Java website protection solutions (especially XSS)

Question

I'm developing a web application, and facing some security problems.

In my app users can send messages and see other's (a bulletin board like app). I'm validating all the form fields that users can send to my app.

There are some very easy fields, like "nick name", that can be 6-10 alpabetical characters, or message sending time, which is sended to the users as a string, and then (when users ask for messages, that are "younger" or "older" than a date) I parse this with SimpleDateFormat (I'm developing in java, but my question is not related to only java).

The big problem is the message field. I can't restrict it to only alphabetical characters (upper or lowercase), because I have to deal with some often use characters like ",',/,{,} etc... (users would not be satisfied if the system didn't allow them to use these stuff)

According to this http://ha.ckers.org/xss.html, there are a lot of ways people can "hack" my site. But I'm wondering, is there any way I can do to prevent that? Not all, because there is no 100% protection, but I'd like a solution that can protect my site.

I'm using servlets on the server side, and jQuery, on the client side. My app is "full" AJAX, so users open 1 JSP, then all the data is downloaded and rendered by jQuery using JSON. (yeah, I know it's not "users-without-javascript" friendly, but it's 2010, right? :-) ) I know front end validation is not enough. I'd like to use 3 layer validation: - 1. front end, javascript validate the data, then send to the server - 2. server side, the same validation, if there is anything, that shouldn't be there (because of client side javascript), I BAN the user - 3. if there is anything that I wasn't able to catch earlier, the rendering process handle and render appropriately

Is there any "out of the box" solution, especially for java? Or other solution that I can use?

Answer 1

The easiest way is to do a simple replacement for the following < with &lt;
> with &gt;
' with \'

That will solve most database vulnerability.

Answer 2

To minimize XSS attacks important thing is to encode any field data before putting it back on the page. Like change > to > and so on. This would never allow any malicious code to execute when being added to the page.

I think you are doing lot of right things by white listing the data you expect for different fields. Beyond that for fields which can allow other characters which can be problematic encoding would fix the issue for you.

Further since you are using Ajax it gives you some protection as people cannot override values in URL parameters etc.

Answer 3

Look at the AntiSamy library. It allows you to define rulesets for your application, then run your user input through AntiSamy to clean it per your rules.