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<div class="section" id="module-xml">
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<span id="xml-processing-modules"></span><span id="xml"></span><h1>19.5. XML Processing Modules<a class="headerlink" href="#module-xml" title="Permalink to this headline">¶</a></h1>
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<p>Python’s interfaces for processing XML are grouped in the <code class="docutils literal"><span class="pre">xml</span></code> package.</p>
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<div class="admonition warning">
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<p class="first admonition-title">Warning</p>
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<p class="last">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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<a class="reference internal" href="#xml-vulnerabilities"><span>XML vulnerabilities</span></a>.</p>
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<p>It is important to note that modules in the <a class="reference internal" href="#module-xml" title="xml: Package containing XML processing modules"><code class="xref py py-mod docutils literal"><span class="pre">xml</span></code></a> 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 <a class="reference internal" href="pyexpat.html#module-xml.parsers.expat" title="xml.parsers.expat: An interface to the Expat non-validating XML parser."><code class="xref py py-mod docutils literal"><span class="pre">xml.parsers.expat</span></code></a> module will always be
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<p>The documentation for the <a class="reference internal" href="xml.dom.html#module-xml.dom" title="xml.dom: Document Object Model API for Python."><code class="xref py py-mod docutils literal"><span class="pre">xml.dom</span></code></a> and <a class="reference internal" href="xml.sax.html#module-xml.sax" title="xml.sax: Package containing SAX2 base classes and convenience functions."><code class="xref py py-mod docutils literal"><span class="pre">xml.sax</span></code></a> packages are the
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definition of the Python bindings for the DOM and SAX interfaces.</p>
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<p>The XML handling submodules are:</p>
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<li><a class="reference internal" href="xml.etree.elementtree.html#module-xml.etree.ElementTree" title="xml.etree.ElementTree: Implementation of the ElementTree API."><code class="xref py py-mod docutils literal"><span class="pre">xml.etree.ElementTree</span></code></a>: the ElementTree API, a simple and lightweight
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<li><a class="reference internal" href="xml.dom.html#module-xml.dom" title="xml.dom: Document Object Model API for Python."><code class="xref py py-mod docutils literal"><span class="pre">xml.dom</span></code></a>: the DOM API definition</li>
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<li><a class="reference internal" href="xml.dom.minidom.html#module-xml.dom.minidom" title="xml.dom.minidom: Minimal Document Object Model (DOM) implementation."><code class="xref py py-mod docutils literal"><span class="pre">xml.dom.minidom</span></code></a>: a minimal DOM implementation</li>
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<li><a class="reference internal" href="xml.dom.pulldom.html#module-xml.dom.pulldom" title="xml.dom.pulldom: Support for building partial DOM trees from SAX events."><code class="xref py py-mod docutils literal"><span class="pre">xml.dom.pulldom</span></code></a>: support for building partial DOM trees</li>
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<li><a class="reference internal" href="xml.sax.html#module-xml.sax" title="xml.sax: Package containing SAX2 base classes and convenience functions."><code class="xref py py-mod docutils literal"><span class="pre">xml.sax</span></code></a>: SAX2 base classes and convenience functions</li>
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<li><a class="reference internal" href="pyexpat.html#module-xml.parsers.expat" title="xml.parsers.expat: An interface to the Expat non-validating XML parser."><code class="xref py py-mod docutils literal"><span class="pre">xml.parsers.expat</span></code></a>: the Expat parser binding</li>
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<div class="section" id="xml-vulnerabilities">
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<span id="id1"></span><h1>19.6. XML vulnerabilities<a class="headerlink" href="#xml-vulnerabilities" title="Permalink to this headline">¶</a></h1>
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<p>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 <a class="reference external" href="https://en.wikipedia.org/wiki/Document_type_definition">DTD</a> (document type definition) with entities.</p>
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<p>The following table gives an overview of the known attacks and if the various
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modules are vulnerable to them.</p>
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<table border="1" class="docutils">
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<thead valign="bottom">
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<tr class="row-odd"><th class="head">kind</th>
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<th class="head">sax</th>
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<th class="head">etree</th>
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<th class="head">minidom</th>
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<th class="head">pulldom</th>
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<th class="head">xmlrpc</th>
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<tr class="row-even"><td>billion laughs</td>
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<td><strong>Vulnerable</strong></td>
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<td><strong>Vulnerable</strong></td>
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<td><strong>Vulnerable</strong></td>
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<td><strong>Vulnerable</strong></td>
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<td><strong>Vulnerable</strong></td>
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<tr class="row-odd"><td>quadratic blowup</td>
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<td><strong>Vulnerable</strong></td>
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<td><strong>Vulnerable</strong></td>
