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Subject: FW-1 IP Fragmentation Vulnerability
From: Lance Spitzner (lanceSPITZNER.NET)
Date: Mon Jun 05 2000 - 19:08:42 CDT

It was never my intent to identify a DoS attack on FW-1.
I was attempting to research and understand how FW-1 handles
IP Fragmentation. Everthing that follows is a result of
that research. Full findings of my research can be found at

On Saturday, May 27, I identified a major DoS attack for FW-1.
CheckPoint was immediately notified. Since then, they have
developed a short term solution and are currently working
on a long term solution (see details below).

CPU mysteriously hits 100% utilization, system locks up.
Some systems may also crash, depending on OS type.

Installations Vulnerable
1. I have reason to believe that every installation of FW-1 is
vulnerable, regardless of Operating System type or version/patch
level of the FW-1 installation. However, this has only been tested
and confirmed with ver 4.1 SP1 on the Nokia, and ver 4.1 on NT and
Solarix x86 platform.

2. There is NO way to protect against it. Your rulebase
cannot stop this attack. If your rulebase is denying everything,
you are still vulnerable.

3. FW-1 does NOT log these attacks in the firewall logs. Not
only will the firewall will be taken out, but it is difficult to
determine why. Illegally fragmented packets (such as those
generated by jolt2) may be logged by Unix systems to

Most frag based attacks that use incomplete or illegal fragments
will work, including jolt2. The firewall does not have to be
attacked directly, if the frags are routed through the firewall
for a system behind the firewall, FW-1 is still taken out.

FW-1 does not inspect, nor does it log, fragmented packets untill the
packet has first been completely reassembled. Since these exploit packets
are never fully assembled, they are never inspected nor logged. Thus, the
firewall's own rulebase cannot be used to protect against the attack.
For more information on FW-1 IP Fragmentation reassembley, see

The actual CPU utilization is most likely the result of the application
attempting to reassemble hundreds or thousands of incomplete and
illegally fragmented packets. As stated above, the firewall rulebase
cannot block these packets, as they are never inspected.

Other firewalls may have the same problem and vulnerability.

1. CheckPoint has developed a short term solution to the problem. A
percentage of CPU utilization is due to console error messages on
some Unix systems. By disabling FW-1 kernel logging, some CPU
utilization will be saved. However, all FW-1 kernel logging is
disabled, you will have no capability for logging any firewall
kernel events. At the command line on the Firewall, type as root:
                 fw ctl debug -buf

2. Ensure the operating system has the latest patches. Most operating
system have recently released patches that help protect against fragment

3. Run an IDS module (such as snort). When you detect frag attacks
block the Src at the router (remember, the firewall CANNOT stop the
attack, its rulebase is powerless). However, this method may not
work with spoofed Src packets.

4. CheckPoint is developing a long term solution, which will be
distributed as part of a later Service Pack. However, this fix
was not available for testing at the time of this post.

I appreciate the help and involvement from the following people
in helping with this issue.

Chris Brenton, Dartmouth's Institute for Security Technology Studies
Dameon Welch-Abernathy, http://www.phoneboy.com/fw1

Joe DiPietro, CheckPoint
Robert Slayton, CheckPoint
Mark Elliott, CheckPoint

Lance Spitzner