The HM Books Gallery plugin for WordPress is vulnerable to Missing Authorization in versions up to and including 4.8.0. This is due to the absence of capability checks and nonce verification in the admin_init hook that handles the permalink settings update at line 205-209 of wp-books-gallery.php. The vulnerable code checks only for the presence of the 'permalink_structure' POST parameter before updating the 'wbg_cpt_slug' option, without verifying that the request comes from an authenticated administrator. This makes it possible for unauthenticated attackers to modify the custom post type slug for the books gallery, which changes the URL structure for all book entries and can break existing links and SEO rankings.