How to prevent xxe vulnerabilities?

XML eXternal Entity injection (XXE), which is now part of the OWASP Top 10 via the point A4, is a type of attack against an application that parses XML input.

XXE issue is referenced under the ID 611 in the Common Weakness Enumeration referential.

This attack occurs when untrusted 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 (SSRF), port scanning from the perspective of the machine where the parser is located, and other system impacts. The following guide provides concise information to prevent this vulnerability.

For more information on XXE, please visit XML External Entity (XXE).

The safest way to prevent XXE is always to disable DTDs (External Entities) completely. Depending on the parser, the method should be similar to the following:

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 document type declarations must be disabled in the way that's specific to each parser.

Detailed XXE Prevention guidance for a number of languages and commonly used XML parsers in those languages is provided below.

The Enum xmlParserOption should not have the following options defined:

Note:

Per: According to this post, starting with libxml2 version 2.9, XXE has been disabled by default as committed by the following patch.

Search for the usage of the following APIs to ensure there is no XML_PARSE_NOENT and XML_PARSE_DTDLOAD defined in the parameters:

Use of XercesDOMParser do this to prevent XXE:

Use of SAXParser, do this to prevent XXE:

Use of SAX2XMLReader, do this to prevent XXE:

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.

DocumentBuilderFactory, SAXParserFactory and DOM4J 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. By disabling DTD processing entirely, most XXE attacks can be averted, although it is also necessary to disable or verify that XInclude is not enabled.

Since the JDK 6, the flag FEATURE_SECURE_PROCESSING can be used to instruct the implementation of the parser to process XML securely. Its behaviour is implementation dependent. Even if it can help tackling resource exhaustion, it may not always mitigate entity expansion. More details on this flag can be found here.

For a syntax highlighted example code snippet using SAXParserFactory, look here. Example code disabling DTDs (doctypes) altogether:

If you can't completely disable DTDs:

Xerces 1 Features:

Xerces 2 Features:

Note: The above defenses require Java 7 update 67, Java 8 update 20, or above, because the above countermeasures for DocumentBuilderFactory and SAXParserFactory are broken in earlier Java versions, per: CVE-2014-6517.

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

To protect a Java XMLInputFactory from XXE, disable DTDs (doctypes) altogether:

or if you can't completely disable DTDs:

The setting xmlInputFactory.setProperty(XMLConstants.ACCESS_EXTERNAL_SCHEMA, ""); is not required, as XMLInputFactory is dependent on Validator to perform XML validation against Schemas. Check the Validator section for the specific configuration.

Follow Oracle recommendation e.g.:

To protect a javax.xml.transform.TransformerFactory from XXE, do this:

To protect a javax.xml.validation.Validator from XXE, do this:

To protect a javax.xml.validation.SchemaFactory from XXE, do this:

To protect a javax.xml.transform.sax.SAXTransformerFactory from XXE, do this:

Note: Use of the following XMLConstants requires JAXP 1.5, which was added to Java in 7u40 and Java 8:

To protect a Java org.xml.sax.XMLReader from XXE, do this:

To protect a Java org.dom4j.io.SAXReader from XXE, do this:

Based on testing, if you are missing one of these, you can still be vulnerable to an XXE attack.

To protect a Java org.jdom2.input.SAXBuilder from XXE, disallow DTDs (doctypes) entirely:

Alternatively, if DTDs can't be completely disabled, disable external entities and entity expansion:

For APIs that take an EntityResolver, you can neutralize an XML parser's ability to resolve entities by supplying a no-op implementation:

or more simply:

Since a javax.xml.bind.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:

A javax.xml.xpath.XPathExpression can not be configured securely by itself, so the untrusted data must be parsed through another securable XML parser first.

For example:

The readObject() method in this class is fundamentally unsafe.

Not only is the XML it parses subject to XXE, but the method can be used to construct any Java object, and execute arbitrary code as described here.

And there is no way to make use of this class safe except to trust or properly validate the input being passed into it.

As such, we'd strongly recommend completely avoiding the use of this class and replacing it with a safe or properly configured XML parser as described elsewhere in this cheat sheet.

There are many third-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 third-party parser similar to how the Unmarshaller is secured.

