Java API for XML Processing (JAXP) Security Guide (original) (raw)

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The JDK and Java XML APIs have been improved over the years with various measures and tools that can help prevent applications from being exploited by XML-related attacks. This guide shows you how to use the secure processing features of Java API for XML Processing (JAXP) to safeguard your applications and systems.

Topics

• Potential Attacks During XML Processing
  • XML External Entity Injection Attack
    * External Resources Supported by XML, Schema, and XSLT Standards
  • Exponential Entity Expansion Attack
• Feature for Secure Processing (FSP)
  • JAXP Properties for Processing Limits
    * elementAttributeLimit
    * entityExpansionLimit
    * entityReplacementLimit
    * maxElementDepth
    * maxGeneralEntitySizeLimit
    * maxOccurLimit
    * maxParameterEntitySizeLimit
    * maxXMLNameLimit
    * totalEntitySizeLimit
    * xpathExprGrpLimit
    * xpathExprOpLimit
    * xpathTotalOpLimit
    * Legacy System Properties
  • JAXP Properties for External Access Restrictions
    * ACCESS_EXTERNAL_DTD
    * ACCESS_EXTERNAL_SCHEMA
    * ACCESS_EXTERNAL_STYLESHEET
    * Values of External Access Restrictions JAXP Properties
  • Scope and Order
    * Scope of Setting Feature for Secure Processing
    * Scope and Order of Setting JAXP Properties
    * Scope of Setting External Access Restrictions
  • Relationship with Security Manager
  • When to Use Processing Limits
    * Memory Setting and Limits
    * Estimating the Limits Using the getEntityCountInfo Property
  • When to Use External Access Restrictions
  • Using JAXP Properties
    * Setting Properties Through JAXP Factories
    * Using System Properties
    * Using the jaxp.properties File
  • Handling Errors from JAXP Properties
  • Streaming API for XML and JAXP Properties
    * StAX and Processing Limits
    * StAX and External Access Restrictions
  • Extension Functions
    * enableExtensionFunctions
• Disabling DTD Processing
  • Disabling DTD Processing for SAX and DOM Parsers
  • Disabling DTD Processing for StAX Parsers
• Using Java XML Resolvers
  • Entity Resolvers for SAX and DOM
  • XMLResolver for StAX
  • URIResolver for javax.xml.transform
  • LSResourceResolver for javax.xml.validation
• Third-Party Parsers
  • overrideDefaultParser
  • Example of Using a Third-Party Parser
• General Recommendations for JAXP Security
• Appendix A: Glossary of Java API for XML Processing Terms and Definitions
• Appendix B: Java and JDK XML Features and Properties Naming Convention

Potential Attacks During XML Processing

XML processing can expose applications to certain vulnerabilities. Among the most prominent and well-known attacks are the XML External Entity (XXE) injection attack and the exponential entity expansion attack, also know as the XML bomb or billion laughs attack. These attacks can potentially cause serious damage to a system by denying its services or worse, lead to the loss of sensitive data.

You should evaluate your applications' requirements and operating environment to assess the level of potential threat, for example, whether or to what extent the applications are exposed to untrusted XML sources.

XML External Entity Injection Attack

The XML, XML Schema, and XSLT standards define a number of structures that enable the embedding of external content in XML documents through system identifiers that reference external resources. In general, XML processors resolve and retrieve almost all of these external resources; see External Resources Supported by XML, Schema, and XSLT Standards for a list of constructs that support the inclusion of external resources. In addition, some constructs enable the execution of applications through external functions. XML External Entity (XXE) injection attacks exploit XML processors that have not been secured by restricting the external resources that it may resolve, retrieve, or execute. This can result in disclosing sensitive data such as passwords or enabling arbitrary execution of code.

External Resources Supported by XML, Schema, and XSLT Standards

XML, Schema, and XSLT standards support the following constructs that require external resources. The default behavior of the JDK XML processors is to make a connection and fetch the external resources as specified.

