Apple Resurrects Dead Mac Ethernet Ports Following Botched OS X Security Update
If you find that Ethernet connectivity is no longer working on your Mac, Apple says to check System Information to find out which version of "Incompatible Kernel Extension Configuration Data" is installed, which you can access in System Information under Software. If you have version 3.28.1, you need to update your Mac.
Here's where things can get tricky. If you're able to use Wi-Fi, your Mac should update to version 3.28.2 automatically. To force the issue, open the Terminal app and type sudo softwareupdate --background and then quit Terminal and restart your Mac. While it's restarting, grab a bottle of your favorite beverage and toast a job well done.
If you can't connect to a wireless network, the steps required to update are a little more involved. First, you'll need to restart your Mac in OS X Recovery, select Disk Utility, and then select your drive from the list of internal drives in the sidebar (default name is "Macintosh HD"). If the drive is gray, select File > Unlock from the Disk Utility menu and enter your FileVault password. Then select File > Mount in Disk Utility (if it's not already mounted).
From there, quit Disk Utility and launch the Terminal app from Utilities > Terminal. Then in one line, type the following command (change Macintosh HD to whatever your drive name is, if it's different):
rm -rf “/Volumes/Macintosh HD/System/Library/Extensions/AppleKextExcludeList.kext”
Quit the Terminal app and restart your Mac. Your Ethernet connection should work at this point, however you're not finished—Apple still suggests updating to 3.28.2. To do that, open the Terminal app again and type sudo softwareupdate --background then quit and restart your Mac one more time.