ZyXEL Prestige routers have a minimum password length for the admin account that is too small, which makes it easier for remote attackers to guess passwords via brute force methods.
https://exchange.xforce.ibmcloud.com/vulnerabilities/41513
http://www.securityfocus.com/archive/1/489009/100/0/threaded
http://www.procheckup.com/Hacking_ZyXEL_Gateways.pdf
http://www.gnucitizen.org/projects/router-hacking-challenge/