The WP Compress WordPress plugin before 7.10.04 does not validate the value of a query parameter that controls the asset CDN host before using it to build the URLs of JavaScript files emitted on the page, leading to Reflected XSS. When a visitor follows a crafted link, the WP Compress WordPress plugin before 7.10.04's loader injects script elements pointing to an attacker-controlled origin, which lets the attacker execute arbitrary JavaScript in the visitor's session on the target site.