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#20237: make a revision pass over the XML vulnerabilities section

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Andrew Kuchling 2014-02-15 15:33:44 -05:00
parent 29352c436c
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1 changed files with 35 additions and 38 deletions
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Doc/library/xml.rst
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@ -14,9 +14,9 @@ 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`.
constructed data. If you need to parse untrusted or
unauthenticated data see the :ref:`xml-vulnerabilities` and
:ref:`defused-packages` sections.
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
@ -46,16 +46,15 @@ The XML handling submodules are:
.. _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.
An attacker can abuse XML features to carry out denial of service attacks,
access local files, generate network connections to other machines, or
circumvent firewalls.
The following table gives an overview of the known attacks and if the various
modules are vulnerable to them.
The following table gives an overview of the known attacks and whether
the various modules are vulnerable to them.
========================= ======== ========= ========= ======== =========
kind sax etree minidom pulldom xmlrpc
@ -68,7 +67,7 @@ decompression bomb No No No No **Yes**
========================= ======== ========= ========= ======== =========
1. :mod:`xml.etree.ElementTree` doesn't expand external entities and raises a
ParserError when an entity occurs.
:exc:`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.
@ -77,23 +76,21 @@ decompression bomb No No No No **Yes**
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.
several times, and the final entity definition contains a small string.
The exponential expansion results in several gigabytes of text and
consumes lots of memory and CPU time.
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.
efficient as the exponential case but it avoids triggering parser countermeasures
that forbid deeply-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.
System identifiers are standard URIs or can refer to local files. The XML
parser retrieves the resource with e.g. HTTP or FTP requests and embeds the
content into the XML document.
also point to external resources or local files. The XML
parser accesses the resource and embeds the content into the XML document.
DTD retrieval
Some XML libraries like Python's :mod:`xml.dom.pulldom` retrieve document type
@ -101,31 +98,32 @@ DTD retrieval
implications as the external entity expansion issue.
decompression bomb
The issue of decompression bombs (aka `ZIP bomb`_) apply to all XML libraries
that can parse compressed XML stream like gzipped HTTP streams or LZMA-ed
Decompression bombs (aka `ZIP bomb`_) apply to all XML libraries
that can parse compressed XML streams such as gzipped HTTP streams or
LZMA-compressed
files. For an attacker it can reduce the amount of transmitted data by three
magnitudes or more.
The documentation of `defusedxml`_ on PyPI has further information about
The documentation for `defusedxml`_ on PyPI has further information about
all known attack vectors with examples and references.
defused packages
----------------
.. _defused-packages:
The :mod:`defusedxml` and :mod:`defusedexpat` Packages
------------------------------------------------------
`defusedxml`_ is a pure Python package with modified subclasses of all stdlib
XML parsers that prevent any potentially malicious operation. The courses of
action are recommended for any server code that parses untrusted XML data. The
package also ships with example exploits and an extended documentation on more
XML exploits like xpath injection.
XML parsers that prevent any potentially malicious operation. Use of this
package is recommended for any server code that parses untrusted XML data. The
package also ships with example exploits and extended documentation on more
XML exploits such as XPath injection.
`defusedexpat`_ provides a modified libexpat and patched replacment
:mod:`pyexpat` extension module with countermeasures against entity expansion
DoS attacks. Defusedexpat still allows a sane and configurable amount of entity
expansions. The modifications will be merged into future releases of Python.
The workarounds and modifications are not included in patch releases as they
break backward compatibility. After all inline DTD and entity expansion are
well-definied XML features.
`defusedexpat`_ provides a modified libexpat and a patched
:mod:`pyexpat` module that have countermeasures against entity expansion
DoS attacks. The :mod:`defusedexpat` module still allows a sane and configurable amount of entity
expansions. The modifications may be included in some future release of Python,
but will not be included in any bugfix releases of
Python because they break backward compatibility.
.. _defusedxml: https://pypi.python.org/pypi/defusedxml/
@ -133,4 +131,3 @@ well-definied XML features.
.. _Billion Laughs: http://en.wikipedia.org/wiki/Billion_laughs
.. _ZIP bomb: http://en.wikipedia.org/wiki/Zip_bomb
.. _DTD: http://en.wikipedia.org/wiki/Document_Type_Definition