19.5. XML Processing Modules

Python’s interfaces for processing XML are grouped in the xml package.

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.