For example, some XXE vulnerabilities were found in Spring OXM and Spring MVC. The following versions of the Spring Framework are vulnerable to XXE:

There were other issues as well that were fixed later, so to fully address these issues, Spring recommends you upgrade to Spring Framework 3.2.8+ or 4.0.2+.

For Spring OXM, this is referring to the use of org.springframework.oxm.jaxb.Jaxb2Marshaller. Note that the CVE for Spring OXM specifically indicates that 2 XML parsing situations are up to the developer to get right, and 2 are the responsibility of Spring and were fixed to address this CVE.

Here's what they say:

Two situations developers must handle:

The issue Spring fixed:

For SAXSource and StreamSource instances, Spring processed external entities by default thereby creating this vulnerability.

Here's an example of using a StreamSource that was vulnerable, but is now safe, if you are using a fixed version of Spring OXM or Spring MVC:

So, per the Spring OXM CVE writeup, the above is now safe. But if you were to use a DOMSource or StAXSource instead, it would be up to you to configure those sources to be safe from XXE.

Castor is a data binding framework for Java. It allows conversion between Java objects, XML, and relational tables. The XML features in Castor prior to version 1.3.3 are vulnerable to XXE, and should be upgraded to the latest version. For additional information, check the official XML configuration file

The following, up to date information for XXE injection in .NET is directly from this web application of unit tests by Dean Fleming. This web application covers all currently supported .NET XML parsers, and has test cases for each demonstrating when they are safe from XXE injection and when they are not, but tests are only with injection from file and not direct DTD (used by DoS attacks).

For DoS attacks using a direct DTD (such as the Billion laughs attack), a separate testing application from Josh Grossman at Bounce Security has been created to verify that .NET >=4.5.2 is safe from these attacks.

Previously, this information was based on some older articles which may not be 100% accurate including:

The following table lists all supported .NET XML parsers and their default safety levels. Note that in .NET Framework ≥4.5.2 in all cases if a DoS attempt is performed, an exception is thrown due to the expanded XML being too many characters.

Table explanation:

* For .NET Framework Versions ≥4.5.2, these libraries won't even process the in-line DTD by default. Even if you change the default to allow processing a DTD, if a DoS attempt is performed an exception will still be thrown as documented above.

Both the XElement and XDocument objects in the System.Xml.Linq library are safe from XXE injection from external file and DoS attack by default. XElement parses only the elements within the XML file, so DTDs are ignored altogether. XDocument has XmlResolver disabled by default so it's safe from SSRF. Whilst DTDs are enabled by default, from Framework versions ≥4.5.2, it is not vulnerable to DoS as noted but it may be vulnerable in earlier Framework versions. For more information, see Microsoft's guidance on how to prevent XXE and XML Denial of Service in .NET

System.Xml.XmlDictionaryReader is safe by default, as when it attempts to parse the DTD, the compiler throws an exception saying that "CData elements not valid at top level of an XML document". It becomes unsafe if constructed with a different unsafe XML parser.

Prior to .NET Framework version 4.5.2, System.Xml.XmlDocument is unsafe by default. The XmlDocument object has an XmlResolver object within it that needs to be set to null in versions prior to 4.5.2. In versions 4.5.2 and up, this XmlResolver is set to null by default.

The following example shows how it is made safe:

For .NET Framework version ≥4.5.2, this is safe by default.

XmlDocument can become unsafe if you create your own nonnull XmlResolver with default or unsafe settings. If you need to enable DTD processing, instructions on how to do so safely are described in detail in the referenced MSDN article.

System.Xml.XmlNodeReader objects are safe by default and will ignore DTDs even when constructed with an unsafe parser or wrapped in another unsafe parser.

System.Xml.XmlReader objects are safe by default.

They are set by default to have their ProhibitDtd property set to false in .NET Framework versions 4.0 and earlier, or their DtdProcessing property set to Prohibit in .NET versions 4.0 and later.

Additionally, in .NET versions 4.5.2 and later, the XmlReaderSettings belonging to the XmlReader has its XmlResolver set to null by default, which provides an additional layer of safety.

