The W3 Total Cache WordPress plugin before 2.1.5 was affected by a reflected Cross-Site Scripting (XSS) issue within the "extension" parameter in the Extensions dashboard, when the 'Anonymously track usage to improve product quality' setting is enabled, as the parameter is output in a JavaScript context without proper escaping. This could 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.