Greetings all,
        Yesterday I was discussing one of my favorite topics with a
friend who works at Enterasys. We were discussing intrusion detection
systems and active response; the use of IDS sensors to detect attacks
and either make a policy change on a firewall or actively respond to
intrusions itself (through the use of TCP resets, ICMP port and network
unreachable's, etc). While discussing the benefits and drawbacks we
both feel come along with this technology, I mentioned a specific issue
I had with a sensor directly responding to detects, and he said it was
something that he had never thought of before. After poking around for
a while in the list archives, I can't find anywhere where it's
mentioned, even when discussing this particular topic. I find it hard
to believe that I'm the first one to think of this, because there are
much smarter people on this list than me, but I'm curious to know what
the community here thinks about this...
        Basically, it's possible for an attacker to calculate where an
IDS sits on an organization's network by looking at the TTL in the IP
header of the TCP reset or ICMP error message he receives in response to
an attack. For example, let's say we have the following network setup:

[Server]--[Router]--[Router]--[IDS]--[Firewall]--[Router]--[Router]--[At
tacker]

        The attacker is trying to break into the server and the sensor
has a signature that resets the connection when it sees the exploit he's
trying to use. When the attacker sends his exploit to the target
server, it doesn't work. Since this is a smart attacker, he grabs a
packet capture to find out exactly what's happening and sees that his
connection is being reset. He notices that in the resets the TTL in the
IP header is 252 compared to 250 for normal responses. Knowing now that
an IDS must be using active response to keep him from exploiting the
target server, he wants to find out where this sensor resides.
Referencing the source code of his favorite IDS (and mine - Snort 1.9.0
from http://www.snort.org/ (SHAMELESS PLUG)), he finds the following
bits of code in sp_respond.c:

libnet_build_ip(TCP_H, 0,
                libnet_get_prand(PRu16) /* IP ID */ ,
                0 /* fragmentation */ , 255 /* TTL */ , IPPROTO_TCP,
                0, 0, NULL, 0, tcp_pkt);

libnet_build_ip(ICMP_UNREACH_H, 0,
                libnet_get_prand(PRu16) /* IP ID */ ,
                0 /* fragmentation */ , 255 /* TTL */ , IPPROTO_ICMP,
                0, 0, NULL, 0, icmp_pkt);

        He sees that these bits of code build the IP header for the TCP
reset and ICMP unreachable messages that the IDS uses for active
response. Knowing from this code that the TTL is statically set to 255
and hence, that's what it was when the reset left the NIC of the IDS, he
can then easily trace the path backwards through each hop (assuming
there's no asymmetric routing happening) and determine on what segment
the sensor resides by using simple addition! This information is
invaluable to the attacker for future attacks against the network, and
he now knows where he should focus his attack if he wants to disable the
sensor itself.
        I posted a message about this on the Snort developers list quite
some time ago, which got a good discussion going, but we couldn't come
up with a good solution to this problem. I believe the best idea that
we can up with was to randomize the TTL, though if an attacker would see
a whole bunch of resets come back with TTL's that wildly jump around,
that would be a clue that the organization he is attacking is using
Snort... and telling an attacker what IDS you're using, is of course, a
bad thing. Another good idea was to let the user specify (in a
configuration file somewhere for those that don't build from source) a
TTL that they wanted to use... obviously you'd want to use some
off-the-wall number like 213 or so. The attacker wouldn't know what hop
to count back too because he wouldn't know what the TTL was originally
set too.
        Please note that I'm only using Snort as an example here because
it's the only IDS software that I have the source code for and could
easily pull an example from. I believe, but am not _sure_, that
probably all IDS software is affected by this specific issue. Of
course, this is just another good reason _not_ to use active response...
or if you must, just break the connection on the internal side. The
attacker could manipulate his TCP stack not to honor resets anyway.
Thoughts?

Thanks,
Abe

--
Abe L. Getchell
Security Engineer
abegetchellqx.net

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