Warning

The XML modules are not secure against erroneous or maliciouslyconstructed data. If you need to parse untrusted orunauthenticated data see theXML vulnerabilities andThe defusedxml and defusedexpat Packages sections.

20.4.1. 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, orcircumvent firewalls.

The following table gives an overview of the known attacks and whetherthe various modules are vulnerable to them.

1. xml.etree.ElementTree doesn’t expand external entities and raises aParserError when an entity occurs.
2. xml.dom.minidom doesn’t expand external entities and simply returnsthe unexpanded entity verbatim.
3. xmlrpclib doesn’t expand external entities and omits them.

billion laughs / exponential entity expansion

TheBillion Laughs attack – also known as exponential entity expansion –uses multiple levels of nested entities. Each entity refers to another entityseveral times, and the final entity definition contains a small string.The exponential expansion results in several gigabytes of text andconsumes lots of memory and CPU time.

quadratic blowup entity expansion

A quadratic blowup attack is similar to aBillion Laughs attack; it abusesentity expansion, too. Instead of nested entities it repeats one large entitywith a couple of thousand chars over and over again. The attack isn’t asefficient as the exponential case but it avoids triggering parser countermeasuresthat forbid deeply-nested entities.

external entity expansion

Entity declarations can contain more than just text for replacement. They canalso point to external resources or local files. The XMLparser accesses the resource and embeds the content into the XML document.

DTD retrieval

Some XML libraries like Python’sxml.dom.pulldom retrieve document typedefinitions from remote or local locations. The feature has similarimplications as the external entity expansion issue.

decompression bomb

Decompression bombs (akaZIP bomb) apply to all XML librariesthat can parse compressed XML streams such as gzipped HTTP streams orLZMA-compressedfiles. For an attacker it can reduce the amount of transmitted data by threemagnitudes or more.

The documentation fordefusedxml on PyPI has further information aboutall known attack vectors with examples and references.

20.4.2. Thedefusedxml anddefusedexpat Packages¶

defusedxml is a pure Python package with modified subclasses of all stdlibXML parsers that prevent any potentially malicious operation. Use of thispackage is recommended for any server code that parses untrusted XML data. Thepackage also ships with example exploits and extended documentation on moreXML exploits such as XPath injection.

defusedexpat provides a modified libexpat and a patchedpyexpat module that have countermeasures against entity expansionDoS attacks. Thedefusedexpat module still allows a sane and configurable amount of entityexpansions. The modifications may be included in some future release of Python,but will not be included in any bugfix releases ofPython because they break backward compatibility.