The Klaviyo WordPress plugin before 3.0.10 does not sanitize and escape some of its settings, which could allow high-privilege users such as admin to perform Stored Cross-Site Scripting attacks even when the unfiltered_html capability is disallowed (for example in multisite setup).
https://wpscan.com/vulnerability/495e39db-793d-454b-9ef1-dd91cae2c49b