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Re: PIX DOS (config problem) - Similar to NetScreen ScreenOS...
From: "David P. Maynard" <dpm () flametree com>
Date: Sat, 02 Feb 2002 18:34:13 -0600


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
--



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