A stored XSS vulnerability exists in Umbraco CMS <= 8.9.1 or current. An authenticated user can inject arbitrary JavaScript code into iframes when editing content using the TinyMCE rich-text editor, as TinyMCE is configured to allow iframes by default in Umbraco CMS.
https://www.tenable.com/security/research/tra-2020-59
https://www.tenable.com/security/research/tra-2020-59
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-5809