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135 lines
5.7 KiB
ReStructuredText
135 lines
5.7 KiB
ReStructuredText
.. _xml:
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XML Processing Modules
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======================
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.. module:: xml
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:synopsis: Package containing XML processing modules
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.. sectionauthor:: Christian Heimes <christian@python.org>
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.. sectionauthor:: Georg Brandl <georg@python.org>
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Python's interfaces for processing XML are grouped in the ``xml`` package.
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.. warning::
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The XML modules are not secure against erroneous or maliciously
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constructed data. If you need to parse untrusted or unauthenticated data see
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:ref:`xml-vulnerabilities`.
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It is important to note that modules in the :mod:`xml` package require that
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there be at least one SAX-compliant XML parser available. The Expat parser is
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included with Python, so the :mod:`xml.parsers.expat` module will always be
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available.
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The documentation for the :mod:`xml.dom` and :mod:`xml.sax` packages are the
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definition of the Python bindings for the DOM and SAX interfaces.
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The XML handling submodules are:
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* :mod:`xml.etree.ElementTree`: the ElementTree API, a simple and lightweight
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..
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* :mod:`xml.dom`: the DOM API definition
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* :mod:`xml.dom.minidom`: a lightweight DOM implementation
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* :mod:`xml.dom.pulldom`: support for building partial DOM trees
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..
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* :mod:`xml.sax`: SAX2 base classes and convenience functions
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* :mod:`xml.parsers.expat`: the Expat parser binding
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.. _xml-vulnerabilities:
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XML vulnerabilities
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===================
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The XML processing modules are not secure against maliciously constructed data.
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An attacker can abuse vulnerabilities for e.g. denial of service attacks, to
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access local files, to generate network connections to other machines, or
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to or circumvent firewalls. The attacks on XML abuse unfamiliar features
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like inline `DTD`_ (document type definition) with entities.
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The following table gives an overview of the known attacks and if the various
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modules are vulnerable to them.
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========================= ======== ========= ========= ======== =========
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kind sax etree minidom pulldom xmlrpc
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========================= ======== ========= ========= ======== =========
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billion laughs **Yes** **Yes** **Yes** **Yes** **Yes**
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quadratic blowup **Yes** **Yes** **Yes** **Yes** **Yes**
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external entity expansion **Yes** No (1) No (2) **Yes** No (3)
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DTD retrieval **Yes** No No **Yes** No
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decompression bomb No No No No **Yes**
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========================= ======== ========= ========= ======== =========
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1. :mod:`xml.etree.ElementTree` doesn't expand external entities and raises a
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ParserError when an entity occurs.
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2. :mod:`xml.dom.minidom` doesn't expand external entities and simply returns
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the unexpanded entity verbatim.
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3. :mod:`xmlrpclib` doesn't expand external entities and omits them.
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billion laughs / exponential entity expansion
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The `Billion Laughs`_ attack -- also known as exponential entity expansion --
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uses multiple levels of nested entities. Each entity refers to another entity
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several times, the final entity definition contains a small string. Eventually
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the small string is expanded to several gigabytes. The exponential expansion
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consumes lots of CPU time, too.
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quadratic blowup entity expansion
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A quadratic blowup attack is similar to a `Billion Laughs`_ attack; it abuses
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entity expansion, too. Instead of nested entities it repeats one large entity
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with a couple of thousand chars over and over again. The attack isn't as
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efficient as the exponential case but it avoids triggering countermeasures of
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parsers against heavily nested entities.
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external entity expansion
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Entity declarations can contain more than just text for replacement. They can
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also point to external resources by public identifiers or system identifiers.
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System identifiers are standard URIs or can refer to local files. The XML
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parser retrieves the resource with e.g. HTTP or FTP requests and embeds the
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content into the XML document.
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DTD retrieval
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Some XML libraries like Python's mod:'xml.dom.pulldom' retrieve document type
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definitions from remote or local locations. The feature has similar
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implications as the external entity expansion issue.
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decompression bomb
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The issue of decompression bombs (aka `ZIP bomb`_) apply to all XML libraries
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that can parse compressed XML stream like gzipped HTTP streams or LZMA-ed
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files. For an attacker it can reduce the amount of transmitted data by three
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magnitudes or more.
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The documentation of `defusedxml`_ on PyPI has further information about
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all known attack vectors with examples and references.
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defused packages
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----------------
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`defusedxml`_ is a pure Python package with modified subclasses of all stdlib
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XML parsers that prevent any potentially malicious operation. The courses of
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action are recommended for any server code that parses untrusted XML data. The
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package also ships with example exploits and an extended documentation on more
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XML exploits like xpath injection.
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`defusedexpat`_ provides a modified libexpat and patched replacment
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:mod:`pyexpat` extension module with countermeasures against entity expansion
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DoS attacks. Defusedexpat still allows a sane and configurable amount of entity
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expansions. The modifications will be merged into future releases of Python.
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The workarounds and modifications are not included in patch releases as they
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break backward compatibility. After all inline DTD and entity expansion are
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well-definied XML features.
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.. _defusedxml: https://pypi.python.org/pypi/defusedxml/
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.. _defusedexpat: https://pypi.python.org/pypi/defusedexpat/
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.. _Billion Laughs: http://en.wikipedia.org/wiki/Billion_laughs
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.. _ZIP bomb: http://en.wikipedia.org/wiki/Zip_bomb
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.. _DTD: http://en.wikipedia.org/wiki/Document_Type_Definition
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