The WP Private Message WordPress plugin (bundled with the Superio theme as a required plugin) before 1.0.6 does not ensure that private messages to be accessed belong to the user making the requests. This allowing any authenticated users to access private messages belonging to other users by tampering the ID.
https://wpscan.com/vulnerability/f915e5ac-e216-4d1c-aec1-c3be11e2a6de
https://themeforest.net/item/superio-job-board-wordpress-theme/32180231