19.5. XML Processing Modules
****************************

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 XML vulnerabilities.

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


19.6. 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.

The following table gives an overview of the known attacks and if 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, 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. 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.

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
   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 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
all known attack vectors with examples and references.


19.6.1. defused packages
========================

These external packages are recommended for any code that parses
untrusted XML data.

defusedxml is a pure Python package with modified subclasses of all
stdlib XML parsers that prevent any potentially malicious operation.
The package also ships with example exploits and extended
documentation on more XML exploits like xpath injection.

defusedexpat provides a modified libexpat and patched replacement
"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-defined XML features.
