Stored XSS in Direct debit name

How it was found

1. Make new or edit old Direct debit (for example https://mobilevikings.be/en/account/easypay/correct-direct-debit-mandate/111366/)
2. Fill owners name with payload asdf'"><script>alert(document.cookie)</script>
3. Save form. We got Stored XSS in pages: https://mobilevikings.be/en/account/easypay/ https://mobilevikings.be/en/account/easypay/history/111366/ https://mobilevikings.be/en/account/easypay/auto-sms-topup/?req_subscription=1030418 And admin area pages may be affected.

The really interesting thing is if we press https://mobilevikings.be/en/account/easypay/287740/suspend/ link we got this cookie setted: Set-Cookie messages="e052df5f3af892c7a61d74d0d9a6ab14c7f1631c$[["__json_message"\0540\05425\054"Successfully suspended asdf'\"><script>alert(document.cookie)</script> <span>BE61310126985517</span>"]]"; Path=/

So we have a properly signed cookie with XSS payload. And if we found some way to setup it (it may be some xss on mobilevikings.be subdomain or CRLF issue in some data which used in header) we can use this xss vector too.