This could be purely incidental to have shown up in 10.13, but yes, the problem comes from the fact that the first hardware ID in the list of devices happens to be an EUI-64 address with 64 bits now. This is the Firewire interface of my Mac Pro and maybe one of the few that actually uses a EUI-64 addresses, hence relatively rare. Maybe the order changed in 10.13 and this now happens to be the first entry returned from 'ifconfig' when previously maybe it was the Ethernet adapter, but given the lookup algorithm in uuid reads in line by line until it finds the first hw id, this is bound to fail on my machine. Clearly, I don't think uuid should make assumptions about order of interfaces that could have mixed EUI-48 and EUI-64 addresses.