Update the WordPress Spiffy Calendar plugin to the latest available version (at least 4.9.1).
FearZzZz discovered and reported this Cross Site Scripting (XSS) vulnerability in WordPress Spiffy Calendar Plugin. This could allow a malicious actor to inject malicious scripts, such as redirects, advertisements, and other HTML payloads into your website which will be executed when guests visit your site. This vulnerability has been fixed in version 4.9.1.
Event deletion via CrossSite Request Forgery (CSRF) vulnerability
Admin+ Persistent CrossSite Scripting (XSS) vulnerability
Multiple Authenticated Reflected CrossSite Scripting (XSS) vulnerabilities
Edit/Delete event via IDOR vulnerability