"It's just XML, what could probably go wrong?"

Christian Heimes <[email protected]>

The results of an attack on a vulnerable XML library can be fairly dramatic. With just a few hundred Bytes of XML data an attacker can occupy several Gigabytes of memory within seconds. An attacker can also keep CPUs busy for a long time with a small to medium size request. Under some circumstances it is even possible to access local files on your server, to circumvent a firewall, or to abuse services to rebound attacks to third parties.

The attacks use and abuse less common features of XML and its parsers. The majority of developers are unacquainted with features such as processing instructions and entity expansions that XML inherited from SGML. At best they know about <!DOCTYPE> from experience with HTML but they are not aware that a document type definition (DTD) can generate an HTTP request or load a file from the file system.

None of the issues is new. They have been known for a long time. Billion laughs was first reported in 2003. Nevertheless some XML libraries and applications are still vulnerable and even heavy users of XML are surprised by these features. It's hard to say whom to blame for the situation. It's too short sighted to shift all blame on XML parsers and XML libraries for using insecure default settings. After all they properly implement XML specifications. Application developers must not rely that a library is always configured for security and potential harmful data by default.

The Billion Laughs attack -- also known as exponential entity expansion --uses multiple levels of nested entities. The original example uses 9 levels of 10 expansions in each level to expand the string lol to a string of 3 * 10 ⁹ bytes, hence the name "billion laughs". The resulting string occupies 3 GB (2.79 GiB) of memory; intermediate strings require additional memory. Because most parsers don't cache the intermediate step for every expansion it is repeated over and over again. It increases the CPU load even more.

An XML document of just a few hundred bytes can disrupt all services on a machine within seconds.

Example XML:

<!DOCTYPE xmlbomb [
<!ENTITY a "1234567890" >
<!ENTITY b "&a;&a;&a;&a;&a;&a;&a;&a;">
<!ENTITY c "&b;&b;&b;&b;&b;&b;&b;&b;">
<!ENTITY d "&c;&c;&c;&c;&c;&c;&c;&c;">
]>
<bomb>&d;</bomb>

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. Some parsers limit the depth and breadth of a single entity but not the total amount of expanded text throughout an entire XML document.

A medium-sized XML document with a couple of hundred kilobytes can require a couple of hundred MB to several GB of memory. When the attack is combined with some level of nested expansion an attacker is able to achieve a higher ratio of success.

<!DOCTYPE bomb [
<!ENTITY a "xxxxxxx... a couple of ten thousand chars">
]>
<bomb>&a;&a;&a;... repeat</bomb>

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. When the URI is a URL (e.g. a http:// locator) some parsers download the resource from the remote location and embed them into the XML document verbatim.

Simple example of a parsed external entity:

<!DOCTYPE external [
<!ENTITY ee SYSTEM "http://www.python.org/some.xml">
]>
<root>&ee;</root>

The case of parsed external entities works only for valid XML content. The XML standard also supports unparsed external entities with a NData declaration.

External entity expansion opens the door to plenty of exploits. An attacker can abuse a vulnerable XML library and application to rebound and forward network requests with the IP address of the server. It highly depends on the parser and the application what kind of exploit is possible. For example:

• An attacker can circumvent firewalls and gain access to restricted resources as all the requests are made from an internal and trustworthy IP address, not from the outside.
• An attacker can abuse a service to attack, spy on or DoS your servers but also third party services. The attack is disguised with the IP address of the server and the attacker is able to utilize the high bandwidth of a big machine.
• An attacker can exhaust additional resources on the machine, e.g. with requests to a service that doesn't respond or responds with very large files.
• An attacker may gain knowledge, when, how often and from which IP address an XML document is accessed.
• An attacker could send mail from inside your network if the URL handler supports smtp:// URIs.

External entities with references to local files are a sub-case of external entity expansion. It's listed as an extra attack because it deserves extra attention. Some XML libraries such as lxml disable network access by default but still allow entity expansion with local file access by default. Local files are either referenced with a file:// URL or by a file path (either relative or absolute). Additionally, lxml's libxml2 has catalog support. XML catalogs like /etc/xml/catalog are XML files, which map schema URIs to local files.

An attacker may be able to access and download all files that can be read by the application process. This may include critical configuration files, too.

<!DOCTYPE external [
<!ENTITY ee SYSTEM "file:///PATH/TO/simple.xml">
]>
<root>&ee;</root>

This case is similar to external entity expansion, too. Some XML libraries like Python's xml.dom.pulldom retrieve document type definitions from remote or local locations. Several attack scenarios from the external entity case apply to this issue as well.

