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Date: Mon, 27 Feb 2012 11:55:30 +0100
From: Sebastian Krahmer <krahmer@...e.de>
To: oss-security@...ts.openwall.com
Subject: Re: Attack on badly configured Netfilter-based firewalls
Hi,
I know that the 127.0.0.1 trick worked in past, but for loopback
addresses this isnt working anymore since quite a while.
You will get a 'martian destination', regardless of routing
or rp_filter's set. If we talk about a Linux kernel:
ip_route_input_slow()
{
[...]
if (ipv4_is_lbcast(daddr) || ipv4_is_zeronet(daddr) ||
ipv4_is_loopback(daddr))
goto martian_destination;
[...]
}
Or I am doing something seriously wrong. No idea what Solaris
or BSD's are doing.
For 'real' NIC's this trick is however still working, even if
the machine is a host (not a router). This leaves some room for
accessing internal admin interfaces from outside. :)
However, playing with source addresses to defeat firewalls should be
difficult, since most dists enable rp_filter.
my 2ct's
Sebastian
On Mon, Feb 27, 2012 at 01:53:29AM +0400, Solar Designer wrote:
> On Sun, Feb 26, 2012 at 10:05:55PM +0100, Eric Leblond wrote:
> > On Sun, 2012-02-26 at 12:17 -0700, Kurt Seifried wrote:
> > > 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.
>
> I guess by "internal clients" Kurt was referring to machines behind the
> firewall (e.g., someone clicking an URL that has a string looking like
> an FTP command embedded in it, thereby triggering the FTP helper to open
> a hole - stuff that was discussed in late 1990s and partially mitigated
> by hardening the helpers at the time), whereas by "attacker on a network
> directly connected to the firewall" Eric means that the attacker may be
> _outside_ the firewall (behind its WAN interface), but on the same
> network segment (e.g., the attacker might have compromised a nearby
> server, such as of another customer at a colocation facility).
>
> It is known that a machine will generally receive and process a packet
> routed to one of its NICs by MAC address even if the destination IP
> address is that of another NIC or even loopback (e.g., it is possible to
> access services bound to 127.0.0.1 in this way - but only from directly
> connected machines). Without rp_filter or equivalent, it is possible to
> have these packets' source addresses match the other NIC's network
> segment. My _guess_ (based solely on the info posted in here so far) is
> that the gist of Eric et al.'s new attack is to apply this approach
> against a protocol helper. The novelty is thus in combining these known
> things together to arrive at something that to the best of my knowledge
> has not yet been discussed.
>
> I suppose Eric will tell us if this is the correct guess or not. ;-)
>
> Alexander
--
~ perl self.pl
~ $_='print"\$_=\47$_\47;eval"';eval
~ krahmer@...e.de - SuSE Security Team
---
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