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* Remove the table * Replace warnings with notes Latest releases of Python 3.9-3.15 include expat 2.7.1 which is not vulnerable. expat 2.6.0 was released in February 2024.
94 lines
3.2 KiB
ReStructuredText
94 lines
3.2 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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**Source code:** :source:`Lib/xml/`
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--------------
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Python's interfaces for processing XML are grouped in the ``xml`` package.
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.. note::
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If you need to parse untrusted or unauthenticated data, see
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:ref:`xml-security`.
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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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XML processor
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..
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* :mod:`xml.dom`: the DOM API definition
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* :mod:`xml.dom.minidom`: a minimal 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-security:
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.. _xml-vulnerabilities:
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XML security
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------------
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An attacker can abuse XML features to carry out denial of service attacks,
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access local files, generate network connections to other machines, or
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circumvent firewalls.
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Expat versions lower that 2.6.0 may be vulnerable to "billion laughs",
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"quadratic blowup" and "large tokens". Python may be vulnerable if it uses such
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older versions of Expat as a system-provided library.
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Check :const:`!pyexpat.EXPAT_VERSION`.
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:mod:`xmlrpc` is **vulnerable** to the "decompression bomb" attack.
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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, and the final entity definition contains a small string.
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The exponential expansion results in several gigabytes of text and
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consumes lots of memory and CPU time.
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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 parser countermeasures
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that forbid deeply nested entities.
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decompression bomb
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Decompression bombs (aka `ZIP bomb`_) apply to all XML libraries
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that can parse compressed XML streams such as gzipped HTTP streams or
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LZMA-compressed
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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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large tokens
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Expat needs to re-parse unfinished tokens; without the protection
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introduced in Expat 2.6.0, this can lead to quadratic runtime that can
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be used to cause denial of service in the application parsing XML.
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The issue is known as :cve:`2023-52425`.
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.. _Billion Laughs: https://en.wikipedia.org/wiki/Billion_laughs
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.. _ZIP bomb: https://en.wikipedia.org/wiki/Zip_bomb
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