One Way to Fix the "System Extension Cannot Be Used, AppleUSBEthernetHost.kext" Error...

Question

I found (one) way to fix this error...well, at least it worked for me (see below):
There seem to be various threads addressing this issue and no "one" way is the right way for everyone. I'm running my Macbook on OS X 10.6.8 (Snow Leopard). Once I installed the iTunes update, two things happened: my iPhone became corrupted when I sync'd via USB and then my iTunes stopped recognizing the iPhone as a "sync-able" device -- meaning it would show up on iTunes but when you went to click sync, the sync button was unavailable.
I tried locating the .kext file, deleting it, repairing it, replacing with an older version, entered different commands in terminal but I kept getting the same error. Ultimately, I fixed the problem fairly simply, but it may not be something everyone is willing to do. If you have an external hard-drive and had your computer hooked up to Time Machine, this is what you should do:
Hopefully you'd been backing your computer up automatically fairly recently, and you have some this stored on an external hard drive via Time Machine from before iTunes 11.4 was downloaded onto your computer.
If you haven't done so already, and your iTunes library isn't corrupt, Backup your entire iTunes library to an external hard-drive (if you don't care about your library, ignore this step...this procedure might not work, however, if your first backup of your iTunes library is via version 11.4. If you go through all of these steps and you encounter the same problem, restore your itunes to an older version via Time Machine, then redo steps)
To do this, create a new Folder on your hard drive and call it "iTunes"
Go into your iTunes preferences > Advanced > under "iTunes Media folder location" we want it to say /Volumes/(Your Hard Drive Name)/iTunesNext to the "iTunes Media folder location" dialogue box, it should read CHANGE or RESET. Click CHANGE, navigate to your external hard drive and locate the newly created iTunes folder...your computer should automatically start to transfer your library to the new destination.       
Backup and "new files" on your computer that you create since updating your iTunes to version 11.4 (i.e. new iCal entries, documents)
Backup your important files (I have mine in dropbox but chose to be safe and backed them up on an external hard drive as well)
Once you have everything saved, you're going to want to go through the process of completely restoring your entire computer (essentially wiping it clean).
To do this on Snow Leopard, you need to have the Mac OS X Snow Leopard Installing disc.
Insert disc > when the Install screen pops up, select Utilities... > it will then prompt you to Restart your computer
After your computer restarts, it will prompt you to reinstall Snow Leopard or select a different option from Utilities which can now be found at the top of your screen on the toolbar. From there, select the option that is along the lines of Restore from Backup Using Time Machine and then select which backup you want to restore your computer to ... since iTunes 11.4 went live on Sept. 10th, I would restore to any date before that.
When your computer restores itself to your chosen backup, your computer should be exactly as it was on that date, iTunes included. It's as if nothing ever happened.
To do this on any later OS X, I believe you have to restart your computer and then hold COMMAND+R before the grey screen appears. From here, I believe it will prompt you similarly to the Snow Leopard procedures 1-3 as listed just above this bullet point.
When your computer prompts you to update iTunes to version 11.4 from whatever version you reverted back to, DO NOT UPDATE. Keep your old iTunes version and wait for this thing to be addressed by Apple.
This procedure worked for me and although it's a bit tedious, it fixed both my iTunes and iPhone errors. Note: I did have to erase and restore my iPhone through an old backup via iTunes once iTunes was working again. Try it out. See if it works for you. MAKE SURE YOU BACKUP YOUR FILES BEFORE ATTEMPTING ANY FULL SYSTEM RESTORE.

Answer 1

There's more to it than this IMHO.  Here is my explanation:
If I go back in my Time Machine backups and restore the previous version before I installed iTunes 11.4, the version of that same kext was also listed as being "7.0" despite being older and twice the size (120400 bytes vs. 63024 bytes in the 11.4-installed version).
(The older version is "BuildVersion 305" and "SourceVersion 122000000000000"; the new 11.4 version is "BuildVersion 417" and "SourceVersion 113000000000000".)
The real problem is this:
/System/Library/Extensions/AppleUSBEthernetHost.kext/Contents/_CodeSignature used to contain
-rw-r--r--+ 1 root  wheel  172 Jan 31  2014 CodeDirectory
-rw-r--r--+ 1 root  wheel    92 Jan 31  2014 CodeRequirements
-rw-r--r--+ 1 root  wheel  1745 Jan 31  2014 CodeResources
-rw-r--r--+ 1 root  wheel  4064 Jan 31  2014 CodeSignature
/System/Library/Extensions/AppleUSBEthernetHost.kext/Contents used to contain symbolic links down into there:
lrwxr-xr-x+ 1 root  wheel    28 Jun 12  2012 CodeDirectory@ -> _CodeSignature/CodeDirectory
lrwxr-xr-x+ 1 root  wheel    31 Jun 12  2012 CodeRequirements@ -> _CodeSignature/CodeRequirements
lrwxr-xr-x+ 1 root  wheel    28 Jun 12  2012 CodeSignature@ -> _CodeSignature/CodeSignature
When iTunes 11.4 was installed via Software Update/App Store, it removed CodeDirectory/CodeRequirements/CodeSignature but it did not remove the corresponding symbolic links.  Presumably that is what triggered the "cannot be used" pop-up message.
Trashing the kext and installing it from the downloaded 11.4 .dmg does not create the symbolic links, thus avoiding the problem.
Obviously rolling back to the previous (Jan 2014) kext will fix the problem as well as the symbolic links point to files that exist.
(I'm really curious to know what happened between BuildVersion 305 and BuildVersion 417 that caused it to shrink nearly by half.  It looks to me like the Code Signing stuff got ripped out - Lord knows why.)