Typical severity: High. Likelihood of attack: High. An adversary embeds malicious script code in the parameters of an HTTP query string and convinces a victim to submit the HTTP request that contains the query string to a vulnerable web application. The web application then procedes to use the values parameters without properly validation them first and generates the HTML code that will be executed by the victim's browser.
Design: Use browser technologies that do not allow client side scripting. | Design: Utilize strict type, character, and encoding enforcement | Design: Server side developers should not proxy content via XHR or other means, if a http proxy for remote content is setup on the server side, the client's browser has no way of discerning where the data is originating from. | Implementation: Ensure all content that is delivered to client is sanitized against an acceptable content specification. | Implementation: Perform input validation for all remote content, including remote and user-generated content