19.5. Modules de traitement XML

Les interfaces de Python de traitement de xml sont regroupées dans le paquet xml.

Avertissement

The XML modules are not secure against erroneous or maliciously constructed data. If you need to parse untrusted or unauthenticated data see Vulnérabilités XML.

Il est important de noter que les modules dans le paquet xml nécessitent qu’au moins un parseur compatible SAX soit disponible. Le parseur Expat est inclu dans Python, ainsi le module xml.parsers.expat est toujours disponible.

La documentation des bindings des interfaces DOM et SAX se trouve dans xml.dom et xml.sax.

Les sous-modules de traitement XML sont :

19.6. Vulnérabilités XML

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.

type sax etree minidom pulldom xmlrpc
billion laughs Vulnérable Vulnérable Vulnérable Vulnérable Vulnérable
quadratic blowup Vulnérable Vulnérable Vulnérable Vulnérable Vulnérable
external entity expansion Vulnérable Sûr (1) Sûr (2) Vulnérable Sûr (3)
Récupération de DTD Vulnérable Sûr Sûr Vulnérable Sûr
decompression bomb Sûr Sûr Sûr Sûr Vulnérable
  1. xml.etree.ElementTree doesn’t expand external entities and raises a ParserError when an entity occurs.
  2. xml.dom.minidom n’étend pas les entités externe et renvoie simplement le verbatim de l’entité non étendu.
  3. xmlrpclib n’étend pas les entités externes et les omet.
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.
Récupération de DTD
Certaines bibliothèques XML comme xml.dom.pulldom de Python récupère les documents de définitions de types (DTD) depuis des emplacements distants ou locaux. La fonctionnalité a des implications similaires que le problème d’extension d’entités externes.
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.