XML External Entity (XXE) Processing

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Vulnerabilities Table of Contents

Description
Processing of an external entity containing tainted data may lead to disclosure of confidential information and other system impacts.

The XML 1.0 standard defines the structure of an XML document. The standard defines a concept called an entity, which is a storage unit of some type. There exists a specific type of entity, an external general parsed entity often shortened to an external entity, that can access local or remote content via a declared system identifier. The system identifier is assumed to be a URI that can be dereferenced (accessed) by the XML processor when processing the entity. The XML processor then replaces occurrences of the named external entity with the contents dereferenced by the system identifier. If the system identifier contains tainted data and the XML processor dereferences this tainted data, the XML processor may disclose confidential information normally not accessible by the application.

Attacks can include disclosing local files, which may contain sensitive data such as passwords or private user data, using file: schemes or relative paths in the system identifier. Since the attack occurs relative to the application processing the XML document, an attacker may use this trusted application to pivot to other internal systems, possibly disclosing other internal content via http(s) requests. In some situations, an XML processor library that is vulnerable to client-side memory corruption issues may be exploited by dereferencing a malicious URI, possibly allowing arbitrary code execution under the application account. Other attacks can access local resources that may not stop returning data, possibly impacting application availability if too many threads or processes are not released.

Risk Factors

 * The application parses XML documents.
 * Tainted data is allowed within the system identifier portion of the entity, within the document type declaration (DTD).
 * The XML processor is configured to validate and process the DTD.
 * The XML processor is configured to resolve external entities within the DTD.

Examples
The examples below are from Testing for XML Injection (OWASP-DV-008).

Accessing a local resource that may not return
'''  <!DOCTYPE foo > &xxe; '''

Disclosing /etc/passwd or other targeted files
'''  <!DOCTYPE foo > &xxe;

 <!DOCTYPE foo > &xxe;

 <!DOCTYPE foo > &xxe;

 <!DOCTYPE foo [ <!ELEMENT foo ANY > <!ENTITY xxe SYSTEM "http://www.attacker.com/text.txt" >]> &xxe; '''

Related Attacks

 * SQL Injection
 * Blind SQL Injection

Related Vulnerabilities

 * Missing XML Validation

Related Controls
Since the whole XML document is communicated from an untrusted client, it's not usually possible to selectively validate or escape tainted data within the system identifier in the DTD. Therefore, the XML processor should be configured to use a local static DTD and disallow any declared DTD included in the XML document.

Testing ought to occur with specific implementations for any controls documented below.

libxml2
The Enum xmlParserOption should not have the following options defined:


 * XML_PARSE_NOENT: Expands entities and substitutes them with replacement text
 * XML_PARSE_DTDLOAD: Load the external DTD

Note: Per: https://mail.gnome.org/archives/xml/2012-October/msg00045.html, starting with libxml2 version 2.9, XXE has been disabled by default as committed by the following patch: http://git.gnome.org/browse/libxml2/commit/?id=4629ee02ac649c27f9c0cf98ba017c6b5526070f.

Java
Java applications using XML libraries are particularly vulnerable to XXE because the default settings for most Java XML parsers is to have XXE enabled. To use these parsers safely, you have to explicitly disable XXE in the parser you use. The following describes how to disable XXE in the most commonly used XML parsers for Java.

JAXP DOM and DocumentBuilderFactory
The DocumentBuilderFactory setFeature method allows a developer to control which implementation-specific XML processor features are enabled or disabled. Each XML processor implementation has its own features that govern how DTDs and external entities are processed.

For a syntax highlighted code snippet, click here.