• External DTD: references an external Document Type Definition (DTD), for example:
• External Entity Reference: Refers to external data, the following is the syntax:
• General entity reference, for example: ]> &otherFile;
• External Parameter Entities: The following is the syntax: The following is an example:

  %ent1;  

  ]>
• XInclude: Includes an external infoset in an XML document, for example:
• References to XML Schema components using theschemaLocation attribute and import andinclude elements, for example:
  <xs:schema xmlns:xs="" title="undefined" rel="noopener noreferrer">http://www.w3.org/2001/XMLSchema">
  <xs:include schemaLocation="" title="undefined" rel="noopener noreferrer">http://www.example.com/schema/schema1.xsd"/>
• Combining style sheets using import or include elements, the following is the syntax:
  <xsl:include href="include.xsl"/>
• xml-stylesheet processing instruction: Used to include a stylesheet in an XML document, for example:
• XSLT document() function: Used to access nodes in an external XML document, for example:
  <xsl:variable select="document('DocumentFunc2.xml')"/>

Exponential Entity Expansion Attack

The exponential entity expansion attack, also know as the XML bomb or billion laughs attack, is a denial-of-service attack that involves XML parsers. The basic exploit is to have several layers of nested entities, each referring to a number of entities of the next layer. The following is a sample SOAP document that contains deeply nested entity references:

<!ENTITY  x99 "&x100;&x100;">
<!ENTITY  x98 "&x99;&x99;">
...
<!ENTITY   x2 "&x3;&x3;">
<!ENTITY   x1 "&x2;&x2;">

]> <SOAP-ENV:Envelope xmlns:SOAP-ENV=...> SOAP-ENV:Body <ns1:aaa xmlns:ns1="urn:aaa" SOAP-ENV:encodingStyle="..."> &x1;

When an XML parser encounters such a document, it will attempt to resolve the entity declaration by expanding the references. Because the references are nested, the expansion becomes exponential by the number of entities each refers to. Such a process can lead the XML parser to consume 100% of CPU time and a large amount of memory, and eventually the system runs out of memory.

Feature for Secure Processing (FSP)

Feature for Secure Processing (FSP), which is defined as javax.xml.XMLConstants.FEATURE_SECURE_PROCESSING, is the central mechanism to help safeguard XML processing. It instructs XML processors, such as parsers and transformers, to try and process XML securely.

By default, the JDK turns on FSP for DOM and SAX parsers and XML schema validators, which sets a number of processing limits on the processors. Conversely, by default, the JDK turns off FSP for transformers and XPath, which enables extension functions for XSLT and XPath.

Turn on and off FSP by calling the setFeature method on factories and setting XMLConstants.FEATURE_SECURE_PROCESSING to either true or false. For example, the following code snippet turns on FSP for SAX parsers that are created by the factory spf by setting XMLConstants.FEATURE_SECURE_PROCESSING to true:

SAXParserFactory spf = SAXParserFactory.newInstance();
spf.setFeature(XMLConstants.FEATURE_SECURE_PROCESSING, true);

While FSP can be turned on and off through factories, it is always on when a Java Security Manager is present and cannot be turned off. The Java XML processors therefore will enforce limits and restrictions when a Java Security Manager is present. You can however adjust individual properties depending on the specific needs of your applications. There are two types of JAXP properties:

• Processing limits: Helps to guard against excessive memory consumption from XML processing.
• External access restrictions: Controls the fetching of external resources.

The following sections describe these two types of JAXP properties.

JAXP Properties for Processing Limits

XML processing can sometimes be a memory intensive operation. Applications, especially those that accept XML, XSD and XSL from untrusted sources, should take steps to guard against excessive memory consumption by using JAXP properties for processing limits.

Evaluate your application's requirements and operating environment to determine the acceptable processing limits for your system configurations and set these limits accordingly. For example, use size-related limits to prevent malformed XML sources from consuming large amounts of memory. UseEntityExpansionLimit to enable an application to control memory consumption under an acceptable level.

The JDK XML parsers observe processing limits by default. Both DOM and SAX parsers have Feature for Secure Processing (FSP) turned on by default and therefore turn on the limits. The StAX parser also observes processing limits by default even though it doesn't support FSP.

The JDK XML processors enable you to adjust processing limits individually in three ways:

• Through standard XML APIs
• By using System properties
• In the jaxp.properties file

See Using JAXP Properties and Scope and Order.

The following table describes the JAXP properties for processing limits supported in the JDK.

Legacy System Properties

These properties, which were introduced in JDK 5.0 and 6, continue to be supported for backward compatibility.

JAXP Properties for External Access Restrictions

The JAXP Properties for external access restrictions, along with their corresponding System properties, enable you to regulate external connections.

