"Multiple Users" Control Panel in Mac OS 9 allows Normal users to gain Owner privileges by removing the Users & Groups Data File, which effectively removes the Owner password and allows the Normal user to log in as the Owner account without a password.
https://exchange.xforce.ibmcloud.com/vulnerabilities/5830
https://euvd.enisa.europa.eu/vulnerability/EUVD-2001-0102
http://archives.neohapsis.com/archives/bugtraq/2000-12/0497.html