bugtraq - Re: Checkpoint SYN DoS Vulnerability

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Date: Tue, 16 May 2006 16:36:09 -0400
From: "sanjay naik" <sanjaynaik@...mail.com>
To: cbrenton@...isbrenton.org, sanjaynaik@...e.org
Cc: bugtraq@...urityfocus.com
Subject: Re: Checkpoint SYN DoS Vulnerability

Hi Chris,

I have tested this with a complete TCPdump on the checkpoint side and 
Tethereal on the scanner side. The scan used is NMAP TCP Connect scan, which 
actually does the full 3-way handshake. So, according to your response, the 
scan should have always succeeded.
The scan does succeed sometimes and at other times we get bogus information 
from the firewall. If this was a feature, it should have consistently 
provided bogus information.

SYNDefender is disabled on the firewalls. Also, the firewall performance 
starts degrading as we start getting these bogus results. The State Table 
definely gets affected due to this scan which is really a valid permitted 
scan with proper rules in place for the scanner. I have seen issues with ACK 
scans and invalid SYN scans, but this is a valid TCP connect scan that we 
are trying.

Nokia's response is that even if SYNDefender is disabld, it still works in 
the background! Authorized Scanning is not allowed by Checkpoint firewall as 
that is a product limitation.

Regards,
Sanjay Naik

----Original Message Follows----
From: Chris Brenton <cbrenton@...isbrenton.org>
Reply-To: cbrenton@...isbrenton.org
To: sanjaynaik@...e.org
CC: bugtraq@...urityfocus.com
Subject: Re: Checkpoint SYN DoS Vulnerability
Date: Tue, 16 May 2006 16:14:09 -0400

On Tue, 2006-05-16 at 11:09 -0400, sanjay naik wrote:
 >
 > When a scan is intiated from the Inside interface of Checkpoint firewall,
 > the firewall responds with bogus information intermittently.

Sounds like you are triggering the SYN flood protection. Typically the
firewall will respond with a SYN/ACK to ensure the source is not just
generating a SYN flood. If you close the handshake, the connection is
passed through to the target host if it is permitted in the rules. If
not, the connection is simply deleted from the state table and ignored.

Not sure why you are calling this a DoS as it does not sound like
regular connectivity is being effected. The exception would be if you
generated enough bogus SYN packets to fill up the state table so legit
connections could not get through. I seem to remember Lance posting info
about that to this list 4-5 years ago.

 > In both cases, the scans results were inconsistent. Both SYN and ACK
 > scans had similar issues.

IMHO this is a feature. I would certainly rather see a port scanner
receiving bogus results rather than accurate info that would assist in a
compromise. Make them work a bit harder and earn it. ;-)

HTH,
Chris

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