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
  "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
    <head/>
    <body>text</body>
</html>

vulnerabilities and features

1. expat parser >= 2.4.0 has billion laughs protection against XML bombs (CVE-2013-0340). The parser has sensible defaults for XML_SetBillionLaughsAttackProtectionMaximumAmplification and XML_SetBillionLaughsAttackProtectionActivationThreshold.
2. Python >= 3.6.8, >= 3.7.1, and >= 3.8 no longer retrieve local and remote resources with urllib, see bpo-17239.
3. xml.etree doesn't expand entities and raises a ParserError when an entity occurs.
4. minidom doesn't expand entities and simply returns the unexpanded entity verbatim.
5. Library has (limited) XInclude support but requires an additional step to process inclusion.
6. These are features but they may introduce exploitable holes, see Other things to consider

feature_external_ges (http://xml.org/sax/features/external-general-entities)
disables external entity expansion

feature_external_pes (http://xml.org/sax/features/external-parameter-entities)
the option is ignored and doesn't modify any functionality

external_parameter_entities
ignored

external_general_entities
ignored

external_dtd_subset
ignored

entities
unsure

The defusedxml package (defusedxml on PyPI) contains several Python-only workarounds and fixes for denial of service and other vulnerabilities in Python's XML libraries. In order to benefit from the protection you just have to import and use the listed functions / classes from the right defusedxml module instead of the original module. Merely defusedxml.xmlrpc is implemented as monkey patch.

Instead of:

>>> from xml.etree.ElementTree import parse
>>> et = parse(xmlfile)

alter code to:

>>> from defusedxml.ElementTree import parse
>>> et = parse(xmlfile)

>>> from defusedxml import ElementTree as DET
>>> from xml.etree.ElementTree as ET

>>> root = DET.fromstring("<root/>")
>>> root.append(ET.Element("item"))
>>> ET.tostring(root)
b'<root><item /></root>'

Additionally the package has an untested function to monkey patch all stdlib modules with defusedxml.defuse_stdlib().

All functions and parser classes accept three additional keyword arguments. They return either the same objects as the original functions or compatible subclasses.

forbid_dtd (default: False)
disallow XML with a <!DOCTYPE> processing instruction and raise a DTDForbidden exception when a DTD processing instruction is found.

forbid_entities (default: True)
disallow XML with <!ENTITY> declarations inside the DTD and raise an EntitiesForbidden exception when an entity is declared.

forbid_external (default: True)
disallow any access to remote or local resources in external entities or DTD and raising an ExternalReferenceForbidden exception when a DTD or entity references an external resource.

DefusedXmlException, DTDForbidden, EntitiesForbidden, ExternalReferenceForbidden, NotSupportedError

defuse_stdlib() (experimental)

NOTE defusedxml.cElementTree is deprecated and will be removed in a future release. Import from defusedxml.ElementTree instead.

parse(), iterparse(), fromstring(), XMLParser

parse(), iterparse(), fromstring(), XMLParser

create_parser(), DefusedExpatParser

parse(), parseString(), make_parser()

parse(), parseString(), DefusedExpatBuilder, DefusedExpatBuilderNS

parse(), parseString()

parse(), parseString()

The fix is implemented as monkey patch for the stdlib's xmlrpc package (3.x) or xmlrpclib module (2.x). The function monkey_patch() enables the fixes, unmonkey_patch() removes the patch and puts the code in its former state.

The monkey patch protects against XML related attacks as well as decompression bombs and excessively large requests or responses. The default setting is 30 MB for requests, responses and gzip decompression. You can modify the default by changing the module variable MAX_DATA. A value of -1 disables the limit.

DEPRECATED The module is deprecated and will be removed in a future release.

lxml is safe against most attack scenarios. lxml uses libxml2 for parsing XML. The library has builtin mitigations against billion laughs and quadratic blowup attacks. The parser allows a limit amount of entity expansions, then fails. lxml also disables network access by default. libxml2 lxml FAQ lists additional recommendations for safe parsing, for example counter measures against compression bombs.

