RE: Active response... some thoughts.

["Ralph Los" <RLos () enteredge com>]

Gentlemen,

        I can't agree more.  I implement and support IDSes at some very
large companies and even some small ones, and TCP-Reset is not a widely
popular nor, IMHO, effective strategy.  First off, as the email mentions
below, the attacker can just simply hack his stack to ignore the
resets...hey, it's possible.  Also, TCP-Resets can create a storm of packets
between your attacker and your IDS, effectively decreasing the effectiveness
of the IDS you have.

        Picture this...you have an attacker who figures our you have an
IDS...woo hoo, right?  Well, the attacker then proceeds to think that it's
better to just wipe you off the 'net than to hack your box, less effort that
way.  How trivial would it be to write a script (for those that can code) to
continue to supply large-quantities of packets at the target host.  These
packets get intercepted by the IDS and it starts to send out huge quantities
of TCP-Resets.  The router in-between starts to see utilization go up, up,
up until you have a saturated circuit - and what's worse, you're partly to
blame.  I can't afford to have an instance where my clients call me to tell
me my IDS has participated in a DoS against their 'net.  For this reason I
stick with NetworkICE's (ISS, who?, heh) Guard product.  It's in-line, fast
and does the trick.  I'm not sure if you guys have used IntruVert's product
large-scale, but I'm working with them to do some testing...sounds like a
competitor to Guard.

        Anyway - the point sir, we well made, and well taken.  But I have to
say that in 75%+ of my managed networks, I don't care because I wouldn't
implement at TCP-Reset product anyway :)

Just my personal, very humble opinion
Ralph

-----Original Message-----
From: detmar.liesen () lds nrw de [mailto:detmar.liesen () lds nrw de] 
Sent: Tuesday, January 21, 2003 2:17 AM
To: abegetchell () qx net; focus-ids () securityfocus com
Subject: AW: Active response... some thoughts.


You already outlined the drawbacks very well.

As you said

* You give valuable information to the hacker
* The attacker could modify his IP-stack such that resets are being ignored

IMHO TCP-reset is a cool technology, but I would always prefer silent packet
dropping by using an inline-device for this purpose, e.g. snort-inline with
iptables or RealSecure Guard.

It's better to create a "blackhole" than flooding the attacker with
tcp-resets anyway.

Some other reasons:
* Generating tcp resets can decrease the performance of your IDS a great
deal, especially on fast links. Depending on the protocol in use you
probably have to reset lots and lots of resets (check out VNC as an
example). To be sure you must reset both client and server, which increases
the performance issues.
* As you outlined, tcp-resets can tell the attacker that your site is
running an IDS, whatever flavour shall be irrelevant right now. If the
attacker knows that your IDS is sending out resets he can use this
information in order to blind the IDS by generating lots and lots of fake
attacks to several hosts. Thus the attacker can decrease the performance of
the IDS, DoS your servers and create so much noise (both on your network and
your IDS) that you will no longer be able to determine what's the real
attack. At least it's getting much more complicated.

IMHO the drawbacks of tcp-reset exceed the pros by far.

Greetings,

Detmar Liesen


 -----Ursprüngliche Nachricht-----
Von:    Abe L. Getchell [mailto:abegetchell () qx net] 
Gesendet:       Donnerstag, 16. Januar 2003 19:37
An:     focus-ids () securityfocus com
Betreff:        Active response... some thoughts.

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
abegetchell () qx net