ZyXEL Prestige 623, 650, and 652 HW Routers, and possibly other versions, with HTTP Remote Administration enabled, does not require a password to access rpFWUpload.html, which allows remote attackers to reset the router configuration file.
https://exchange.xforce.ibmcloud.com/vulnerabilities/18202
https://euvd.enisa.europa.eu/vulnerability/EUVD-2004-1534
http://www.securityfocus.com/bid/11723
http://securitytracker.com/id?1012298
http://secunia.com/advisories/13278