External access restrictions enable you to specify the type of external connections that can or cannot be permitted. The property values are a list of protocols. The JAXP processors check if a given external connection is permitted by matching the protocol with those in the list. Processors will attempt to establish the connection if it is on the list, or reject it if not. Use these JAXP properties along with custom resolvers (seeUsing Java XML Resolvers) to reduce the risk of external connections by rejecting and resolving them with local resources.

Note: Explicitly turning on Feature for Secure Processing (FSP) through the API, for example,factory.setFeature(XMLConstants.FEATURE_SECURE_PROCESSING, true), disables all external connections.

The external access restrictions JAXP properties are defined in javax.xml.XMLConstants as follows:

• javax.xml.XMLConstants.ACCESS_EXTERNAL_DTD
• javax.xml.XMLConstants.ACCESS_EXTERNAL_SCHEMA
• javax.xml.XMLConstants.ACCESS_EXTERNAL_STYLESHEET

ACCESS_EXTERNAL_DTD

ACCESS_EXTERNAL_SCHEMA

ACCESS_EXTERNAL_STYLESHEET

Values of External Access Restrictions JAXP Properties

All JAXP properties for external access restrictions have values of the same format:

• Value: A list of protocols separated by comma. A protocol is the scheme portion of an URI, or in the case of the JAR protocol, jar plus the scheme portion separated by colon. A scheme is defined as:
  _scheme_ = _alpha_ *( _alpha_ |_digit_ | "+" | "-" | "." )
  where _alpha_ = a-z andA-Z.
  The JAR protocol is defined as:jar[:_scheme_]
  Protocols are case-insensitive. Any whitespace characters as defined by Character.isSpaceChar in the value are ignored. Examples of protocols are file,http, and jar:file.
• Default value: The default value is implementation specific. For the JDK, the default value isall, which grants permissions to all protocols.
• Granting all access: The keywordall grants permission to all protocols. For example, specifying javax.xml.accessExternalDTD=all in thejaxp.properties file enables a system to work as before with no restrictions on accessing external DTDs and entity references.
• Denying any access: An empty string ("") means that no permission is granted to any protocol. For example, specifyingjavax.xml.accessExternalDTD="" in thejaxp.properties file instructs JAXP processors to deny any external connections.

Scope and Order

Scope of Setting Feature for Secure Processing

Feature for Secure Processing (FSP) is required for XML processors including DOM, SAX, schema validation, XSLT, and XPath.

When FSP is turned on, then default processing limits (seeJAXP Properties for Processing Limits) are enforced. Turning off FSP does not change the limits.

When FSP is "explicitly" turned on through the API, for example,factory.setFeature(XMLConstants.FEATURE_SECURE_PROCESSING, true), then external access restrictions (see JAXP Properties for External Access Restrictions) are set to the empty string, which means that no permission is granted to any protocol. Although FSP is turned on by default for DOM, SAX and schema validation, it is not treated as if "explicitly" turned on; therefore, the default value for external access restrictions is all, which means that permission is granted to all protocols.

When a Java Security Manager is present, FSP is turned on and cannot be turned off.

Scope and Order of Setting JAXP Properties

In general, JAXP properties set in a smaller scope override those in a larger one:

• JAXP properties specified through JAXP factories or processors take preference over System properties, thejaxp.properties file, and FSP.
• System properties, when set, affect one JDK invocation only and override processing limit and external access restriction values set by default, set in the jaxp.propertiesfile, or set by FSP.
• Properties specified in the jaxp.properties file affect the entire JDK and override processing limit and external access restriction values set by default or set by FSP.

Scope of Setting External Access Restrictions

External access restrictions have no effect on the relevant constructs that they attempt to restrict in the following situations:

• When there is a resolver and the source returned by the resolver is not null: This applies to entity resolvers that may be set on SAX and DOM parsers, XML resolvers on StAX parsers,LSResourceResolver on SchemaFactory, aValidator or ValidatorHandler, orURIResolver on a transformer.
• When a schema is created explicitly by calling thenewSchema method fromSchemaFactory.
• When external resources are not required: For example, the following features and properties are supported by the JDK and may be used to instruct the processor to not load the external DTD or resolve external entities:
  http://apache.org/xml/features/disallow-doctype-decl true
  http://apache.org/xml/features/nonvalidating/load-external-dtd false
  http://xml.org/sax/features/external-general-entities false
  http://xml.org/sax/features/external-parameter-entities false

Relationship with Security Manager

JAXP properties are checked first before a connection is attempted, whether or not a SecurityManager is present. This means that a connection may be blocked even if it is granted permission by the SecurityManager. For example, if the JAXP properties are set to disallow the HTTP protocol, then they will effectively block any connection attempt even when an application has a SocketPermission.