The default parser resolves entities and protects against huge trees and deeply nested entities. To disable entities expansion, use a custom parser object:

from lxml import etree

parser = etree.XMLParser(resolve_entities=False)
root = etree.fromstring("<example/>", parser=parser)

The module acts as an example how you could protect code that uses lxml.etree. It implements a custom Element class that filters out Entity instances, a custom parser factory and a thread local storage for parser instances. It also has a check_docinfo() function which inspects a tree for internal or external DTDs and entity declarations. In order to check for entities lxml > 3.0 is required.

parse(), fromstring() RestrictedElement, GlobalParserTLS, getDefaultParser(), check_docinfo()

The defusedexpat package (defusedexpat on PyPI) is no longer supported. expat parser 2.4.0 and newer come with billion laughs protection against XML bombs.

Update to Python 3.6.8, 3.7.1, or newer. The SAX and DOM parser do not load external entities from files or network resources.

Update to expat to 2.4.0 or newer. It has billion laughs protection with sensible default limits to mitigate billion laughs and quadratic blowup.

Official binaries from python.org use libexpat 2.4.0 since 3.7.12, 3.8.12, 3.9.7, and 3.10.0 (August 2021). Third party vendors may use older or newer versions of expat. pyexpat.version_info contains the current runtime version of libexpat. Vendors may have backported fixes to older versions without bumping the version number.

Example:

import sys
import pyexpat

has_mitigations = (
    sys.version_info >= (3, 7, 1) and
    pyexpat.version_info >= (2, 4, 0)
)

• Don't allow DTDs
• Don't expand entities
• Don't resolve externals
• Limit parse depth
• Limit total input size
• Limit parse time
• Favor a SAX or iterparse-like parser for potential large data
• Validate and properly quote arguments to XSL transformations and XPath queries
• Don't use XPath expression from untrusted sources
• Don't apply XSL transformations that come untrusted sources

(based on Brad Hill's Attacking XML Security)

XML, XML parsers and processing libraries have more features and possible issue that could lead to DoS vulnerabilities or security exploits in applications. I have compiled an incomplete list of theoretical issues that need further research and more attention. The list is deliberately pessimistic and a bit paranoid, too. It contains things that might go wrong under daffy circumstances.

XML parsers may use an algorithm with quadratic runtime O(n ²) to handle attributes and namespaces. If it uses hash tables (dictionaries) to store attributes and namespaces the implementation may be vulnerable to hash collision attacks, thus reducing the performance to O(n ²) again. In either case an attacker is able to forge a denial of service attack with an XML document that contains thousands upon thousands of attributes in a single node.

I haven't researched yet if expat, pyexpat or libxml2 are vulnerable.

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. Gzip is able to compress 1 GiB zeros to roughly 1 MB, lzma is even better:

$ dd if=/dev/zero bs=1M count=1024 | gzip > zeros.gz
$ dd if=/dev/zero bs=1M count=1024 | lzma -z > zeros.xy
$ ls -sh zeros.*
1020K zeros.gz
 148K zeros.xy

None of Python's standard XML libraries decompress streams except for xmlrpclib. The module is vulnerable <https://bugs.python.org/issue16043> to decompression bombs.

lxml can load and process compressed data through libxml2 transparently. libxml2 can handle even very large blobs of compressed data efficiently without using too much memory. But it doesn't protect applications from decompression bombs. A carefully written SAX or iterparse-like approach can be safe.

PI's like:

<?xml-stylesheet type="text/xsl" href="style.xsl"?>

may impose more threats for XML processing. It depends if and how a processor handles processing instructions. The issue of URL retrieval with network or local file access apply to processing instructions, too.

DTD has more features like <!NOTATION>. I haven't researched how these features may be a security threat.

XPath statements may introduce DoS vulnerabilities. Code should never execute queries from untrusted sources. An attacker may also be able to create an XML document that makes certain XPath queries costly or resource hungry.

XPath injeciton attacks pretty much work like SQL injection attacks. Arguments to XPath queries must be quoted and validated properly, especially when they are taken from the user. The page Avoid the dangers of XPath injection list some ramifications of XPath injections.

Python's standard library doesn't have XPath support. Lxml supports parameterized XPath queries which does proper quoting. You just have to use its xpath() method correctly:

# DON'T
>>> tree.xpath("/tag[@id='%s']" % value)

# instead do
>>> tree.xpath("/tag[@id=$tagid]", tagid=name)

XML Inclusion is another way to load and include external files:

<root xmlns:xi="http://www.w3.org/2001/XInclude">
  <xi:include href="filename.txt" parse="text" />
</root>

This feature should be disabled when XML files from an untrusted source are processed. Some Python XML libraries and libxml2 support XInclude but don't have an option to sandbox inclusion and limit it to allowed directories.

