Formie is a Craft CMS plugin for creating forms. Prior to version 2.1.44, it is possible to inject malicious code into the HTML content of an email notification, which is then rendered on the preview. There is no issue when rendering the email via normal means (a delivered email). This would require access to the form's email notification settings. This has been fixed in Formie 2.1.44.

Formie has XSS vulnerability for email notification content for preview