MW WP Form < 5.1.5 - Unauthenticated Arbitrary Shortcode Execution via Completion Message Merge Tags

The MW WP Form WordPress plugin before 5.1.5 does not prevent shortcodes in user-submitted values from being executed when it merges those values into a message that it later processes for shortcodes, allowing unauthenticated users to run any shortcode registered on the site. Exploitation requires the site to have been configured to echo a submitted value back to the visitor after submission.