The Newsletter WordPress plugin before 8.7.1 does not sanitise 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/a6582e14-e21e-48e7-9b4c-0044fb199825/