For the purpose of restricting connections, theSecurityManager can be viewed as being at a lower level. Permissions are checked after JAXP properties are evaluated. For example, if an application does not have aSocketPermission, then aSecurityException will be thrown even if JAXP properties are set to allow HTTP connections.

When a SecurityManager is present, Feature for Secure Processing (FSP) is set to true. This behavior does not turn on any external access restrictions.

When to Use Processing Limits

When determining which processing limits to apply and what values to use, at the system level, consider the amount of memory available for applications and whether XML, XSD, or XSL sources from untrusted sources are accepted and processed. At the application level, consider whether certain constructs such as DTDs are used.

Memory Setting and Limits

XML processing can be very memory intensive. The amount of memory that should be allowed to be consumed depends on the requirements of the applications in a specific environment. Processing of malformed XML data must be prevented from consuming excessive memory.

The default limits are generally set to allow legitimate XML inputs for most applications with memory usage allowed for a small hardware system, such as a PC. It is recommended that the limits are set to the smallest possible values, so that any malformed input can be caught before it consumes large amounts of memory.

The limits are correlated, but not entirely redundant. You should set appropriate values for all of the limits: usually the limits should be set to a much smaller value than the default.

For example, ENTITY_EXPANSION_LIMIT andGENERAL_ENTITY_SIZE_LIMIT can be set to prevent excessive entity references. But when the exact combination of the expansion and entity sizes are unknown,TOTAL_ENTITY_SIZE_LIMIT can serve as a overall control. Similarly, while TOTAL_ENTITY_SIZE_LIMITcontrols the total size of a replacement text, if the text is a very large chunk of XML, ENTITY_REPLACEMENT_LIMITsets a restriction on the total number of nodes that can appear in the text and prevents overloading the system.

Estimating the Limits Using the getEntityCountInfo Property

To help you analyze what values you should set for the limits, a special property calledhttp://www.oracle.com/xml/jaxp/properties/getEntityCountInfois available. The following code snippet, from Processing Limit Samples in The Java Tutorials, shows an example of using the property:

public static final String ORACLE_JAXP_PROPERTY_PREFIX =
    "http://www.oracle.com/xml/jaxp/properties/";
// ...
public static final String JDK_ENTITY_COUNT_INFO = 
    ORACLE_JAXP_PROPERTY_PREFIX + "getEntityCountInfo";
// ...
parser.setProperty(JDK_ENTITY_COUNT_INFO, "yes");

When you run the processing limit sample with the DTD in W3C MathML 3.0, it prints out the following table:

In this example, the total number of entity references, or the entity expansion, is 1417; the default limit is 64000. The total size of all entities is 55425; the default limit is 50000000. The biggest parameter entity is %MultiScriptExpressionwith a length of 7303 after all references are resolved; the default limit is 1000000.

If this is the largest file that the application is expected to process, it is recommended that the limits be set to smaller numbers. For example, 2000 forENTITY_EXPANSION_LIMIT, 100000 forTOTAL_ENTITY_SIZE_LIMIT, and 10000 forPARAMETER_ENTITY_SIZE_LIMIT.

When to Use External Access Restrictions

The XML processors, by default, attempt to connect and read external resources that are referenced in XML sources. Note that this may potentially expose applications and systems to risks posed by external connections. It's therefore recommended that applications consider limiting external connections with external access restriction properties.

Internal applications and systems that handle only trusted XML documents may not need these restrictions. Applications and systems that rely on the Java Security Manager to regulate external connections may also have no need for them. However, keep in mind that external access restrictions are specific to the XML processors and at the top layer of the process, which means that the processors check these restrictions before any connections are made. They may therefore serve as an additional and more direct protection against external connection risks.

You can use external access restriction properties along with custom resolvers and catalogs (see Using Java XML Resolvers) to effectively manage external connections and reduce risks.

