JSP JSTL

Brief Overview of JSP Architecture
JSPs are delivered to a container that provides services like life-cycle management and runtime support. A JSP gets translated to a servlet class which is instantiated at runtime. A request headed for a particular JSP will be directed by the container to its corresponding servlet class (aka jsp implementation object). It then handles requests and generates responses. The default request response objects are HttpServletRequest and HttpServletResponse. JSP makes use of implicit objects that can be considered taint sources, sinks and propagators. I won't discuss further details since that is outside the scope of this project. For further information try this reference guide.

JSP In Light of Security
There’s not much to say here except that JSPs can act as both a model and view. It can operate fairly well without a distinct service or business layer because it doesn’t quite enforce separation of logic and concerns (hence the advent of development frameworks).

I won’t discuss details of web app design here, but one should understand that lack of separation can have negative effects on web-app stability and security. But even so, lack of input validation can lead to easy security vulns in JSP, namely XSS.

Commonly, JavaBeans are used in conjunction with JSP to store parameters and implement business logic. Most of my examples will use beans to demonstrate taint propagation and proper cleansing.

For more data on design with JSP and servlets, see this best practices article

Propagators
 

Sinks
 

JSP Implicit Objects
There are a handful of objects made available in JSPs which are susceptible to security flaws. Their corresponding java class functions are used as is in scriptlets. All the same security rules should apply. Examples: saveinfo.jsp <%    String name = request.getParameter("username"); session.setAttribute("taintedAttribute", name); %> displayinfo.jsp my xss varible: <%=session.getAttribute("taintedAttribute", name)%>

Unified EL
see also [J2EE Bad Practices: JSP Expressions]