Cross-site information disclosure via modal calls — Mozilla

Security researcher Eduardo Vela Nava reported that if a web page opened a new window and used a javascript: URL to make a modal call, such as alert(), then subsequently navigated the page to a different domain, once the modal call returned the opener of the window could get access to objects in the navigated window. This is a violation of the same-origin policy and could be used by an attacker to steal information from another web site.