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.. _xml:
XML Processing Modules
======================
.. module:: xml
:synopsis: Package containing XML processing modules
.. sectionauthor:: Christian Heimes
.. sectionauthor:: Georg Brandl
Python's interfaces for processing XML are grouped in the ``xml`` package.
.. warning::
The XML modules are not secure against erroneous or maliciously
constructed data. If you need to parse untrusted or unauthenticated data see
:ref:`xml-vulnerabilities`.
It is important to note that modules in the :mod:`xml` package require that
there be at least one SAX-compliant XML parser available. The Expat parser is
included with Python, so the :mod:`xml.parsers.expat` module will always be
available.
The documentation for the :mod:`xml.dom` and :mod:`xml.sax` packages are the
definition of the Python bindings for the DOM and SAX interfaces.
The XML handling submodules are:
* :mod:`xml.etree.ElementTree`: the ElementTree API, a simple and lightweight
..
* :mod:`xml.dom`: the DOM API definition
* :mod:`xml.dom.minidom`: a lightweight DOM implementation
* :mod:`xml.dom.pulldom`: support for building partial DOM trees
..
* :mod:`xml.sax`: SAX2 base classes and convenience functions
* :mod:`xml.parsers.expat`: the Expat parser binding
.. _xml-vulnerabilities:
XML vulnerabilities
===================
The XML processing modules are not secure against maliciously constructed data.
An attacker can abuse vulnerabilities for e.g. denial of service attacks, to
access local files, to generate network connections to other machines, or
to or circumvent firewalls. The attacks on XML abuse unfamiliar features
like inline `DTD`_ (document type definition) with entities.
========================= ======== ========= ========= ======== =========
kind sax etree minidom pulldom xmlrpc
========================= ======== ========= ========= ======== =========
billion laughs **True** **True** **True** **True** **True**
quadratic blowup **True** **True** **True** **True** **True**
external entity expansion **True** False (1) False (2) **True** False (3)
DTD retrieval **True** False False **True** False
decompression bomb False False False False **True**
========================= ======== ========= ========= ======== =========
1. :mod:`xml.etree.ElementTree` doesn't expand external entities and raises a
ParserError when an entity occurs.
2. :mod:`xml.dom.minidom` doesn't expand external entities and simply returns
the unexpanded entity verbatim.
3. :mod:`xmlrpclib` doesn't expand external entities and omits them.
billion laughs / exponential entity expansion
The `Billion Laughs`_ attack -- also known as exponential entity expansion --
uses multiple levels of nested entities. Each entity refers to another entity
several times, the final entity definition contains a small string. Eventually
the small string is expanded to several gigabytes. The exponential expansion
consumes lots of CPU time, too.
quadratic blowup entity expansion
A quadratic blowup attack is similar to a `Billion Laughs`_ attack; it abuses
entity expansion, too. Instead of nested entities it repeats one large entity
with a couple of thousand chars over and over again. The attack isn't as
efficient as the exponential case but it avoids triggering countermeasures of
parsers against heavily nested entities.
external entity expansion
Entity declarations can contain more than just text for replacement. They can
also point to external resources by public identifiers or system identifiers.
Sy