The Speed Optimizer WordPress plugin before 7.8.5 does not sanitise or escape some of its settings before outputting them in the head of front-end pages, which could allow high-privilege users such as admins to perform Stored Cross-Site Scripting attacks even when the unfiltered_html capability is disallowed (for example in a multisite setup).
https://wpscan.com/vulnerability/308b6173-139d-4940-869d-c9ff1e83c323/