21.4. XML Processing Modules
****************************

**Source code:** Lib/xml/

======================================================================

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 the XML vulnerabilities and The defusedxml
  and defusedexpat Packages sections.

It is important to note that modules in the "xml" package require that
there be at least one SAX-compliant XML parser available. The Expat
parser is included with Python, so the "xml.parsers.expat" module will
always be available.

The documentation for the "xml.dom" and "xml.sax" packages are the
definition of the Python bindings for the DOM and SAX interfaces.

The XML handling submodules are:

* "xml.etree.ElementTree": the ElementTree API, a simple and
  lightweight XML processor

* "xml.dom": the DOM API definition

* "xml.dom.minidom": a minimal DOM implementation

* "xml.dom.pulldom": support for building partial DOM trees

* "xml.sax": SAX2 base classes and convenience functions

* "xml.parsers.expat": the Expat parser binding


21.4.1. XML vulnerabilities
===========================

The XML processing modules are not secure against maliciously
constructed data. 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 whether
the various modules are vulnerable to them.

+---------------------------+----------------+-----------------+----------------+----------------+----------------+
| kind                      | sax            | etree           | minidom        | pulldom        | xmlrpc         |
+===========================+================+=================+================+================+================+
| billion laughs            | **Vulnerable** | **Vulnerable**  | **Vulnerable** | **Vulnerable** | **Vulnerable** |
+---------------------------+----------------+-----------------+----------------+----------------+----------------+
| quadratic blowup          | **Vulnerable** | **Vulnerable**  | **Vulnerable** | **Vulnerable** | **Vulnerable** |
+---------------------------+----------------+-----------------+----------------+----------------+----------------+
| external entity expansion | **Vulnerable** | Safe    (1)     | Safe    (2)    | **Vulnerable** | Safe    (3)    |
+---------------------------+----------------+-----------------+----------------+----------------+----------------+
| DTD retrieval             | **Vulnerable** | Safe            | Safe           | **Vulnerable** | Safe           |
+---------------------------+----------------+-----------------+----------------+----------------+----------------+
| decompression bomb        | Safe           | Safe            | Safe           | Safe           | **Vulnerable** |
+---------------------------+----------------+-----------------+----------------+----------------+----------------+

1. "xml.etree.ElementTree" doesn’t expand external entities and
   raises a "ParserError" when an entity occurs.

2. "xml.dom.minidom" doesn’t expand external entities and simply
   returns the unexpanded entity verbatim.

3. "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, 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 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 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 "xml.dom.pulldom" retrieve
   document type definitions from remote or local locations. The
   feature has similar implications as the external entity expansion
   issue.

decompression bomb
   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 for defusedxml on PyPI has further information about
all known attack vectors with examples and references.


21.4.2. The "defusedxml" and "defusedexpat" Packages
====================================================

defusedxml is a pure Python package with modified subclasses of all
stdlib 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 a patched "pyexpat"
module that have countermeasures against entity expansion DoS attacks.
The "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.
