The good news is that Apple has since acknowledged the problem and has already provided a fix for it. According to Apple, “If the Ethernet connection on your Mac stopped working recently, check System Information to find out which version of “Incompatible Kernel Extension Configuration Data” is installed. If you have version 3.28.1, you need an update. If you can connect to WiFi, your Mac will update to version 3.28.2 automatically, or you can follow the steps below to restore it manually.”
For those who can connect to WiFi, Apple asks that you launch the Terminal app and type in (minus the quotes) “sudo softwareupdate –background”. This will update the kernel’s version to the one that will fix the Ethernet port. For those who can’t connect to the WiFi or who don’t have WiFi, the steps are a bit longer but basically you will have to reboot your computer into OS X Recovery mode.
The full instructions can be found on Apple’s support page, so head on over if you’d like the step-by-step instructions.