XML External Entity (XXE) Processing

Last revision (mm/dd/yy): //

Vulnerabilities Table of Contents

Description
An XML External Entity attack is a type of attack against an application that parses XML input. This attack occurs when XML input containing a reference to an external entity is processed by a weakly configured XML parser. This attack may lead to the disclosure of confidential data, denial of service, server side request forgery, port scanning from the perspective of the machine where the parser is located, 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 are a few different types of entities, external general/parameter parsed entity often shortened to 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. Similar attack vectors apply the usage of external DTDs, external stylesheets, external schemas, etc. which, when included, allow similar external resource inclusion style attacks.

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 or launching a CSRF attack to any unprotected internal services. 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.

Note that the application does not need to explicitly return the response to the attacker for it to be vulnerable to information disclosures. An attacker can leverage DNS information to exfiltrate data through subdomain names to a DNS server that he/she controls.

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.

General Guidance
There are many XML parsers that come with the JDK, and many more from 3rd party libraries. The safest way to prevent XXE is always to disable DTDs completely. Depending on the parser, the method should be similar to the following:  factory.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true); 

Disabling DTDs also makes the parser secure against denial of services (DOS) attacks such as Billion Laughs. If it is not possible to disable DTDs completely, then external entities and external doctypes must be disabled in the way that’s specific to each parser listed below.

JAXP DocumentBuilderFactory and SAXParserFactory
Both DocumentBuilderFactory and SAXParserFactory XML Parsers can be configured using the same techniques to protect them against XXE. Only the DocumentBuilderFactory example is presented here. The JAXP DocumentBuilderFactory 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 for DocumentBuilderFactory, click here.

For a syntax highlighted code snippet for SAXParserFactory, click here.

''' import javax.xml.parsers.DocumentBuilderFactory; import javax.xml.parsers.ParserConfigurationException; // catching unsupported features ...   DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance; try { // This is the PRIMARY defense. If DTDs (doctypes) are disallowed, almost all XML entity attacks are prevented // Xerces 2 only - http://xerces.apache.org/xerces2-j/features.html#disallow-doctype-decl String FEATURE = "http://apache.org/xml/features/disallow-doctype-decl"; dbf.setFeature(FEATURE, true);

// If you can't completely disable DTDs, then at least do the following: // 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 // JDK7+ - http://xml.org/sax/features/external-general-entities FEATURE = "http://xml.org/sax/features/external-general-entities"; dbf.setFeature(FEATURE, false);

// Xerces 1 - http://xerces.apache.org/xerces-j/features.html#external-parameter-entities // Xerces 2 - http://xerces.apache.org/xerces2-j/features.html#external-parameter-entities // JDK7+ - http://xml.org/sax/features/external-parameter-entities FEATURE = "http://xml.org/sax/features/external-parameter-entities"; dbf.setFeature(FEATURE, false);

// Disable external DTDs as well FEATURE = “http://apache.org/xml/features/nonvalidating/load-external-dtd” dbf.setFeature(FEATURE, false);

// and these as well, per Timothy Morgan's 2014 paper: "XML Schema, DTD, and Entity Attacks" (see reference below) dbf.setXIncludeAware(false); dbf.setExpandEntityReferences(false); // And, per Timothy Morgan: "If for some reason support for inline DOCTYPEs are a requirement, then      // ensure the entity settings are disabled (as shown above) and beware that SSRF attacks      // (http://cwe.mitre.org/data/definitions/918.html) and denial       // of service attacks (such as billion laughs or decompression bombs via "jar:") are a risk."

// remaining parser logic ...       catch (ParserConfigurationException e) { // This should catch a failed setFeature feature logger.info("ParserConfigurationException was thrown. The feature '" +                       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.
 * Do not include parameter entities by setting this feature to.
 * Do not include external DTDs 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.
 * Do not include parameter entities by setting this feature to.
 * Do not include external DTDs by setting this feature to.

StAX and XMLInputFactory
StAX parsers such as XMLInputFactory allow various properties and features to be set.

To protect a Java XMLInputFactory from XXE, do this:


 * xmlInputFactory.setProperty(XMLInputFactory.SUPPORT_DTD, false); // This disables DTDs entirely for that factory
 * xmlInputFactory.setProperty(“javax.xml.stream.isSupportingExternalEntities”, false); // disable external entities

TransformerFactory
To protect a Java TransformerFactory from XXE, do this:
 * TransformerFactory tf = TransformerFactory.newInstance;
 * tf.setAttribute(XMLConstants.ACCESS_EXTERNAL_DTD, "");
 * tf.setAttribute(XMLConstants.ACCESS_EXTERNAL_STYLESHEET, "");

Validator
To protect a Java Validator from XXE, do this:
 * SchemaFactory factory = SchemaFactory.newInstance("http://www.w3.org/2001/XMLSchema");
 * Schema schema = factory.newSchema;
 * Validator validator = schema.newValidator;
 * validator.setProperty(XMLConstants.ACCESS_EXTERNAL_DTD, "");
 * validator.setProperty(XMLConstants.ACCESS_EXTERNAL_SCHEMA, "");

SchemaFactory
To protect a SchemaFactory from XXE, do this:
 * SchemaFactory factory = SchemaFactory.newInstance("http://www.w3.org/2001/XMLSchema");
 * factory.setProperty(XMLConstants.ACCESS_EXTERNAL_DTD, "");
 * factory.setProperty(XMLConstants.ACCESS_EXTERNAL_SCHEMA, "");
 * Schema schema = factory.newSchema(Source);

