Apple iTunes 12.3 update breaks OS X USB tethering

They forgot to codesign

    /System/Library/Extensions/AppleUSBEthernetHost.kext/

    $ codesign -vvv /System/Library/Extensions/AppleUSBEthernetHost.kext
    /System/Library/Extensions/AppleUSBEthernetHost.kext: code object is not signed at all
    $

Solution:

1. sudo mv /Applications/iTunes.app /Applications/iTunes.broken.app

2. Downgrade to 12.2.2 via DMG installer (be sure to remove all iDevices before re/installing iTunes, or that specific kext may also be erased entirely instead of installed... another bug.)

Tested and it works. The old kext used the old style signing, so they probably meant to use the new kind.