XML Processing Modules¶
Source code: Lib/xml/
Python's interfaces for processing XML are grouped in the xml
package.
Peringatan
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 Package 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 definitionxml.dom.minidom
: a minimal DOM implementationxml.dom.pulldom
: support for building partial DOM trees
xml.sax
: SAX2 base classes and convenience functionsxml.parsers.expat
: the Expat parser binding
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 (1) |
Vulnerable (1) |
Vulnerable (1) |
Vulnerable (1) |
Vulnerable (1) |
quadratic blowup |
Vulnerable (1) |
Vulnerable (1) |
Vulnerable (1) |
Vulnerable (1) |
Vulnerable (1) |
external entity expansion |
Safe (5) |
Safe (2) |
Safe (3) |
Safe (5) |
Safe (4) |
DTD retrieval |
Safe (5) |
Safe |
Safe |
Safe (5) |
Safe |
decompression bomb |
Safe |
Safe |
Safe |
Safe |
Rentan |
large tokens |
Vulnerable (6) |
Vulnerable (6) |
Vulnerable (6) |
Vulnerable (6) |
Vulnerable (6) |
Expat 2.4.1 and newer is not vulnerable to the "billion laughs" and "quadratic blowup" vulnerabilities. Items still listed as vulnerable due to potential reliance on system-provided libraries. Check
pyexpat.EXPAT_VERSION
.xml.etree.ElementTree
doesn't expand external entities and raises aParserError
when an entity occurs.xml.dom.minidom
doesn't expand external entities and simply returns the unexpanded entity verbatim.xmlrpclib
doesn't expand external entities and omits them.Since Python 3.7.1, external general entities are no longer processed by default.
Expat 2.6.0 and newer is not vulnerable to denial of service through quadratic runtime caused by parsing large tokens. Items still listed as vulnerable due to potential reliance on system-provided libraries. Check
pyexpat.EXPAT_VERSION
.
- 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.
- large tokens
Expat needs to re-parse unfinished tokens; without the protection introduced in Expat 2.6.0, this can lead to quadratic runtime that can be used to cause denial of service in the application parsing XML. The issue is known as CVE-2023-52425.
The documentation for defusedxml on PyPI has further information about all known attack vectors with examples and references.
The defusedxml
Package¶
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.