[benlivengood]

Without at least some filtering a Gateway NAT appliance is vulnerable to:

* LAN IP address spoofing from the WAN

* Potential for misconfigured "internal" daemons to accept WAN traffic (listening on 0.0.0.0 instead of the LAN or localhost)

* Reflection amplification attacks

[tsimionescu]

LAN IP address spoofing is indeed a valid attack vector, if the ISP is compromised.

Internal daemons on machines other than the router itself in the LAN network listening on 0.0.0.0 are not insecure (unless you have the problem from point 1, malicious/compromised ISP). The router won't route packets with IPs that are not in its LAN to them. Of course, the router itself could be compromised if it accidentally listens on 0.0.0.0 and accepts malicious packets.

Not sure what you mean by reflection amplification attacks, but unless they are attacking the router itself, or they are arriving on WAN with LAN IPs (again, compromised/malicious ISP), I don't see how they would reach LAN machines.

[zajio1am]

You do not need compromised ISP for spoofed LAN IP traffic, the attack could came from other clients on the same WAN segment.