The Simple Sitemap – Create a Responsive HTML Sitemap plugin for WordPress is vulnerable to Cross-Site Request Forgery in all versions up to, and including, 3.5.13. This is due to missing or incorrect nonce validation in the 'admin_notices' hook found in class-settings.php. This makes it possible for unauthenticated attackers to reset the plugin options to a default state via a forged request granted they can trick a site administrator into performing an action such as clicking on a link.