iThemes Sync <= 2.1.13 - Cross-Site Request Forgery and Missing Authorization via 'hide_authenticate_notice'

The iThemes Sync plugin for WordPress is vulnerable to Cross-Site Request Forgery in versions up to, and including, 2.1.13. This is due to missing or incorrect nonce validation on the hide_authenticate_notice function. This makes it possible for unauthenticated attackers to hide admin notices via a forged request granted they can trick a site administrator into performing an action such as clicking on a link. Authorization was also missing, which allowed subscribers to dismiss notices via AJAX request.