WordPress W3 Total Cache plugin before 2.1.5 is susceptible to cross-site scripting via the extension parameter in the Extensions dashboard, when the setting 'Anonymously track usage to improve product quality' is enabled. The parameter is output in a JavaScript context without proper escaping. This can allow an attacker, who can convince an authenticated admin into clicking a link, to run malicious JavaScript within the user's web browser, which could lead to full site compromise.
Successful exploitation of this vulnerability could allow an attacker to inject malicious scripts into the affected website, leading to potential data theft, session hijacking, or defacement.