Allowing external DTD entities in untrusted documents to be processed could lay your systems bare to attackers. Imagine if these entities were parsed:
<!ENTITY xxe SYSTEM "file:///etc/passwd" >]><foo>&xxe;</foo> <!ENTITY xxe SYSTEM "http://www.attacker.com/text.txt" >]><foo>&xxe;</foo>
If you must parse untrusted XML, the best way to protect yourself is to only accept an embedded DTD or, even better, completely ignore any DTD declared in the document.
This rule raises an issue when any of the following are used without first disabling external entity processing:
javax.xml.validation.Validator, JAXP's DocumentBuilderFactory, SAXParserFactory, Xerces 1 and Xerces 2 StAX's
XMLInputFactory and XMLReaderFactory.
To disable external entity processing for XMLInputFactory, configure one of the properties
XMLInputFactory.IS_SUPPORTING_EXTERNAL_ENTITIES or XMLInputFactory.SUPPORT_DTD to false.
To disable external entity processing for SAXParserFactory, XMLReader or DocumentBuilderFactory configure
one of the features XMLConstants.FEATURE_SECURE_PROCESSING or "http://apache.org/xml/features/disallow-doctype-decl" to
true.
To disable external entity processing for Validator, configure both properties XMLConstants.ACCESS_EXTERNAL_DTD,
XMLConstants.ACCESS_EXTERNAL_SCHEMA to the empty string "".
/* Load XML stream and display content */
String maliciousSample = "xxe.xml";
XMLInputFactory factory = XMLInputFactory.newInstance();
try (FileInputStream fis = new FileInputStream(malicousSample)) {
// Load XML stream
XMLStreamReader xmlStreamReader = factory.createXMLStreamReader(fis); // Noncompliant; reader is vulnerable
/* Load XML stream and display content */
String maliciousSample = "xxe.xml";
XMLInputFactory factory = XMLInputFactory.newInstance();
// disable resolving of external DTD entities
factory.setProperty(XMLInputFactory.IS_SUPPORTING_EXTERNAL_ENTITIES, Boolean.FALSE);
// or disallow DTDs entirely
factory.setProperty(XMLInputFactory.SUPPORT_DTD, Boolean.FALSE);
try (FileInputStream fis = new FileInputStream(malicousSample)) {
// Load XML stream
XMLStreamReader xmlStreamReader = factory.createXMLStreamReader(fis);