The Google Tag Manager for WordPress (GTM4WP) plugin is vulnerable to Stored Cross-Site Scripting due to insufficient escaping via the `gtm4wp-options[scroller-contentid]` parameter found in the `~/public/frontend.php` file which allowed attackers with administrative user access to inject arbitrary web scripts, in versions up to and including 1.15.1. This affects multi-site installations where unfiltered_html is disabled for administrators, and sites where unfiltered_html is disabled.