Therefore, XmlReader objects will only become unsafe in version 4.5.2 and up if both the DtdProcessing property is set to Parse and the XmlReaderSetting's XmlResolver is set to a nonnull XmlResolver with default or unsafe settings. If you need to enable DTD processing, instructions on how to do so safely are described in detail in the referenced MSDN article.

System.Xml.XmlTextReader is unsafe by default in .NET Framework versions prior to 4.5.2. Here is how to make it safe in various .NET versions:

In .NET Framework versions prior to 4.0, DTD parsing behavior for XmlReader objects like XmlTextReader are controlled by the Boolean ProhibitDtd property found in the System.Xml.XmlReaderSettings and System.Xml.XmlTextReader classes.

Set these values to true to disable inline DTDs completely.

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.

However, they didn't change the default settings so XmlTextReader is still vulnerable to XXE by default.

Setting DtdProcessing to Prohibit causes the runtime to throw an exception if a element is present in the XML.

To set this value yourself, it looks like this:

Alternatively, you can set the DtdProcessing property to Ignore, which will not throw an exception on encountering a 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.

In .NET Framework versions 4.5.2 and up, XmlTextReader's internal XmlResolver is set to null by default, making the XmlTextReader ignore DTDs by default. The XmlTextReader can become unsafe if you create your own nonnull XmlResolver with default or unsafe settings.

System.Xml.XPath.XPathNavigator is unsafe by default in .NET Framework versions prior to 4.5.2.

This is due to the fact that it implements IXPathNavigable objects like XmlDocument, which are also unsafe by default in versions prior to 4.5.2.

You can make XPathNavigator safe by giving it a safe parser like XmlReader (which is safe by default) in the XPathDocument's constructor.

Here is an example:

For .NET Framework version ≥4.5.2, XPathNavigator is safe by default.

System.Xml.Xsl.XslCompiledTransform (an XML transformer) is safe by default as long as the parser it's given is safe.

It is safe by default because the default parser of the Transform() methods is an XmlReader, which is safe by default (per above).

The source code for this method is here.

Some of the Transform() methods accept an XmlReader or IXPathNavigable (e.g., XmlDocument) as an input, and if you pass in an unsafe XML Parser then the Transform will also be unsafe.

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).

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 this page:

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

When using the default XML parser (based on libxml2), PHP 8.0 and newer prevent XXE by default.

For PHP versions prior to 8.0, per the PHP documentation, the following should be set when using the default PHP XML parser in order to prevent XXE:

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.

The Python 3 official documentation contains a section on xml vulnerabilities. As of the 1st January 2020 Python 2 is no longer supported, however the Python website still contains some legacy documentation.

The following table gives an overview of various modules in Python 3 used for XML parsing and whether or not they are vulnerable.

To protect your application from the applicable attacks, two packages exist to help you sanitize your input and protect your application against DDoS and remote attacks.

Semgrep is a command-line tool for offline static analysis. Use pre-built or custom rules to enforce code and security standards in your codebase.

Below are the rules for different XML parsers in Java

Identifying XXE vulnerability in the org.apache.commons.digester3.Digester library Rule can be played here https://semgrep.dev/s/salecharohit:xxe-Digester

Identifying XXE vulnerability in the javax.xml.parsers.DocumentBuilderFactory library Rule can be played here https://semgrep.dev/s/salecharohit:xxe-dbf

Identifying XXE vulnerability in the org.jdom2.input.SAXBuilder library Rule can be played here https://semgrep.dev/s/salecharohit:xxe-saxbuilder

Identifying XXE vulnerability in the javax.xml.parsers.SAXParserFactory library Rule can be played here https://semgrep.dev/s/salecharohit:xxe-SAXParserFactory

Identifying XXE vulnerability in the org.dom4j.io.SAXReader library Rule can be played here https://semgrep.dev/s/salecharohit:xxe-SAXReader

Identifying XXE vulnerability in the javax.xml.stream.XMLInputFactory library Rule can be played here https://semgrep.dev/s/salecharohit:xxe-XMLInputFactory

Identifying XXE vulnerability in the org.xml.sax.XMLReader library Rule can be played here https://semgrep.dev/s/salecharohit:xxe-XMLReader

• Sanitize user inputs to filter out unacceptable characters.
• Specify which inputs are allowed.
• Keep an eye on your XML parser.
• Implement a content security policy (CSP).