Came across a blog which states that Magento 1 version below 188.8.131.52 and Magento 2 below 2.1.3 are not designed to validate MasterCard credit cards that start with a 2xxx (its a new BIN launched in Jan 2017).
Personally, I just recently found out about this issue and never have though about it.
You can read more on this issue here:
I really hope this message can get to Magento Development team, in hopes they can release official Patch (just like thy did in 2014 with Discovery credit cards)
Interestingly, MOST stores are not able to accept new MasterCard cards as of early 2017, and are not even aware about it. I hope Magento can address this as soon as possible. It's a requirement as of June 2017 for merchants to accept MC's.
A patch is already available as of now. Did you check this SUPEE-8967 ?
Please read this also https://twitter.com/piotrekkaminski/status/859485929211396098