FireWall-1 Denial of Service
Reported June 6 by Lance Spritzer
Firewall-1 v4.0 and v4.1
A denial of service condition exists in versions 4.0 of FireWall-1 which is caused by
fragmented IP packets. According to Checkpoint, if a person uses the
"jolt2" program to send a stream of extremely large IP fragments to a FireWall-1
gateway, in some cases the action will cause the write mechanism to consume all CPU
resources on the firewall system.
The discoverer"s full report is listed below.
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
CPU mysteriously hits 100% utilization, system locks up. Some systems may also crash,
depending on OS type.
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 \[ note: see Checkpoint"s
workaround below which stops this attack \] . 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 http://www.enteract.com/~lspitz/fwtable.html
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
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 attacks.
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
I appreciate the help and involvement from the following people in helping with this
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
Web page regarding this matter states that they are working on service packs for
versions 4.0 and 4.1. It also states that "As an interim
workaround, customers can disable the console logging, thereby mitigating this issue by
using the following command line on their FireWall-1 module(s):
$FWDIR/bin/fw ctl debug -buf
This takes effect immediately. This command can be added to the $FWDIR/bin/fw/fwstart
command in order to be enabled when the firewall software is restarted. It should be noted
that although this command will disable fragmentation console output messages, standard
log messages (e.g., Long, Short, control messages, etc.) will continue to operate in their
Discovered and reported by Lance