Many routers have port 0 open as a way of doing NAT. Any attempt to open port 0 from the WAN will result in the packet being redirected to an available port greater than 1025. There's no intrusion issue on having port 0 open, but there is one involving "fingerprinting" or identifying the router from the WAN side using the way port 0 is handled.'m not sure if one should consider this a security risk. Probably not.