Secure SAXParserFactory that prevents XXE

SecureXMLParser.java

import javax.xml.parsers.ParserConfigurationException;  // catching unsupported features
import javax.xml.parsers.SAXParser;
import javax.xml.parsers.SAXParserFactory;
 
import org.xml.sax.SAXNotRecognizedException;  // catching unknown features
import org.xml.sax.SAXNotSupportedException;  // catching known but unsupported features
import org.xml.sax.XMLReader;
 
...
 
    SAXParserFactory spf = SAXParserFactory.newInstance();
    SAXParser saxParser = spf.newSAXParser();
    XMLReader reader = saxParser.getXMLReader();
 
    try {
      // 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
 
      // Using the SAXParserFactory's setFeature
      spf.setFeature("http://xml.org/sax/features/external-general-entities", false);
      // Using the XMLReader's setFeature
      reader.setFeature("http://xml.org/sax/features/external-general-entities", false);
 
 
      // Xerces 2 only - http://xerces.apache.org/xerces-j/features.html#external-general-entities
      spf.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true);
 
      // remaining parser logic
      ...
 
    } catch (ParserConfigurationException e) {
      // Tried an unsupported feature.
 
    } catch (SAXNotRecognizedException e) {
      // Tried an unknown feature.
 
    } catch (SAXNotSupportedException e) {
      // Tried a feature known to the parser but unsupported.
 
    } catch ... {
      
    }
...

Got it,it's useful for me!

If you came here from https://www.owasp.org/index.php/XML_External_Entity_(XXE)_Prevention_Cheat_Sheet#JAXP_DocumentBuilderFactory.2C_SAXParserFactory_and_DOM4J and are wondering about DocumentBuilderFactory, I have a Gist for that.

DocumentBuilderFactory that mitigates XXE using OWASP guidance:
https://gist.github.com/AlainODea/1779a7c6a26a5c135280bc9b3b71868f

Do I have to configure the parser and reader instance seperately or do they get configured if the instances are created after the factory is configured?

The configuration is only for the factory. You don't need any additional configuration steps.

Hi,
Where should I place this class so that it works?
I want to prevent XXE in my java SOAP project

Hi @Sohaibgit - You may not be able to use this 'as-is'. Take a look at your code in the class where the SaxParserFactory is instantiated, and ensure that you're setting the features to ensure it is not susceptible to XXE

The code is a bit missleading, I would not request a parser and reader from the factory before configuring the factory. (and it should not nead to configure the reader from a configured factory in that case, right?)

Two comments, if you can't spf.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true); because this means your parser throws an error if a DTD is included in the document (which might be there from legacy documents).

So we found we had to disable the external loading using spf.setFeature("http://apache.org/xml/features/nonvalidating/load-external-dtd", false);

This feature gets ignored though by xerces if you set the setValidating on SAXParserFactory to true, so make sure that is set to false or xerces will always try to load the DTD and external DTDs.