clathem@skyhawke.com said: > Problem: NetScreen ScreenOS 2.6.1 subject to Trust Interface DoS > Attack >... > Exploit: Someone within the trusted side of the network can attempt a > portscan on an external IP address. When the scan runs it appears to > consume all of the available sessions. This, in turn, causes a DoS > to the entire trusted interface. For what it's worth, the instructions that Cisco publishes on how to configure the PIX firewall will make many users subject to a similar DOS attack. Cisco's published examples (at least the ones I have seen) on how to configure NAT for the PIX all show the following command: nat (inside) 1 0.0.0.0 0.0.0.0 0 0 The "1" is the global NAT pool identifier and the "0.0.0.0 0.0.0.0" is the address and netmask of addresses that are allowed to use the pool. In other words, any source IP on the inside interface is allowed to use global NAT pool 1. Given this configuration and a limited NAT pool, any machine on the inside network can create a DOS situation by launching a large number of outbound connections using random source IPs. Each random source IP will occupy one slot on the NAT table until they are all exhausted. Adding an "overload" or "PAT" address will mitigate the situation, but still isn't a "fix." A much better configuration is to restrict access to the NAT pool to valid source IPs on your local network. For example, if your inside network uses 192.168.0.0/24 and 192.168.5.0/24, then use: nat (inside) 1 192.168.0.0 255.255.255.0 0 0 nat (inside) 1 192.168.5.0 255.255.255.0 0 0 With all of the publicity over the past few years about proper egress filtering at border routers, you would think that more people would catch this problem. Unfortunately, I can safely say that I have never seen a PIX configured by anyone else that restricted NAT access to valid source IPs. Some of these boxes had been configured by end-users who were just reading the docs and wouldn't know any better. Unfortunately, a fair number of them had been configured by high-dollar network consultants (who apparently didn't know any better either). It is possible that PIX OS has a recent feature that can mitigate the impact of this problem, but I have seen it take down entire sites back when smurf attacks first came around. In any event, it is always a good idea to validate the source IPs leaving your network. -dpm -- David P. Maynard, CTO OutServ.net, Inc. -- Managed IT Operations Solutions [TM] EMail: dmaynard@outserv.net, Tel: +1 512 977 8918, Fax: +1 512 977 0986 --
