Does this set of regular expressions FULLY protect against cross site scripting?

Question

What\'s an example of something dangerous that would not be caught by the code below?

EDIT: After some of the comments I added another line, commented below. See V

Answer 1

I still have not figured out why developers want to massage bad input into good input with a regular expression replace. Unless your site is a blog and needs to allow embedded html or javascript or any other sort of code, reject the bad input and return an error. The old saying is Garbage In - Garbage Out, why would you want to take in a nice steaming pile of poo and make it edible?

If your site is not internationalized, why accept any unicode?

If your site only does POST, why accept any URL encoded values?

Why accept any hex? Why accept html entities? What user inputs ' ' or '"' ?

As for regular expressions, using them is fine, however, you do not have to code a separate regular expression for the full attack string. You can reject many different attack signatures with just a few well constructed regex patterns:

patterns.put("xssAttack1", Pattern.compile("


    // hmtl allows embedded tabs...
 // hmtl allows embedded newline...
 // hmtl allows embedded carriage return...

Notice that my patterns are not the full attack signature, just enough to detect if the value is malicious. It is unlikely that a user would enter 'SRC=' or 'pt:al' This allows my regex patterns to detect unknown attacks that have any of these tokens in them.

Many developers will tell you that you cannot protect a site with a blacklist. Since the set of attacks is infinite, that is basically true, however, if you parse the entire request (params, param values, headers, cookies) with a blacklist constructed based on tokens, you will be able to figure out what is an attack and what is valid. Remember, the attacker will most likely be shotgunning exploits at you from a tool. If you have properly hardened your server, he will not know what environment you are running and will have to blast you with lists of exploits. If he pesters you enough, put the attacker, or his IP on a quarantine list. If he has a tool with 50k exploits ready to hit your site, how long will it take him if you quarantine his id or ip for 30 min for each violation? Admittedly there is still exposure if the attacker uses a botnet to multiplex his attack. Still your site ends up being a much tougher nugget to crack.

Now having checked the entire request for malicious content you can now use whitelist type checks against length, referential/ logical, naming to determine validity of the request

Don't forget to implement some sort of CSRF protection. Maybe a honey token, and check the user-agent string from previous requests to see if it has changed.