RE: IP Spoofs in the log - not sure what to do next

["Curt Purdy" <purdy () tecman com>]

You did not specify whether the logs were ingress or egress, but considering
your notes, I will assume outgoing.  Although it is relatively easy to spoof
ip addresses, it is more difficult, though not impossible to spoof mac
addresses.  Therefore, I would assume that the Apple is the likely culprit,
whether compromised or spoofing at the direction of it's user.  Either way,
you could confirm this box is the culprit by sniffing it's port on the
switch with tcpdump/ethereal/windoze sniffer.

If this is not the box, you have a bigger problem on your hands.  Also, I am
not sure why you are unable to stop the user from acessing AOL webmail.  You
should be able to put an ACL in your router/firewall to prevent this.

Curt Purdy CISSP, MCSE+I, CNE, CCDA
Information Security Engineer
DP Solutions
cpurdy () dpsol com

----------------------------------------

If you spend more on coffee than on IT security, you will be hacked.
What's more, you deserve to be hacked.
-- White House cybersecurity adviser Richard Clarke


-----Original Message-----
From: Chris Corbett [mailto:ccorbett () aspenwood com]
Sent: Thursday, April 17, 2003 5:18 PM
To: incidents () securityfocus org
Subject: IP Spoofs in the log - not sure what to do next


I have been observing this list for a while and believe this is the right
forum for this post. If not, direct me elsewhere
I am seeing a steady stream of IP Spoofs in a firewall log we track for a
client. Here is a sample
04/16/2003 10:08:15.624 - IP spoof detected - Source:172.175.86.24, LAN-
Destination:24.191.183.249, WAN - MAC address: 00.90.27.xx.xx.xx

All of the sources lead back to 172.128.x.x, 172.162.x.x, 172.138.x.x or
172.175.x.x which show up as AOL registered IP addresses (whois lookup)

The destination addresses seem to be random,  24.191.183.249,   64.1.1.34,
216.160.20.203 .....nothing I can decipher as a pattern and nothing close to
the network this firewall is "protecting".

The MAC address listed in the spoof is the same every time, ironically an
Apple computer on this network. This user (on the Apple) will occasionally
use AOL mail via the web (I can't stop them), but they are not using AOL as
their ISP. It's a DSL circuit and ISP services from another provider.

I am still learning about IP Spoofing and I don't want to overreact, but
from what I read, spoofs should be investigated further and I am at a point
where I am not sure what to look at next. The spoof is being detected by the
firewall and therefore denied, but what else should I be looking for to make
sure this is harmless?

Is it someone trying to use this network to spoof another network?

Could it be possible that this Apple machine is being compromised in some
way and being used for spoof attempts?

Chris Corbett
Aspenwood Technologies, LTD
ccorbett () aspenwood com
Denver, CO

Chris Corbett
Aspenwood Technologies, LTD
Denver, CO
303-733-0044 x 303
303-733-4466




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