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XML bomb protection with modified expat parser

Project description

defusedexpat protects the XML packages of Python’s standard library from several denial of service vulnerabilities and external entity exploits. It contains

  • a modified and enhanced version of expat parser library

  • replacements for pyexpat and cElementTree’s _elementtree extension modules

  • loader code that replaces built-in extensions with the modified extensions

  • monkey patches for xml.sax and xml.dom to prevent external entity expansions

In order to protect your application you have to import the defusedxml module before any of the stdlib’s XML modules.


  • limited entity expansion level to antagonize billion laugh attacks

  • limited total length of expansions to prevent quadratic blowups

  • monkey patch to prevent retrieval of external entities and DTDs


Modifications in pyexpat

Parser object

New parser attributes (r/w)

  • max_entity_indirections

  • max_entity_expansions

  • reset_dtd

Module constants




Modules functions

  • get_reset_dtd(), set_reset_dtd(bool)

  • get_max_entity_expansions(), set_max_entity_expansions(int)

  • get_max_entity_indirections(), et_max_entity_indirections(int)

New CAPI members

  • capi.GetFeature

  • capi.SetFeature

  • capi.GetFeatureDefault

  • capi.SetFeatureDefault

Modifications in _elementtree


New arguments and r/o attributes

  • max_entity_indirections

  • max_entity_expansions

  • ignore_dtd

Modifications in expat

new definitions:


new XML_FeatureEnum members:


new XML_Error members:


new API functions:

int XML_GetFeature(XML_Parser parser,
                   enum XML_FeatureEnum feature,
                   long *value);
int XML_SetFeature(XML_Parser parser,
                   enum XML_FeatureEnum feature,
                   long value);
int XML_GetFeatureDefault(enum XML_FeatureEnum feature,
                          long *value);
int XML_SetFeatureDefault(enum XML_FeatureEnum feature,
                          long value);

Limit the amount of indirections that are allowed to occur during the expansion of a nested entity. A counter starts when an entity reference is encountered. It resets after the entity is fully expanded. The limit protects the parser against exponential entity expansion attacks (aka billion laughs attack). When the limit is exceeded the parser stops and fails with XML_ERROR_ENTITY_INDIRECTIONS. A value of 0 disables the protection.

Supported range





Limit the total length of all entity expansions throughout the entire document. The lengths of all entities are accumulated in a parser variable. The setting protects against quadratic blowup attacks (lots of expansions of a large entity declaration). When the sum of all entities exceeds the limit, the parser stops and fails with XML_ERROR_ENTITY_EXPANSION. A value of 0 disables the protection.

Supported range



8 MiB


Reset all DTD information after the <!DOCTYPE> block has been parsed. When the flag is set (default: false) all DTD information after the endDoctypeDeclHandler has been called. The flag can be set inside the endDoctypeDeclHandler. Without DTD information any entity reference in the document body leads to XML_ERROR_UNDEFINED_ENTITY.

Supported range

0, 1




  • Python 2.6.6 or newer (2.6.8 or newer for randomized hashing)

  • Python 2.7 (2.7.3 or newer for randomized hashing and Windows binaries)

  • Python 3.1 (3.1.5 or newer for randomized hashing and Windows binaries)

  • Python 3.2 (3.2.3 or newer for randomized hashing and Windows binaries)

  • Python 3.3.0 or newer

Windows binaries are compatible to 2.6.6, 2.7.3, 3.1.5, 3.2.3 or 3.3.0 or newer micro releases. They don’t work on older versions.


Copyright (c) 2013 by Christian Heimes <>

Licensed to PSF under a Contributor Agreement.

See for licensing details.


Antoine Pitrou

code review

Brett Cannon

code review


defusedexpat 0.4

Release date: 25-Feb-2013

  • Make code source compatible to Python versions without hash randomization.

  • Include latest version of expat patch

defusedexpat 0.3

Release date: 19-Feb-2013

  • Disable some tests on Windows because proxy trick doesn’t work

  • Remove ‘bomb protection’ suffix from CAPI MAGIC

  • Don’t support additional kwargs in _elementtree when XML_BOMB_PROTECTION is not available.

  • Implement better and more flexibel get/set feature API in expat

  • Add module functions to set global settings

defusedexpat 0.2

Release date: 15-Feb-2013

  • Python 3.1 support

  • Misc fixes and improvements

defusedexpat 0.1

Release date: 11-Feb-2013

  • Initial and internal release for PSRT review

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