A validating XML parser may download schema files from the information in a xsi:schemaLocation attribute.

<ead xmlns="urn:isbn:1-931666-22-9"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:schemaLocation="urn:isbn:1-931666-22-9 http://www.loc.gov/ead/ead.xsd">
</ead>

You should keep in mind that XSLT is a Turing complete language. Never process XSLT code from unknown or untrusted source! XSLT processors may allow you to interact with external resources in ways you can't even imagine. Some processors even support extensions that allow read/write access to file system, access to JRE objects or scripting with Jython.

Example from Attacking XML Security for Xalan-J:

<xsl:stylesheet version="1.0"
 xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
 xmlns:rt="http://xml.apache.org/xalan/java/java.lang.Runtime"
 xmlns:ob="http://xml.apache.org/xalan/java/java.lang.Object"
 exclude-result-prefixes= "rt ob">
 <xsl:template match="/">
   <xsl:variable name="runtimeObject" select="rt:getRuntime()"/>
   <xsl:variable name="command"
     select="rt:exec($runtimeObject, &apos;c:\Windows\system32\cmd.exe&apos;)"/>
   <xsl:variable name="commandAsString" select="ob:toString($command)"/>
   <xsl:value-of select="$commandAsString"/>
 </xsl:template>
</xsl:stylesheet>

CVE-2013-1664
Unrestricted entity expansion induces DoS vulnerabilities in Python XML libraries (XML bomb)

CVE-2013-1665
External entity expansion in Python XML libraries inflicts potential security flaws and DoS vulnerabilities

Several other programming languages and frameworks are vulnerable as well. A couple of them are affected by the fact that libxml2 up to 2.9.0 has no protection against quadratic blowup attacks. Most of them have potential dangerous default settings for entity expansion and external entities, too.

Perl's XML::Simple is vulnerable to quadratic entity expansion and external entity expansion (both local and remote).

Ruby's REXML document parser is vulnerable to entity expansion attacks (both quadratic and exponential) but it doesn't do external entity expansion by default. In order to counteract entity expansion you have to disable the feature:

REXML::Document.entity_expansion_limit = 0

libxml-ruby and hpricot don't expand entities in their default configuration.

PHP's SimpleXML API is vulnerable to quadratic entity expansion and loads entities from local and remote resources. The option LIBXML_NONET disables network access but still allows local file access. LIBXML_NOENT seems to have no effect on entity expansion in PHP 5.4.6.

Information in XML DoS and Defenses (MSDN) suggest that .NET is vulnerable with its default settings. The article contains code snippets how to create a secure XML reader:

XmlReaderSettings settings = new XmlReaderSettings();
settings.ProhibitDtd = false;
settings.MaxCharactersFromEntities = 1024;
settings.XmlResolver = null;
XmlReader reader = XmlReader.Create(stream, settings);

Untested. The documentation of Xerces and its Xerces SecurityMananger sounds like Xerces is also vulnerable to billion laugh attacks with its default settings. It also does entity resolving when an org.xml.sax.EntityResolver is configured. I'm not yet sure about the default setting here.

Java specialists suggest to have a custom builder factory:

DocumentBuilderFactory builderFactory = DocumentBuilderFactory.newInstance();
builderFactory.setXIncludeAware(False);
builderFactory.setExpandEntityReferences(False);
builderFactory.setFeature(XMLConstants.FEATURE_SECURE_PROCESSING, True);
# either
builderFactory.setFeature("http://apache.org/xml/features/disallow-doctype-decl", True);
# or if you need DTDs
builderFactory.setFeature("http://xml.org/sax/features/external-general-entities", False);
builderFactory.setFeature("http://xml.org/sax/features/external-parameter-entities", False);
builderFactory.setFeature("http://apache.org/xml/features/nonvalidating/load-external-dtd", False);
builderFactory.setFeature("http://apache.org/xml/features/nonvalidating/load-dtd-grammar", False);

• DOM: Use xml.dom.xmlbuilder options for entity handling
• SAX: take feature_external_ges and feature_external_pes (?) into account
• test experimental monkey patching of stdlib modules
• improve documentation

Copyright (c) 2013-2023 by Christian Heimes <[email protected]>

Licensed to PSF under a Contributor Agreement.

See https://www.python.org/psf/license for licensing details.

