I noticed on a website that when I entered my credit card number, the entry box was highlighted blue:
I then tried a dummy credit card number, which made the box highlight red, indicating that credit card number can not be used:
I then tried entering other credit card numbers from expired credit cards and the box highlighted blue again, so the page is checking if entered credit cards belong to an actual card.
Is this a secure practice? Because I could iterate all number combinations and discover which numbers belong to an existing credit card.