Had the same problem here today with this popping up after installing the update to iTunes 11.4.

System extension cannot be used
The system extension “//System/Library/Extensions/AppleUSBEthernetHost.kext” was installed improperly and cannot be used. Please try reinstalling it, or contact the product’s vendor for an update.


OSX 10.8.5
3.06 GHz Intel Core 13
4 GB 1333 MHz RAM

I don't 'tether' anything to my desktop but would still like to know what, if anything, I need to do. Will iTunes still be able to install future updates?