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.