Kirki < 6.2.1 - Unauthenticated Arbitrary Shortcode Execution via Form Email Actions

The Kirki WordPress plugin before 6.2.1 does not properly authorise its front-end form submission REST routes and passes attacker-controlled input through shortcode execution, allowing unauthenticated users to run any shortcode registered on the site, which on a default install leads to disclosure of the site administrator's email address and an arbitrary-recipient mail relay from the victim's domain.