Today, I applied the latest security patch to my Mac Studio, and on reboot, I had no networking. It appears to have been a system extension issue.
At one point, I needed to "Allow" Apple system software in System Settings. I found that strange.
I thought I'd document the issue and my resolution in case someone else runs into this.
(1) I did the usual - reboot, shutdown & restart, reboot my Eero mesh; changed from Wi-Fi to wired Ethernet. Nothing worked.
(2) I do have my own application that uses a network system extension, so I went through the system extension uninstall process (using the API). Still no joy.
I then tried to reinstall the network extensions, but that didn't seem to work. I was never prompted to open the System Settings app. I think the network system extension had not actually been removed.
I deleted the app (which should remove the network system extension). Still no joy.
Interestingly, launchctl still showed a crashed network system extension (no PID, status -9)
(3) I then disabled SIP, rebooted, and used systemextensionsctl to remove the network system extension.
While doing this, I discovered an old network system extension from several years ago tied to one of my old organizations and may have been built for Intel CPU. I deleted that too.
(If I had to guess, it might have been that old network system extension that caused the problem.)
Reenabled SIP
Rebooted.
(4) At some point I got an interesting alert from Apple about System Extension errors.
And when I opened System Settings, I had to allow an extension from Apple?!
(5) Networking is now working.
I reinstalled my application from TestFlight, installed the network system extension, and everything is still working.
(6) Summary
I lost networking after applying the security update.
Worried that it might be my program, I tried uninstalling the network system extension, but I could not cleanly uninstall and reinstall my network system extension as I've done many times before.
I found an old network system extension; deleted both network system extensions with SIP disabled.
I had to Allow Apple software.
Everything works (including my app with its network system extension installed).
I am not sure what the root cause was. My old network system extension? The fact I needed to Allow Apple software? My current app and its network system extension?