Mould it as per your need

We had a discussion with our colleagues over XSS issue found in one application. Initially there was not input validation at all-you can insert simple script tag and execute XSS. Following our recommendations they filtered out certain special characters like (>,<," etc) also they encoded them at time of output. Fair enough? No. Actually they implemented half of the recommendations- ie. they worked on blacklisting and left out whitelisting. There are a number of models to think about when designing a data validation strategy, which are listed from the strongest to the weakest as follows. 1.Exact Match (Constrain) 2.Known Good (Accept) 3.Reject Known bad (Reject) 4.Encode Known bad (Sanitize) They were implementing last two of strategies only. So the application was now filtering out normal XSS vectors like "><script>alert(...);</script> based attacks. But what happens when we provide eventhalders like onmouseover,onload etc-XSS executed. When we brought this to customers' notice they said that alphabets and " (double quotes) are valid inputs in the comment fileds, how can we filter them out, not even whitelisting approach will work here as these are valid characters. So after a brainstorming session with them we advised them to mould as per their need. It's not like that you blindly follow strategies mentioned above for whole application. We suggested them for that specific case where " (double quotes) and alphabets were valid inputs (in comments fields) don't filter " (double quotes) but atleast filter even handlers-onload,onfocus etc by using this sample script: .replaceAll("(?i)<.*?\\s+on.*?>.*?", "");

It removes on* attributes like onLoad or onClick

My point is that in some cases you need to shape your strategies as per your need to strike a fine balance between security and user-friendliness.


530 Geeks said…
What you are recommending is incorrect.

The only way to solve XSS is Escaping Output. Input Validation has NOTHING to do with XSS prevention.

Refer to OWASP's XSS Prevention cheat sheet. There are specific rules for encoding attribute values. If you follow OWASP's recommendation, your application can accept any string and still display it back to the user without any XSS.

There is no need to mould security on an application basis. OWASPs recommendations for XSS are good enough to fix XSS in all kinds of websites.
Nilesh Kumar said…
Hi 530 Geeks,

Thanks for your valuable comments!

I can understand what you are saying. I have already mentioned the about encoding (4.Encode Known bad (Sanitise)) in my blog. They are doing that also. But in the case of their comment field if they encode " it comes as &quotes; which doesn't look good.
What I want to say is sometimes you have to make a balance between security and user-friendliness.

530 Geeks said…
If it appears as & q u o t ; , they are not escaping/encoding it properly. I suspect they are double encoding. It perhaps appears as & a m p ; q u o t ; in the HTML.

I am trying to say that "Escaping Output" is alone sufficient for a usable and secure website. Input Validation helps, but it neither necessary nor fool-proof.

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