SAXTransformerFactory
To protect a Java SAXTransformerFactory from XXE, do this:
 * SAXTransformerFactory sf = SAXTransformerFactory.newInstance;
 * sf.setAttribute(XMLConstants.ACCESS_EXTERNAL_DTD, "");
 * sf.setAttribute(XMLConstants.ACCESS_EXTERNAL_STYLESHEET, "");
 * sf.newXMLFilter(Source);

XMLReader
To protect a Java XMLReader from XXE, do this:
 * XMLReader spf = XMLReaderFactory.createXMLReader;
 * spf.setFeature(“http://xml.org/sax/features/external-general-entities”, false);
 * spf.setFeature("http://xml.org/sax/features/external-parameter-entities", false);
 * spf.setFeature(“http://apache.org/xml/features/nonvalidating/load-external-dtd”,false);

Unmarshaller
Since an Unmarshaller parses XML and does not support any flags for disabling XXE, it’s imperative to parse the untrusted XML through a configurable secure parser first, generate a Source object as a result, and pass the source object to the Unmarshaller. For example:
 * SAXParserFactory spf = SAXParserFactory.newInstance;
 * spf.setFeature(“http://xml.org/sax/features/external-general-entities”, false);
 * spf.setFeature("http://xml.org/sax/features/external-parameter-entities", false);
 * spf.setFeature("http://apache.org/xml/features/nonvalidating/load-external-dtd", false);


 * Source xmlSource = new SAXSource(spf.newSAXParser.getXMLReader, new InputSource(new StringReader(xml)));
 * JAXBContext jc = JAXBContext.newInstance(Object.class);
 * Unmarshaller um = jc.createUnmarshaller;
 * um.unmarshal(xmlSource);

XPathExpression
An XPathExpression is similar to an Unmarshaller where it can’t be configured securely by itself, so the untrusted data must be parsed through another securable XML parser first. For example:
 * DocumentBuilderFactory df =DocumentBuilderFactory.newInstance;
 * df.setAttribute(XMLConstants.ACCESS_EXTERNAL_DTD, "");
 * df.setAttribute(XMLConstants.ACCESS_EXTERNAL_SCHEMA, "");
 * builder = df.newDocumentBuilder;
 * xPathExpression.evaluate( builder.parse(new ByteArrayInputStream(xml.getBytes)) );

Non-JDK XML Parsers
There are many 3rd party libraries that parse XML either directly or through their use of other libraries. Please test and verify their XML parser is secure against XXE by default. If the parser is not secure by default, look for flags supported by the parser to disable all possible external resource inclusions like the examples given above. If there’s no control exposed to the outside, make sure the untrusted content is passed through a secure parser first and then passed to insecure 3rd party parser similar to how the Unmarshaller is secured.

.NET
The following information for .NET are almost direct quotes from this great article on how to prevent XXE and XML Denial of Service in .NET:http://msdn.microsoft.com/en-us/magazine/ee335713.aspx.

In the .NET Framework, you can prevent XmlReader from resolving external entities while still allowing it to resolve inline entities by setting the XmlResolver property of XmlReaderSettings to null.

settings.XmlResolver = null;

To protect your app from XML Denial of Service attack you should prohibit inline entities resolving by changing DTD parsing settings.

.NET 3.5
In .NET Framework versions 3.5 and earlier, DTD parsing behavior is controlled by the Boolean ProhibitDtd property found in the System.Xml.XmlTextReader and System.Xml.XmlReaderSettings classes. Set this value to true to disable inline DTDs completely:

XmlTextReader reader = new XmlTextReader(stream); reader.ProhibitDtd = true;

or

XmlReaderSettings settings = new XmlReaderSettings; settings.ProhibitDtd = true; XmlReader reader = XmlReader.Create(stream, settings);

The default value of ProhibitDtd in XmlReaderSettings is true, but the default value of ProhibitDtd in XmlTextReader is false, which means that you have to explicitly set it to true to disable inline DTDs.

If you need DTD parsing enabled, but need to know how to do it safely, the above referenced MSDN article has detailed instructions on how to do that too.

.NET 4.0, .NET 4.5
In .NET Framework version 4.0, DTD parsing behavior has been changed. The ProhibitDtd property has been deprecated in favor of the new DtdProcessing property, whose default value is Prohibit. This means that .NET 4.0 apps should be immune to XXE by default, if they are using an XmlReader to parse their XML.

Setting DtdProcessing to Prohibit causes the runtime to throw an exception if a <!DOCTYPE> element is present in the XML. To set this value yourself, it looks like this:

XmlReaderSettings settings = new XmlReaderSettings; settings.DtdProcessing = DtdProcessing.Prohibit; XmlReader reader = XmlReader.Create(stream, settings);

Alternatively, you can set the DtdProcessing property to Ignore, which will not throw an exception on encountering a <!DOCTYPE> element but will simply skip over it and not process it. Finally, you can set DtdProcessing to Parse if you do want to allow and process inline DTDs.

Again, if you need to enable DTD processing, instructions on how to do so safely are described in detail in the referenced MSDN article.

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.

PHP
Per the PHP documentation, the following should be set when using the default PHP XML parser in order to prevent XXE:

libxml_disable_entity_loader(true);

A description of how to abuse this in PHP is presented in a good SensePost article describing a cool PHP based XXE vulnerability that was fixed in Facebook.