElementTree API, a simple and lightweight 31 XML processor 32 33 .. 34 35 * :mod:`xml.dom`: the DOM API definition 36 * :mod:`xml.dom.minidom`: a minimal DOM implementation 37 * :mod:`xml.dom.pulldom`: support for building partial DOM trees 38 39 .. 40 41 * :mod:`xml.sax`: SAX2 base classes and convenience functions 42 * :mod:`xml.parsers.expat`: the Expat parser binding 43 44 45 .. _xml-vulnerabilities: 46 47 XML vulnerabilities 48 =================== 49 50 The XML processing modules are not secure against maliciously constructed data. 51 An attacker can abuse vulnerabilities for e.g. denial of service attacks, to 52 access local files, to generate network connections to other machines, or 53 to or circumvent firewalls. The attacks on XML abuse unfamiliar features 54 like inline `DTD`_ (document type definition) with entities. 55 56 The following table gives an overview of the known attacks and if the various 57 modules are vulnerable to them. 58 59 ========================= ============== =============== ============== ============== ============== 60 kind sax etree minidom pulldom xmlrpc 61 ========================= ============== =============== ============== ============== ============== 62 billion laughs **Vulnerable** **Vulnerable** **Vulnerable** **Vulnerable** **Vulnerable** 63 quadratic blowup **Vulnerable** **Vulnerable** **Vulnerable** **Vulnerable** **Vulnerable** 64 external entity expansion **Vulnerable** Safe (1) Safe (2) **Vulnerable** Safe (3) 65 `DTD`_ retrieval **Vulnerable** Safe Safe **Vulnerable** Safe 66 decompression bomb Safe Safe Safe Safe **Vulnerable** 67 ========================= ============== =============== ============== ============== ============== 68 69 1. :mod:`xml.etree.ElementTree` doesn't expand external entities and raises a 70 ParserError when an entity occurs. 71 2. :mod:`xml.dom.minidom` doesn't expand external entities and simply returns 72 the unexpanded entity verbatim. 73 3. :mod:`xmlrpclib` doesn't expand external entities and omits them. 74 75 76 billion laughs / exponential entity expansion 77 The `Billion Laughs`_ attack -- also known as exponential entity expansion -- 78 uses multiple levels of nested entities. Each entity refers to another entity 79 several times, the final entity definition contains a small string. Eventually 80 the small string is expanded to several gigabytes. The exponential expansion 81 consumes lots of CPU time, too. 82 83 quadratic blowup entity expansion 84 A quadratic blowup attack is similar to a `Billion Laughs`_ attack; it abuses 85 entity expansion, too. Instead of nested entities it repeats one large entity 86 with a couple of thousand chars over and over again. The attack isn't as 87 efficient as the exponential case but it avoids triggering countermeasures of 88 parsers against heavily nested entities. 89 90 external entity expansion 91 Entity declarations can contain more than just text for replacement. They can 92 also point to external resources by public identifiers or system identifiers. 93 System identifiers are standard URIs or can refer to local files. The XML 94 parser retrieves the resource with e.g. HTTP or FTP requests and embeds the 95 content into the XML document. 96 97 `DTD`_ retrieval 98 Some XML libraries like Python's :mod:`xml.dom.pulldom` retrieve document type 99 definitions from remote or local locations. The feature has similar 100 implications as the external entity expansion issue. 101 102 decompression bomb 103 The issue of decompression bombs (aka `ZIP bomb`_) apply to all XML libraries 104 that can parse compressed XML stream like gzipped HTTP streams or LZMA-ed 105 files. For an attacker it can reduce the amount of transmitted data by three 106 magnitudes or more. 107 108 The documentation of `defusedxml`_ on PyPI has further information about 109 all known attack vectors with examples and references. 110 111 defused packages 112 ---------------- 113 114 These external packages are recommended for any code that parses 115 untrusted XML data. 116 117 `defusedxml`_ is a pure Python package with modified subclasses of all stdlib 118 XML parsers that prevent any potentially malicious operation. The 119 package also ships with example exploits and extended documentation on more 120 XML exploits like xpath injection. 121 122 `defusedexpat`_ provides a modified libexpat and patched replacement 123 :mod:`pyexpat` extension module with countermeasures against entity expansion 124 DoS attacks. Defusedexpat still allows a sane and configurable amount of entity 125 expansions. The modifications will be merged into future releases of Python. 126 127 The workarounds and modifications are not included in patch releases as they 128 break backward compatibility. After all inline DTD and entity expansion are 129 well-defined XML features. 130 131 132 .. _defusedxml: https://pypi.python.org/pypi/defusedxml/ 133 .. _defusedexpat: https://pypi.python.org/pypi/defusedexpat/ 134 .. _Billion Laughs: https://en.wikipedia.org/wiki/Billion_laughs 135 .. _ZIP bomb: https://en.wikipedia.org/wiki/Zip_bomb 136 .. _DTD: https://en.wikipedia.org/wiki/Document_type_definition 137