Even in a trusted environment with trusted sources, it's recommended that you use both external access restrictions and resolvers to minimize dependencies on external sources.

Using JAXP Properties

Setting Properties Through JAXP Factories

If you can modify your application's code, or you're creating a new application, then setting JAXP properties through JAXP factories or a parser is the preferred method. Set these properties through the following interfaces:

DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
dbf.setAttribute(_name_, _value_);

SAXParserFactory spf = SAXParserFactory.newInstance();
SAXParser parser = spf.newSAXParser();
parser.setProperty(_name_, _value_);

SchemaFactory schemaFactory = SchemaFactory.newInstance(schemaLanguage);
schemaFactory.setProperty(_name_, _value_);

TransformerFactory factory = TransformerFactory.newInstance();
factory.setAttribute(_name_, _value_);

XMLInputFactory xif = XMLInputFactory.newInstance();
xif.setProperty(_name_, _value_);

XPathFactory xf = XPathFactory.newInstance();
xf.setProperty(_name_, _value_);

The following is an example of setting processing limits:

dbf.setAttribute("http://www.oracle.com/xml/jaxp/properties/entityExpansionLimit", "2000");
dbf.setAttribute("http://www.oracle.com/xml/jaxp/properties/maxGeneralEntitySizeLimit", "100000");
dbf.setAttribute("http://www.oracle.com/xml/jaxp/properties/maxParameterEntitySizeLimit", "10000"); 
dbf.setAttribute("http://www.oracle.com/xml/jaxp/properties/maxElementDepth", "100");

factory.setAttribute("jdk.xml.xpathTotalOpLimit", "1000"); 
xf.setProperty("jdk.xml.xpathExprGrpLimit", "20");    

The following is an example of limiting a DOM parser to only local connections for external DTDs:

dbf.setAttribute(XMLConstants.ACCESS_EXTERNAL_DTD, "file, jar:file");

If a parser module within the application handles untrusted sources, it may further restrict access. The following code overrides those in the jaxp.properties file and those specified by System properties and enables the XML processor to read local files only:

DocumentBuilderFactory dbf =
DocumentBuilderFactory.newInstance();   
dbf.setAttribute(XMLConstants.ACCESS_EXTERNAL_DTD, "file");
// ...   
SchemaFactory schemaFactory = SchemaFactory.newInstance(XMLConstants.W3C_XML_SCHEMA_NS_URI);   
schemaFactory.setProperty(XMLConstants.ACCESS_EXTERNAL_DTD, "file");   
schemaFactory.setProperty(XMLConstants.ACCESS_EXTERNAL_SCHEMA, "file");

As described in Scope and Order of Setting JAXP Properties, JAXP properties specified through JAXP factories have the narrowest scope, affecting only the processors created by the factories, and therefore override any default settings, System properties, and those in thejaxp.properties file. By setting JAXP properties through JAXP factories, you can ensure that your applications behave the same way regardless of which JDK release you're using or whether JAXP properties are set through other means.

Using System Properties

System properties may be useful if you can't modify your application's code.

To set JAXP properties for an entire JDK invocation, set their corresponding System properties on the command line.

To set JAXP properties for only a portion of the application, set their corresponding System properties before the portion, and then clear them afterward. For example, if your application requires access to external DTDs and schemas, then add these lines to your application's initialization code block:

System.setProperty("javax.xml.accessExternalDTD", "file, http");
System.setProperty("javax.xml.accessExternalSchema", "file, http");

Then, once your application is done processing XML documents or before it exits, clear out the properties as follows:

System.clearProperty("javax.xml.accessExternalDTD");   
System.clearProperty("javax.xml.accessExternalSchema");

The following code, from Processing Limit Samples in The Java Tutorials, is another example that shows how to do this for the processing limitmaxGeneralEntitySizeLimit:

public static final String SP_GENERAL_ENTITY_SIZE_LIMIT = "jdk.xml.maxGeneralEntitySizeLimit";

// Set limits using System property; // this setting will affect all processing after it's set System.setProperty(SP_GENERAL_ENTITY_SIZE_LIMIT, "2000");

// Perform some processing here

// After it is done, clear the property System.clearProperty(SP_GENERAL_ENTITY_SIZE_LIMIT);

Note that processing limit values are integers. ANumberFormatException is thrown if a processing limit's value is not a parsable integer; see the methodjava.lang.Integer.parseInt(String).