Brett Cannon (Python Core developer)
review and code cleanup

Antoine Pitrou (Python Core developer)
code review

Aaron Patterson, Ben Murphy and Michael Koziarski (Ruby community)
Many thanks to Aaron, Ben and Michael from the Ruby community for their report and assistance.

Thierry Carrez (OpenStack)
Many thanks to Thierry for his report to the Python Security Response Team on behalf of the OpenStack security team.

Carl Meyer (Django)
Many thanks to Carl for his report to PSRT on behalf of the Django security team.

Daniel Veillard (libxml2)
Many thanks to Daniel for his insight and assistance with libxml2.

semantics GmbH (https://www.semantics.de/)
Many thanks to my employer semantics for letting me work on the issue during working hours as part of semantics's open source initiative.

• XML DoS and Defenses (MSDN)
• Billion Laughs on Wikipedia
• ZIP bomb on Wikipedia
• Configure SAX parsers for secure processing
• Testing for XML Injection

Release date: 2023

• Fix testing without lxml
• Test on 3.13-dev and PyPy 3.9

Release date: 29-Sep-2023

• Silence deprecation warning in defuse_stdlib.
• Update lxml safety information

Release date: 26-Sep-2023

• Drop support for Python 2.7, 3.4, and 3.5.
• Test on 3.10, 3.11, and 3.12.
• Add defusedxml.ElementTree.fromstringlist()
• Update vulnerabilities and features table in README.
• Pending removal The defusedxml.lxml module has been unmaintained and deprecated since 2019. The module will be removed in the next version.
• Pending removal The defusedxml.cElementTree will be removed in the next version. Please use defusedxml.ElementTree instead.

Release date: 08-Mar-2021

• Fix regression defusedxml.ElementTree.ParseError (#63) The ParseError exception is now the same class object as xml.etree.ElementTree.ParseError again.

Release date: 4-Mar-2021

• No changes

Release date: 12-Jan-2021

• Re-add and deprecate defusedxml.cElementTree
• Use GitHub Actions instead of TravisCI
• Restore ElementTree attribute of xml.etree module after patching

Release date: 04-May-2020

• Add support for Python 3.9
• defusedxml.cElementTree is not available with Python 3.9.
• Python 2 is deprecate. Support for Python 2 will be removed in 0.8.0.

Release date: 17-Apr-2019

• Increase test coverage.
• Add badges to README.

Release date: 14-Apr-2019

• Test on Python 3.7 stable and 3.8-dev
• Drop support for Python 3.4
• No longer pass html argument to XMLParse. It has been deprecated and ignored for a long time. The DefusedXMLParser still takes a html argument. A deprecation warning is issued when the argument is False and a TypeError when it's True.
• defusedxml now fails early when pyexpat stdlib module is not available or broken.
• defusedxml.ElementTree.__all__ now lists ParseError as public attribute.
• The defusedxml.ElementTree and defusedxml.cElementTree modules had a typo and used XMLParse instead of XMLParser as an alias for DefusedXMLParser. Both the old and fixed name are now available.

Release date: 07-Feb-2017

• No changes

Release date: 28-Jan-2017

• Add compatibility with Python 3.6
• Drop support for Python 2.6, 3.1, 3.2, 3.3
• Fix lxml tests (XMLSyntaxError: Detected an entity reference loop)

Release date: 28-Mar-2013

• Add more demo exploits, e.g. python_external.py and Xalan XSLT demos.
• Improved documentation.

Release date: 25-Feb-2013

• As per http://seclists.org/oss-sec/2013/q1/340 please REJECT CVE-2013-0278, CVE-2013-0279 and CVE-2013-0280 and use CVE-2013-1664, CVE-2013-1665 for OpenStack/etc.
• Add missing parser_list argument to sax.make_parser(). The argument is ignored, though. (thanks to Florian Apolloner)
• Add demo exploit for external entity attack on Python's SAX parser, XML-RPC and WebDAV.

Release date: 19-Feb-2013

• Improve documentation

Release date: 15-Feb-2013

• Rename ExternalEntitiesForbidden to ExternalReferenceForbidden
• Rename defusedxml.lxml.check_dtd() to check_docinfo()
• Unify argument names in callbacks
• Add arguments and formatted representation to exceptions
• Add forbid_external argument to all functions and classes
• More tests
• LOTS of documentation
• Add example code for other languages (Ruby, Perl, PHP) and parsers (Genshi)
• Add protection against XML and gzip attacks to xmlrpclib

Release date: 08-Feb-2013

• Initial and internal release for PSRT review