I was curious as to how many (and which) services support multifactor authentication beyond 2FA.
Googling, I found references to:
* Three factor authentication. The TechTarget piece I read suggests that the third factor is commonly a biometric.
* Four factor authentication.
* Up to six factor authentication.
To clarify my questions a little:
* What kind of services are four-factor authentication and above used to protect?
* Are there any common consumer products that rely on such elaborate protective measures?
* I assume the answer is obviously ‘no’, but is there theoretical limit as to how many cumulative layers of authentication a provider could enforce to protect a login / access?