The plugin 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).
1. Go to Klaviyo Settings, and at Klaviyo Settings in thePublic API Key field, put: "autofocus onfocus=alert(1)// 2. Submit and the alert will trigger.