There are many web applications which are designed to permit the input of html tags for displaying 
the html formatted data. these tags can be used by malicious users to attack other users by inserting scripts or malicious applets etc.this called cross site scripting or XSS. such attacks are result of poor input validations. it uses the combination of html and scripting languages. with the proper combination of html and java script a intruder can misguide the client and perform various attack from DOS(by opening enormous amount of window on client side) or By embedding malicious FORM tags at the right place, an mailicious user may be able to trick users into revealing sensitive information by modifying the behavior of an existing form or by embedding scripts, an intruder can cause various problems.
This is by no means a complete list of problems, but hopefully this is enough to convince you that this is a serious problem. Infosecwriters.com
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