ApostropheCMS is an open-source Node.js content management system. Versions up to and including 1.4.2 of the "@apostrophecms/seo" package injects the Google Analytics Tracking ID ("seoGoogleTrackingId") and Google Tag Manager ID ("seoGoogleTagManager") directly into "<script>" tag bodies using JavaScript template literals without any sanitization or validation. Any user with editor-level access (the default role for content managers) can set these fields to a malicious value, resulting in stored XSS that executes on every page for every visitor of the site. As of time of publication, no known patched versions are available.