Už jsem na to linkoval... In fact, no YubiKey firmware can be upgraded since the device security is designed so nothing (including malware) can write to the firmware. The Security Key has U2F capabilities as its only supported protocol.
(Samozřejmě když si to koupíš vod Ťing-ťonga za 2 dolary včetně poštovnýho, tak to chráněné nebude nijak...)