Zen Cart 1.5.6d allows reflected XSS via the main_page parameter to includes/templates/template_default/common/tpl_main_page.php or includes/templates/responsive_classic/common/tpl_main_page.php.
https://github.com/advisories/GHSA-wxxx-2x6v-979f
https://herolab.usd.de/security-advisories/usd-2019-0069/