The following example allows the resolution of external schemas for a portion of an application:

// Allow resolution of external schemas // This setting will affect all processing after it's set System.setProperty("javax.xml.accessExternalSchema", "file, http");

// Perform some processing here

// After it's done, clear the property System.clearProperty("javax.xml.accessExternalSchema");

Using the jaxp.properties File

If you want to specify a JAXP property that affects every JDK invocation, then create a configuration file named_<java-home>_/lib/jaxp.propertiesand specify names and values of JAXP properties in it, one name-value pair on each line. For example, the followingjaxp.properties file sets themaxGeneralEntitySizeLimit processing limit property to 2000 and restricts access to the file and HTTP protocols for external references set by the stylesheet processing instruction, document function, and theimport and include elements.

jdk.xml.maxGeneralEntitySizeLimit=2000 javax.xml.accessExternalStylesheet=file, http

If you don't want to allow any external connection by XML processors, you can set all access external restrictions tofile only:

javax.xml.accessExternalDTD=file javax.xml.accessExternalSchema=file javax.xml.accessExternalStylesheet=file

If you want to prevent applications from accidentally reading external files through an XML processor, set the external access restrictions as follows in the jaxp.properties file as follows:

javax.xml.accessExternalDTD=""
javax.xml.accessExternalSchema=""
javax.xml.accessExternalStylesheet=""

Note:

• Use the corresponding System property in thejaxp.properties file. Processing limit System properties have the prefix jdk.xml. External access restriction System properties have the prefixjavax.xml.
• Processing limit values are integers. ANumberFormatException is thrown if a processing limit's value is not a parsable integer; see the methodjava.lang.Integer.parseInt(String).

Handling Errors from JAXP Properties

It is recommended that applications catchorg.xml.sax.SAXNotRecognizedException when setting JAXP properties so that the applications will work properly on older releases that don't support them.

For example, the following method,isNewPropertySupported, from Processing Limit Samples in The Java Tutorials, detects if the sample is run with a version of the JDK that supports theJDK_GENERAL_ENTITY_SIZE_LIMIT property:

public boolean isNewPropertySupported() {
    try {
        SAXParser parser = getSAXParser(false, false, false);
        parser.setProperty(JDK_GENERAL_ENTITY_SIZE_LIMIT, "10000");
    } catch (ParserConfigurationException ex) {
        fail(ex.getMessage());
    } catch (SAXException ex) {
        String err = ex.getMessage();
        if (err.indexOf("Property '" + JDK_GENERAL_ENTITY_SIZE_LIMIT +
                                       "' is not recognized.") > -1) {
            // expected before this patch
            debugPrint("New limit properties not supported. Samples not run.");
            return false;
        }
    }
    return true;
}

When input files contain constructs that cause an over-the-limit exception, applications may check the error code to determine the nature of the failure. The following error codes are defined for processing limits:

• EntityExpansionLimit: JAXP00010001
• ElementAttributeLimit: JAXP00010002
• MaxEntitySizeLimit: JAXP00010003
• TotalEntitySizeLimit: JAXP00010004
• MaxXMLNameLimit: JAXP00010005
• maxElementDepth: JAXP00010006
• EntityReplacementLimit: JAXP00010007

The error code has the following format:

"JAXP" + components (two digits) + error category (two digits) + sequence number

The code JAXP00010001, therefore, represents the JAXP base parser security limit EntityExpansionLimit.

If access to external resources is denied due to the restrictions set by external access restrictions, then an exception will be thrown with an error in the following format:

[type of construct]: Failed to read [type of construct] "[name of the external resource]", because "[type of restriction]" access is not allowed due to restriction set by the [property name] property.

For example, suppose the following:

• The ACCESS_EXTERNAL_DTD JAXP property is set as follows:
  parser.setProperty(
  "http://javax.xml.XMLConstants/property/accessExternalDTD", "file");
• Your application tries to fetch an external DTD with the HTTP protocol.
• The parser parsed an XML file that contains an external reference tohttp://www.example.com/dtd/properties.dtd.

The error message would look like the following:

External DTD: Failed to read external DTD "http://www.example.com/dtd/properties.dtd", because "http" access is not allowed due to restriction set by the accessExternalDTD property.