''' import javax.xml.parsers.DocumentBuilderFactory; import javax.xml.parsers.ParserConfigurationException; // catching unsupported features ...   DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance; try { // Xerces 1 - http://xerces.apache.org/xerces-j/features.html#external-general-entities // Xerces 2 - http://xerces.apache.org/xerces2-j/features.html#external-general-entities dbf.setFeature("http://xml.org/sax/features/external-general-entities", false); // Xerces 2 only - http://xerces.apache.org/xerces2-j/features.html#disallow-doctype-decl dbf.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true); // remaining parser logic ...       catch (ParserConfigurationException e) { // This should catch a failed setFeature feature logger.info("ParserConfigurationException was thrown. The feature '" +                       DISALLOW_FEATURE +                        "' is probably not supported by your XML processor."); ...       }        catch (SAXException e) { // On Apache, this should be thrown when disallowing DOCTYPE logger.warning("A DOCTYPE was passed into the XML document"); ...       }        catch (IOException e) { // XXE that points to a file that doesn't exist logger.error("IOException occurred, XXE may still possible: " + e.getMessage); ...       } '''

Xerces 1 Features:
 * Do not include external entities by setting this feature to.

Xerces 2 Features:
 * Disallow an inline DTD by setting this feature to.
 * Do not include external entities by setting this feature to.

JAXP SAX and SAXParserFactory
The SAXParserFactory setFeature method allows a developer to control which implementation-specific XML processor features are enabled or disabled. The features can either be set on the factory or the underlying XMLReader setFeature method. Each XML processor implementation has its own features that govern how DTDs and external entities are processed.

For a syntax highlighted code snippet, click here.

''' import javax.xml.parsers.ParserConfigurationException; // catching unsupported features import javax.xml.parsers.SAXParser; import javax.xml.parsers.SAXParserFactory;

import org.xml.sax.SAXNotRecognizedException; // catching unknown features import org.xml.sax.SAXNotSupportedException; // catching known but unsupported features import org.xml.sax.XMLReader;

...

SAXParserFactory spf = SAXParserFactory.newInstance; SAXParser saxParser = spf.newSAXParser; XMLReader reader = saxParser.getXMLReader;

try { // Xerces 1 - http://xerces.apache.org/xerces-j/features.html#external-general-entities // Xerces 2 - http://xerces.apache.org/xerces2-j/features.html#external-general-entities

// Using the SAXParserFactory's setFeature spf.setFeature("http://xml.org/sax/features/external-general-entities", false); // Using the XMLReader's setFeature reader.setFeature("http://xml.org/sax/features/external-general-entities", false);

// Xerces 2 only - http://xerces.apache.org/xerces2-j/features.html#disallow-doctype-decl spf.setFeature("http://apache.org/xml/features/disallow-doctype-decl", false);

// remaining parser logic ...

} catch (ParserConfigurationException e) { // Tried an unsupported feature.

} catch (SAXNotRecognizedException e) { // Tried an unknown feature.

} catch (SAXNotSupportedException e) { // Tried a feature known to the parser but unsupported.

} catch ... {   } ... '''

Xerces 1 Features:
 * Do not include external entities by setting this feature to.

Xerces 2 Features:
 * Disallow an inline DTD by setting this feature to.
 * Do not include external entities by setting this feature to.

StAX and XMLInputFactory
The StAX XMLInputFactory can allow properties and features to be set.

Disallow Resolving of External Entities:
 * Set the "javax.xml.stream.isSupportingExternalEntities" property to.

libxml2
iOS includes the C/C++ libxml2 library described above, so that guidance applies if you are using libxml2 directly. However, the version of libxml2 provided up through iOS6 is prior to version 2.9 of libxml2 (which protects against XXE by default).

NSXMLDocument
iOS also provides an NSXMLDocument type, which is built on top of libxml2. However, NSXMLDocument provides some additional protections against XXE that aren't available in libxml2 directly. Per the 'NSXMLDocument External Entity Restriction API' section of: http://developer.apple.com/library/ios/#releasenotes/Foundation/RN-Foundation-iOS/Foundation_iOS5.html:


 * iOS4 and earlier: All external entities are loaded by default.


 * iOS5 and later: Only entities that don't require network access are loaded. (which is safer)

However, to completely disable XXE in an NSXMLDocument in any version of iOS you simply specify NSXMLNodeLoadExternalEntitiesNever when creating the NSXMLDocument.