The Sentence To SEO (keywords, description and tags) plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the 'Permanent keywords' field in all versions up to and including 1.0. This is due to insufficient input sanitization and output escaping. The plugin reads user input via filter_input_array(INPUT_POST) which applies no HTML sanitization (FILTER_DEFAULT), stores it unsanitized to the WordPress options table via update_option(), and then outputs the stored value directly into a textarea element without any escaping using PHP short echo tags (<?= ?>). An attacker can break out of the textarea element using a closing </textarea> tag and inject arbitrary HTML/JavaScript. This makes it possible for authenticated attackers, with administrator-level access and above, to inject arbitrary web scripts in pages that will execute whenever a user accesses the plugin's settings page.