Streaming API for XML and JAXP Properties

Streaming API for XML (StAX), JSR 173, does not support FSP nor does it support external access restrictions. However, JDK's StAX implementation supports processing limits, and StAX in the context of JAXP supports external access restrictions.

StAX and Processing Limits

JDK's StAX implementation supports processing limits and their corresponding System properties. However, because FSP is not supported, you can't turn on or off processing limits for StAX by turning on or off FSP. Processing limits continue to behave as described in JAXP Properties for Processing Limits.

StAX and External Access Restrictions

JDK's StAX implementation supports the JAXP properties related to external access restrictions. Setting them is similar to SAX or DOM, but through the XMLInputFactory class, for example:

XMLInputFactory xif = XMLInputFactory.newInstance();
xif.setProperty(
    "http://javax.xml.XMLConstants/property/accessExternalDTD",
    "file");

For compatibility, StAX properties and features take precedence over the processing limit and external access restriction properties. For example, the SupportDTD property, when set to false, causes a program to throw an exception when an input file contains a DTD before it can be parsed. Therefore, processing limits and external access restrictions on DTDs will have no effect on applications that have disabled DTDs by setting the SupportDTD property to false.

Extension Functions

Because Feature for Secure Processing (FSP) is off by default for Transformer and XPath, extension functions are allowed. For applications processing documents from untrusted sources, it is recommended to turn off the extension functions feature. There are two ways to do so:

• By setting FSP to true, for example:
  TransformerFactory tf = TransformerFactory.newInstance();
  tf.setFeature(XMLConstants.FEATURE_SECURE_PROCESSING, true);
• By setting the enableExtensionFunctions property to false:
  final static String ENABLE_EXTENSION_FUNCTIONS =
  "http://www.oracle.com/xml/jaxp/properties/enableExtensionFunctions";
  // ...
  TransformerFactory tf = TransformerFactory.newInstance();
  tf.setFeature(ENABLE_EXTENSION_FUNCTIONS, false);

In cases where extension functions are disabled as a result of installing a Java Security Manager, applications may also choose to re-enable the extension functions feature by setting the property enableExtensionFunctions to true. The following table defines this property:

enableExtensionFunctions

Disabling DTD Processing

If your applications don't require DTDs, then consider disabling DTD processing to safeguard against many common DTD-related attacks, including denial-of-service, XML external entity (XXE), and server-side request forgery (SSRF).

Disabling DTD Processing for SAX and DOM Parsers

To disable DTD processing for SAX and DOM parsers, set the featurehttp://apache.org/xml/features/disallow-doctype-declto true through a factory. The following code snippet disables DTDs for SAX parsers. A fatal error is thrown if the incoming XML document contains a DOCTYPE declaration.

final static String DISALLOW_DTD =
    "http://apache.org/xml/features/disallow-doctype-decl";
// ...
SAXParserFactory spf = SAXParserFactory.newInstance();      
spf.setFeature(DISALLOW_DTD, true);

Disabling DTD Processing for StAX Parsers

To disable DTD processing for StAX parsers, set the propertySupportDTD with theXMLInputFactory.setProperty method:

XMLInputFactory xif = XMLInputFactory.newInstance();
xif.setProperty(XMLInputFactory.SUPPORT_DTD, Boolean.FALSE);

Using Java XML Resolvers

You can register custom resolvers on a JDK XML processor to intercept any references to external resources and resolve them with local ones. This feature eliminates the need to read and access external resources, thus helping to remove a source of potential risk.

The Java XML API supports various resolvers that you can register on JDK XML processors to resolve external resources. These resolvers includes entity resolvers for SAX and DOM parsers, XML resolvers for StAX parsers, LSResourceResolver for validation, and URIResolver for transformation.

