UI Redressing - The modem (like most modems) does not have a clickjacking protection. thus, can be used to modify settings, override admin accounts by a simple clickjack. forexample by using http://192.168.1.1/adminpasswd.html it is possible into tricking an admin submit a form with our credintials (since it doesn't require current password)
not using SSL - The modem does not use HTTPS, so anyone can use MiTM to sniff on going actions, possibly gain user credintials.
Unrestricted privileges - anyone who is connected to the modem with Telnet or tftp is root. simply telneting and authenticating as admin:admin and typing sh and echo $USER would prove that.