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Real Example: Adding Input Validation to a Legacy Application

Of course, using AOP to its full potential on existing applications would entail a near rewrite of the application - something that is often not feasible. What should be more interesting to the security community is how to use AOP in an existing code base for security functions that are not already implemented.

Let's take a look at an existing Customer Relationship Management (CRM) application that has several Cross-site Scripting (XSS) vulnerabilities. When I try to enter in a new sales lead in one of the web forms with the description <script>alert(document.cookie)</script>.

I get:

This form is vulnerable to XSS. Since I know there are several parts to the code that share this vulnerability, I decide that I want to protect the NewLead object in the business layer by adding input validation to any time that a setter method on a String attribute is called. Here is a snippet of the NewLead class that has several setter methods:

I created the following code in programming language AspectJ, which can be easily integrated into existing Java applications - to mitigate the XSS in the application. Don't worry if this looks confusing. I'll explain the example in detail:

public aspect InputValidator-

An Aspect in AspectJ closely resembles a class. It can have instance variables and methods inside of it. There are some key differences, however, including pointcuts and advice.

pointcut myPointcut(String argument): execution(* NewLeadBean.set*(String)) && args(argument)-

A pointcut in AOP defines where we want our aspects to interact with the rest of the code. In this case, our pointcut myPoint() is invoked anytime anyone calls setName(), setSalesStage(), etc. from the NewLeadBean class. The syntax NewLead.set*(String) is intuitive: any method that is part of the NewLead class, begins with "set", and has a single string parameter.

The && args(argument) part is saying that we want to save the string parameter to a variable named argument (e.g. if somebody called myNewLeadObject.setName("rohit") then rohit would be saved to the variable argument). Pointcuts can get quite complicated - for a detailed discussion please see the AspectJ programming guide[4].

before (String argument): myPointcut(argument)

This is what we call "advice". It is saying, before the invocation of myPointcut (i.e. before NewLead.set*() is called), execute the following code. Note we could also have specified after or around (i.e. both before and after) innovation of the pointcut. The argument is the string that's passed in as a parameter. This example shows that we are taking the string argument (in our case, "rohit") and checking it to see if it passes white-list validation [7] with a regular expression checker. If I had entered in something along the lines of <script>alert(document.cookie)</script> as the name of my sales lead, I would have seen the following page on screen:

This shows that the aspect successfully stopped the attack.

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