Of course, the whole problem with JSP EL could be solved if Tomcat (and other containers) would allow a flag to turn on XML escaping by default. IMO, it's badly needed to make JSP-based webapps safe from XSS.
A couple months later, I proposed a Tomcat enhancement to escape JSP's EL by default. I also entered an enhancement request for this feature and attached a patch. That issue has remained open and unfixed for 3 and 1/2 years.
Yesterday, Chin Huang posted a handy-dandy ELResolver that XML-escapes EL values.
I tried Chin's resolver in AppFuse today and it works as well as advertised. To do this, I copied his EscapeXML*.java files into my project, changed the JSP API's Maven coordinates from javax.servlet:jsp-api:2.0 to javax.servlet.jsp:jsp-api:2.1 and added the listener to web.xml.
With Struts 2 and Spring MVC, I was previously able to have ${param.xss} and pass in ?xss=<script>alert('gotcha')</script> and it would show a JavaScript alert. After using Chin's ELResolver, it prints the string on the page instead of displaying an alert.
Thanks to Chin Huang for this patch! If you're using JSP, I highly recommend you add this to your projects as well.
