[Attack] Cross Site Scripting (XSS) [Responsibility of] Page Owner [Prevention] Sanitize all input [Future Standards] Content Security Policy [Future Responsibility of] Server Owner [Example] 
Site A includes a particular URL from site B in a page, and makes the user agent request it. The URL is created in such a way that it will cause site B to include a script of site A's choosing. As the page is loaded with the user agents credentials, the script is able to perform actions at site B in the user's name.