BSK(bishan4u@yahoo.co.uk)@Mon, Oct 24, 2005 at 12:34:30PM +0100:
Just wanted some feedback from you people. I'm doing a
Firewall Assessment for a CISCO PIX firewall. The
firewall allows SYN, FIN, NULL and XMAS scans but
blocks ACK scans (largely means its a stateful
firewall).
Now what do we do to block the scans that are allowed.
I think it should be easy to block FIN, NULL and XMAS
scans but how do we block or limit or workaround a SYN
scan. 1 way that I think is probably blocking or
limiting the packets from the source (using IDS/IPS)
You most likely don't really want to. Scans that are not part of a
stateful connection can have their source IP easily spoofed. If you
start blocking IP's based on the appearance of bad traffic that can be
spoofed, you'll open yourself up for some pretty impressive targeted DoS
situations.
When you auto-block based on perceived "badness", you're allowing that
bad person to write your rules. Basically, when you allow a "bad"
person to start writing your firewall rules for you, you've taken a big
step in the wrong direction.
Furthermore, I would say that there is an incorrect assumption that
trying to block port scans increases your security. Unfortunately, if
the resource is still available, you've not increased your security in
any measurable way. You've only added an additional set of DoS
conditions.
As a security tester you have a finite amount of resources (IP's to come
from, bandwidth, computers, etc), and a limited amount of time (1-4
weeks, etc). It should be assumed that an attacker has an unlimited
amount of resources (He's attacking you, surely he's compromised other
machines too). Even if he did port scan, he could break it into small
enough chunks from throw away IP's.
If it's a targeted exploit payload for a specific host/service, you
won't see a port scan coming before the exploit payload.
Robert
--
Robert E. Lee
CIO, Dyad Security, Inc.
W - http://www.dyadsecurity.com
E - robert@dyadsecurity.com
M - (949) 394-2033
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