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<td><strong>Vulnerable</strong></td>
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<td><strong>Vulnerable</strong></td>
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<td><strong>Vulnerable</strong></td>
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<tr class="row-even"><td>external entity expansion</td>
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<td><strong>Vulnerable</strong></td>
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<td><strong>Vulnerable</strong></td>
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<tr class="row-odd"><td><a class="reference external" href="https://en.wikipedia.org/wiki/Document_type_definition">DTD</a> retrieval</td>
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<td><strong>Vulnerable</strong></td>
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<td><strong>Vulnerable</strong></td>
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<tr class="row-even"><td>decompression bomb</td>
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<td><strong>Vulnerable</strong></td>
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<ol class="arabic simple">
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<li><a class="reference internal" href="xml.etree.elementtree.html#module-xml.etree.ElementTree" title="xml.etree.ElementTree: Implementation of the ElementTree API."><code class="xref py py-mod docutils literal"><span class="pre">xml.etree.ElementTree</span></code></a> doesn’t expand external entities and raises a
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ParserError when an entity occurs.</li>
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<li><a class="reference internal" href="xml.dom.minidom.html#module-xml.dom.minidom" title="xml.dom.minidom: Minimal Document Object Model (DOM) implementation."><code class="xref py py-mod docutils literal"><span class="pre">xml.dom.minidom</span></code></a> doesn’t expand external entities and simply returns
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the unexpanded entity verbatim.</li>
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<li><a class="reference internal" href="xmlrpclib.html#module-xmlrpclib" title="xmlrpclib: XML-RPC client access."><code class="xref py py-mod docutils literal"><span class="pre">xmlrpclib</span></code></a> doesn’t expand external entities and omits them.</li>
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<dl class="docutils">
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<dt>billion laughs / exponential entity expansion</dt>
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<dd>The <a class="reference external" href="https://en.wikipedia.org/wiki/Billion_laughs">Billion Laughs</a> 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.</dd>
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<dt>quadratic blowup entity expansion</dt>
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<dd>A quadratic blowup attack is similar to a <a class="reference external" href="https://en.wikipedia.org/wiki/Billion_laughs">Billion Laughs</a> 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.</dd>
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<dt>external entity expansion</dt>
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<dd>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.</dd>
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<dt><a class="reference external" href="https://en.wikipedia.org/wiki/Document_type_definition">DTD</a> retrieval</dt>
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<dd>Some XML libraries like Python’s <a class="reference internal" href="xml.dom.pulldom.html#module-xml.dom.pulldom" title="xml.dom.pulldom: Support for building partial DOM trees from SAX events."><code class="xref py py-mod docutils literal"><span class="pre">xml.dom.pulldom</span></code></a> 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.</dd>
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<dt>decompression bomb</dt>
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<dd>The issue of decompression bombs (aka <a class="reference external" href="https://en.wikipedia.org/wiki/Zip_bomb">ZIP bomb</a>) 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.</dd>
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<p>The documentation of <a class="reference external" href="https://pypi.python.org/pypi/defusedxml/">defusedxml</a> on PyPI has further information about
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all known attack vectors with examples and references.</p>
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<div class="section" id="defused-packages">
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<h2>19.6.1. defused packages<a class="headerlink" href="#defused-packages" title="Permalink to this headline">¶</a></h2>
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<p>These external packages are recommended for any code that parses
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untrusted XML data.</p>
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<p><a class="reference external" href="https://pypi.python.org/pypi/defusedxml/">defusedxml</a> is a pure Python package with modified subclasses of all stdlib
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XML parsers that prevent any potentially malicious operation. The
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package also ships with example exploits and extended documentation on more
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XML exploits like xpath injection.</p>
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<p><a class="reference external" href="https://pypi.python.org/pypi/defusedexpat/">defusedexpat</a> provides a modified libexpat and patched replacement
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<code class="xref py py-mod docutils literal"><span class="pre">pyexpat</span></code> 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.</p>
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<p>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-defined XML features.</p>
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<h3><a href="../contents.html">Table Of Contents</a></h3>
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<li><a class="reference internal" href="#">19.5. XML Processing Modules</a></li>
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<li><a class="reference internal" href="#xml-vulnerabilities">19.6. XML vulnerabilities</a><ul>
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<li><a class="reference internal" href="#defused-packages">19.6.1. defused packages</a></li>
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