The The Grid Plus – Unlimited grid layout plugin for WordPress is vulnerable to arbitrary shortcode execution via grid_plus_load_by_category AJAX action in all versions up to, and including, 1.3.5. This is due to the software allowing users to execute an action that does not properly validate a value before running do_shortcode. This makes it possible for unauthenticated attackers to execute arbitrary shortcodes.
https://wordpress.org/plugins/grid-plus/#developers
https://plugins.trac.wordpress.org/browser/grid-plus/tags/1.3.5/core/ajax_fe.php#L19