Typical severity: Very High. Likelihood of attack: High. An adversary exploits web applications that generate web content, such as links in a HTML page, based on unvalidated or improperly validated data submitted by other actors. XSS in HTTP Headers attacks target the HTTP headers which are hidden from most users and may not be validated by web applications.
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.