Comment by julia-zheng

There's a number of ways that a fake card will look different from a real card - we aggregate these signals and form a decision on real vs fake (sorry, we know that's a terrible answer - we would disclose more, but it's best practice to keep specifics of fraud detection a secret to maintain efficacy). Surprisingly, the gap between real and fake is wide enough that we can with good precision separate those cases. Of course, someone could build a replica indistinguishable from a real card, but at that point you've raise the barrier of committing fraud much higher than simply having a stolen credit card number, so chances are the fraudsters would migrate to some other platform