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Re: Attack on badly configured Netfilter-based firewalls
From: Eric Leblond <eric () regit org>
Date: Tue, 28 Feb 2012 10:46:58 +0100

Hello,

On Mon, 2012-02-27 at 19:13 +0100, Florian Weimer wrote:
* Eric Leblond:

I've discovered a generic attack on firewall using Application Level
Gateway (like Netfilter or Checkpoint).

This is rediscovered every two to five years.  Here's mine
(from 2005, but it's been proposed before):

<http://www.enyo.de/fw/security/java-firewall/>

Secure use of iptables and connection tracking helpers:
http://home.regit.org/netfilter-en/secure-use-of-helpers/

I think your filters aren't effective against sandboxed Java code on
the client.

Interesting point. On this abnormal usage of helper, I fear more
"malicious" software. I'm even surprised that a wonderful network
evasion software like Shype is not using this technique.


I think there are other client-side sandboxes which allow de-facto
unrestricted access (with server cooperation).  Doesn't Flash require
just a policy file on the server to open up arbitrary ports?

You could exclude the magic Silverlight port range:

| One additional restriction on using the sockets classes is that the
| destination port range that a network application is allowed to
| connect to must be within the range of 4502-4534.

<http://msdn.microsoft.com/en-us/library/cc645032%28v=vs.95%29.aspx>

Regarding Netfilter, the FTP helper will only do the analysis if you
connect to port 21 (or to match params specified by CT target). There is
no need to limit traffic on other ports.
Thus, it seems the approach explained in Netfilter document is correct:
only activate the helpers to trusted server (if you can) or accept the
fact that abnormal usage can be done. Regarding this last point, I will
update the document to be more precise about the potential attack and
evasion.

Best regards,
-- 
Eric Leblond 
Blog: http://home.regit.org/

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