Entity Resolvers for SAX and DOM

SAX defines an interface that DOM also supports,org.xml.sax.EntityResolver. It enables applications to step into the entity resolution process and perform entity resolution on their own terms. The following is the interface's definition:

package org.xml.sax;

public interface EntityResolver { public InputSource resolveEntity(String publicID, String systemID) throws SAXException; }

You can then register an implementation of the interface on a SAX driver:

EntityResolver resolver = ...;
SAXParserFactory factory = SAXParserFactory.newInstance();
factory.setNamespaceAware(true);
XMLReader reader = factory.newSAXParser().getXMLReader();
reader.setEntityResolver(resolver);

Alternatively, you can register it on a DOM builder:

DocumentBuilder builder =
    DocumentBuilderFactory.newInstance().newDocumentBuilder();       
docBuilder.setEntityResolver(resolver);

XMLResolver for StAX

StAX defines a javax.xml.stream.XMLResolverinterface:

package javax.xml.stream;

public interface XMLResolver { public Object resolveEntity( String publicID, String systemID, String baseURI, String namespace) throws XMLStreamException; }

You can register it on a StAX factory:

XMLResolver resolver = ...;
XMLInputFactory xif = XMLInputFactory.newInstance();
xif.setProperty(XMLInputFactory.RESOLVER, resolver);

URIResolver for javax.xml.transform

The javax.xml.transform API supports custom resolution of external resources through theURIResolver interface:

package javax.xml.transform;

public interface URIResolver { public Source resolve(String href, String base) throws TransformerException; }

You can register an implementation of URIResolveron a Transformer as follows:

URIResolver resolver = ...;
TransformerFactory tf = TransformerFactory.newInstance();
Transformer t =
  tf.newTransformer(new StreamSource(
    new StringReader("xsl source")));
t.setURIResolver(resolver);

LSResourceResolver for javax.xml.validation

The javax.xml.validation API supports Document Object Model Level 3 Load and Save (DOM LS) DOM through theLSResourceResolver interface:

package org.w3c.dom.ls;

public interface LSResourceResolver { public LSInput resolveResource( String type, String namespaceURI, String publicId, String systemId, String baseURI); }

You can register an implementation ofLSResourceResolver on a SchemaFactoryas follows:

SchemaFactory schemaFactory =
    SchemaFactory.newInstance("http://www.w3.org/2001/XMLSchema");
LSResourceResolver resolver = ...;
schemaFactory.setResourceResolver(resolver);

Third-Party Parsers

The JDK will always use its system-default parser even when there's a third-party parser on the classpath. To override the JDK system-default parser, set thejdk.xml.overrideDefaultParser property to true.

overrideDefaultParser

Example of Using a Third-Party Parser

The following code snippets instruct the factories to use a third-party parser, if found on the classpath, by setting thejdk.xml.overrideDefaultParser property with thesetFeature method:

static final String JDK_OVERRIDE_PARSER = "jdk.xml.overrideDefaultParser";
...
TransformerFactory tFactory = TransformerFactory.newInstance();
tFactory.setFeature(JDK_OVERRIDE_PARSER, true);
...
XPathFactory xf = XPathFactory.newInstance();
xf.setFeature(JDK_OVERRIDE_PARSER, true);
...
SchemaFactory schemaFactory =
    SchemaFactory.newInstance(XMLConstants.W3C_XML_SCHEMA_NS_URI);
schemaFactory.setFeature(JDK_OVERRIDE_PARSER, true);
...
Schema schema = schemaFactory.newSchema(new File("mySchema.xsd"));
Validator validator = schema.newValidator();
validator.setFeature(JDK_OVERRIDE_PARSER, true);

The following code snippet setsjdk.xml.overrideDefaultParser as a System property:

System.setProperty("jdk.xml.overrideDefaultParser", "true"));

You can add the following line to thejaxp.properties file to enable third-party parsers:

jdk.xml.overrideDefaultParser=true

General Recommendations for JAXP Security

The following are general recommendations for configuring JAXP properties and features to help secure your applications and systems:

• Turn on Feature for Secure Processing (FSP), then adjust individual features and properties in accordance with your specific requirements.
• For processing limits, adjust them so that they are just large enough to accommodate the maximum amount your applications require.
• For external access restrictions, reduce or eliminate your applications' reliance on external resources, including the use of resolvers, then tighten these restrictions.

Appendix A: Glossary of Java API for XML Processing Terms and Definitions

Appendix B: Java and JDK XML Features and Properties Naming Convention

Java and JDK XML features and properties are defined in thejavax.xml.XMLConstants class. The features have a prefix http://javax.xml.XMLConstants/feature, the properties http://javax.xml.XMLConstants/property. If there is a corresponding System property, its prefix isjavax.xml.

The JDK XML properties are JDK implementation-only properties. Their prefixes vary depending on the JDK version. The following table summarizes this naming convention: