As you browse from site to site each new page should start with a clean
slate. shutdown reports a technique that pollutes the global scope of
a window in a way that persists from page to page.
A malicious script could define a setter function
for a variable known to be used by a popular site, and if the user
does browse to that site the malicious script will run in that page.
This would allows the setter script to steal cookies or the contents
of the page, or potentially perform actions on the user's behalf
(such as make purchases or delete webmail) depending heavily on
how the site was designed.The objects persist until the window or tab is closed.