PunBB 1.2.10 and earlier allows remote attackers to cause a denial of service (resource consumption) by registering many user accounts quickly.
https://exchange.xforce.ibmcloud.com/vulnerabilities/24837
https://euvd.enisa.europa.eu/vulnerability/EUVD-2006-0870
http://www.securityfocus.com/archive/1/425630/100/0/threaded