ZyXEL Prestige 642R 2.50(FA.1) and Prestige 310 V3.25(M.01), allows remote attackers to cause a denial of service via an oversized, fragmented "jolt" style ICMP packet.
ZyXEL ZyWALL 10 before 3.50 allows remote attackers to cause a denial of service via an ARP packet with the firewall's IP address and an incorrect MAC address, which causes the firewall to disable the LAN interface.
Zyxel Prestige 681 and 1600 SDSL Routers allow remote attackers to cause a denial of service via malformed packets with (1) an IP length less than actual packet size, or (2) fragmented packets whose size exceeds 64 kilobytes after reassembly.
ZyXEL Prestige 642R and 642R-I routers do not filter the routers' Telnet and FTP ports on the external WAN interface from inside access, allowing someone on an internal computer to reconfigure the router, if the password is known.