Reset Admin password for macOS Mojave

Solution 1:

You can't reset the admin's password but you can create a new admin.

Essentially it's the same procedure as in this answer:

How can I get admin access to a Mac without knowing the current password?

1. Boot to Recovery Mode holding ⌘+R on startup
2. Open Disk Utility and unlock the encrypted volume. The encrypted volume is an sub-item of the internal drive and greyed out. Select it and hit mount/activate. Use the password of the standard user to unlock it.

3. Open Terminal from the Utilities menu and enter (to execute a typed command you have to hit the ⏎ Enter/Return key):

   rm /Volumes/<main_volume_name>/private/var/db/.AppleSetupDone
   

   Replace <⁠main_volume_name⁠> with the name of your main volume (usually Macintosh HD). Example:

   rm /Volumes/Macintosh\ HD/private/var/db/.AppleSetupDone
   

4. Enter reboot to reboot your Mac

5. Login with your standard user (to unlock the disc), then follow the instructions to create a new admin user (similar to configuring a new Mac)
6. The new admin can't unlock the disk - the account has to be added by another admin user, because the new admin can't promote himself to a FileVault unlocker. Check this by entering in Terminal: sudo fdesetup list.
7. With the new admin promote the standard user to admin and reboot

8. Login as old standard/now admin user, open Terminal and enter

   sudo fdesetup add -user exstandarduser -usertoadd newadmin
   

   Example:

   sudo fdesetup add -user lorenzoalfieri -usertoadd adminlorenzo
   

   Then you will be asked for passwords: first enter the password of exstandarduser to authenticate him and second the password (set in step 5) of newadmin.

   Check that the new admin is added to the FV users: sudo fdesetup list

9. Reboot and login as new newadmin
10. Downgrade the exstandarduser admin to a standard user.
11. Clean up the old admin.
12. Finally create new recovery key(s): sudo fdesetup changerecovery -personal and make a (physical) note of it|them and store it at a safe place.

• Resetting the admin password with resetpassword after unlocking the disk is not possible.

• Changing the password of the admin with dscl after unlocking the disk is apparently not possible - the old password is still required.

  Example:

  dscl -f /Volumes/Macintosh\ HD/var/db/dslocal/nodes/Default localonly -passwd /Local/Target/users/<user_name> some_password