20.4. 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.dom: the DOM API definition
• xml.dom.minidom: a lightweight 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

20.5. 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	Yes	Yes	Yes	Yes	Yes
quadratic blowup	Yes	Yes	Yes	Yes	Yes
external entity expansion	Yes	No (1)	No (2)	Yes	No (3)
DTD retrieval	Yes	No	No	Yes	No
decompression bomb	No	No	No	No	Yes

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 mod:’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.

20.5.1. defused 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.

defusedexpat provides a modified libexpat and patched replacment 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.