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Re: Attack on badly configured Netfilter-based firewalls
From: Eric Leblond <eric () regit org>
Date: Sun, 26 Feb 2012 22:05:55 +0100


On Sun, 2012-02-26 at 12:17 -0700, Kurt Seifried wrote:
On 02/25/2012 11:37 AM, Eric Leblond wrote:

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

Impact: An attacker on a local network can open some pinholes in a
firewall which is not correctly protected.

Are there any helpers that can be abused to open holes in the firewall
externally, or is it only internal clients that can cause problems and
trigger the firewall to improperly allow network traffic in/out.

No, attacker has to be on a network directly connected to the firewall.

Fix: None, the issue has to be fixed in the firewall
configuration. Workaround: Apply a strict anti-spoofing policy for
IPv4 and IPv6 as described in the document "Secure use of iptables
and connection tracking helpers" This document was written after
private disclosure of the attack to the Netfilter's team.

Just to confirm: setting net.ipv4.conf.[IFNAME].rp_filter to 1 is
sufficient, it doesn't need to be set globally as well?

It is sufficient for IPv4 but the feature is lacking on IPv6.

This attack will be presented at Cansecwest, March 9th 2012.

I assume you won't be providing any specifics until this date?I can't
assign CVE's without more information so I guess we both just have to

This is a generic attack not related to any version of Netfilter. It
even works with badly configured Checkpoint firewall.

Eric Leblond 
Blog: http://home.regit.org/

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