XML文件的解析与XXE防护
DOM
DOM的全称是Document Object Model，也即文档对象模型。在应用程序中，基于DOM的XML分析器将一个XML文档转换成一个对象模型的集合（通常称DOM树），应用程序正是通过对这个对象模型的操作，来实现对XML文档数据的操作。
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException; // catching unsupported features
...
DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
String FEATURE = null;
try {
// This is the PRIMARY defense. If DTDs (doctypes) are disallowed, almost all XML entity attacks are prevented
// Xerces 2 only - http://xerces.apache.org/xerces2-j/features.html#disallow-doctype-decl
FEATURE = "http://apache.org/xml/features/disallow-doctype-decl";
dbf.setFeature(FEATURE, true);
// If you can't completely disable DTDs, then at least do the following:
// Xerces 1 - http://xerces.apache.org/xerces-j/features.html#external-general-entities
// Xerces 2 - http://xerces.apache.org/xerces2-j/features.html#external-general-entities
// JDK7+ - http://xml.org/sax/features/external-general-entities
FEATURE = "http://xml.org/sax/features/external-general-entities";
dbf.setFeature(FEATURE, false);
// Xerces 1 - http://xerces.apache.org/xerces-j/features.html#external-parameter-entities
// Xerces 2 - http://xerces.apache.org/xerces2-j/features.html#external-parameter-entities
// JDK7+ - http://xml.org/sax/features/external-parameter-entities
FEATURE = "http://xml.org/sax/features/external-parameter-entities";
dbf.setFeature(FEATURE, false);
// Disable external DTDs as well
FEATURE = "http://apache.org/xml/features/nonvalidating/load-external-dtd";
dbf.setFeature(FEATURE, false);
// and these as well, per Timothy Morgan's 2014 paper: "XML Schema, DTD, and Entity Attacks"
dbf.setXIncludeAware(false);
dbf.setExpandEntityReferences(false);
// And, per Timothy Morgan: "If for some reason support for inline DOCTYPEs are a requirement, then
// ensure the entity settings are disabled (as shown above) and beware that SSRF attacks
// (http://cwe.mitre.org/data/definitions/918.html) and denial
// of service attacks (such as billion laughs or decompression bombs via "jar:") are a risk."
// remaining parser logic
...
} catch (ParserConfigurationException e) {
// This should catch a failed setFeature feature
logger.info("ParserConfigurationException was thrown. The feature '" +
FEATURE + "' is probably not supported by your XML processor.");
...
}
catch (SAXException e) {
// On Apache, this should be thrown when disallowing DOCTYPE
logger.warning("A DOCTYPE was passed into the XML document");
...
}
catch (IOException e) {
// XXE that points to a file that doesn't exist
logger.error("IOException occurred, XXE may still possible: " + e.getMessage());
...
}
DocumentBuilder safebuilder = dbf.newDocumentBuilder();
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.
XMLInputFactory (a StAX parser)
StAX parsers such as XMLInputFactory allow various properties and features to be set.
To protect a Java XMLInputFactory from XXE, do this:
xmlInputFactory.setProperty(XMLInputFactory.SUPPORT_DTD, false); // This disables DTDs entirely for that factory
xmlInputFactory.setProperty("javax.xml.stream.isSupportingExternalEntities", false); // disable external entities
TransformerFactory
To protect a javax.xml.transform.TransformerFactory from XXE, do this:
TransformerFactory tf = TransformerFactory.newInstance();
tf.setAttribute(XMLConstants.ACCESS_EXTERNAL_DTD, "");
tf.setAttribute(XMLConstants.ACCESS_EXTERNAL_STYLESHEET, "");
Validator
To protect a javax.xml.validation.Validator from XXE, do this:
SchemaFactory factory = SchemaFactory.newInstance("http://www.w3.org/2001/XMLSchema");Schema schema = factory.newSchema();Validator validator = schema.newValidator();validator.setProperty(XMLConstants.ACCESS_EXTERNAL_DTD, "");validator.setProperty(XMLConstants.ACCESS_EXTERNAL_SCHEMA, "");
SchemaFactory
To protect a javax.xml.validation.SchemaFactory from XXE, do this:
SchemaFactory factory = SchemaFactory.newInstance("http://www.w3.org/2001/XMLSchema");factory.setProperty(XMLConstants.ACCESS_EXTERNAL_DTD, "");factory.setProperty(XMLConstants.ACCESS_EXTERNAL_SCHEMA, "");Schema schema = factory.newSchema(Source);
SAXTransformerFactory
To protect a javax.xml.transform.sax.SAXTransformerFactory from XXE, do this:
SAXTransformerFactory sf = SAXTransformerFactory.newInstance();
sf.setAttribute(XMLConstants.ACCESS_EXTERNAL_DTD, "");
sf.setAttribute(XMLConstants.ACCESS_EXTERNAL_STYLESHEET, "");
sf.newXMLFilter(Source);
Note: Use of the following XMLConstants requires JAXP 1.5, which was added to Java in 7u40 and Java 8:
javax.xml.XMLConstants.ACCESS_EXTERNAL_DTD
javax.xml.XMLConstants.ACCESS_EXTERNAL_SCHEMA
javax.xml.XMLConstants.ACCESS_EXTERNAL_STYLESHEET
XMLReader
To protect a Java org.xml.sax.XMLReader from XXE, do this:
XMLReader reader = XMLReaderFactory.createXMLReader();reader.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true);reader.setFeature("http://apache.org/xml/features/nonvalidating/load-external-dtd", false); // This may not be strictly required as DTDs shouldn't be allowed at all, per previous line.reader.setFeature("http://xml.org/sax/features/external-general-entities", false);reader.setFeature("http://xml.org/sax/features/external-parameter-entities", false);
SAXReader
To protect a Java org.dom4j.io.SAXReader from XXE, do this:
saxReader.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true);saxReader.setFeature("http://xml.org/sax/features/external-general-entities", false);saxReader.setFeature("http://xml.org/sax/features/external-parameter-entities", false);
Based on testing, if you are missing one of these, you can still be vulnerable to an XXE attack.
SAXBuilder
To protect a Java org.jdom2.input.SAXBuilder from XXE, do this:
SAXBuilder builder = new SAXBuilder();builder.setFeature("http://apache.org/xml/features/disallow-doctype-decl",true);builder.setFeature("http://xml.org/sax/features/external-general-entities", false);builder.setFeature("http://xml.org/sax/features/external-parameter-entities", false);Document doc = builder.build(new File(fileName));
SAX
SAX的全称是Simple APIs for XML，也即XML简单应用程序接口。与DOM不同，SAX提供的访问模式是一种顺序模式，这是一种快速读写XML数据的方式。当使用SAX分析器对XML文档进行分析时，会触发一系列事件，并激活相应的事件处理函数，应用程序通过这些事件处理函数实现对XML文档的访问，因而SAX接口也被称作事件驱动接口。
防护参考以上:
官网链接:
https://www.owasp.org/index.php/XML_External_Entity_(XXE)_Prevention_Cheat_Sheet#JAXP_DocumentBuilderFactory.2C_SAXParserFactory_and_DOM4J
