RE: Active response... some thoughts.

["Abe L. Getchell" <abegetchell () qx net>]

        ...and this would solve the problem, sort of.  For all practical
purposes you're entering a "packet race" when you use this technique.
You're trying to get the resets to either endpoint of the connection
before the real packet gets there, thus making sure sequence numbers are
correct so the machines at both ends don't know anything funny is going
on and reject the subsequent "real" packets like you're trying to make
them do.  When you only send resets to one end of the connection you're
technically cutting your chances of this succeeding in half, the way I
see it (which very well may be wrong).  The more resets you send out on
the wire as fast as you can, the better the chance of one of those
getting there before the real packet does and closing the connection
before any damage can be done.  This is why Marty spent a considerable
amount of time manipulating the FelxResp code in Snort a year or so ago
to setup precaching of IP and UDP/TCP headers used in the response
packets... the less time you spend generating these headers on the fly,
the more of them you can send out at a higher rate of speed.
        FWIW, we found that during the original Code Red outbreak, we
saw a 90% success rate when using FlexResp to snipe sessions from
infected hosts.  This, of course, means that 10% of all of those forged
reset packets didn't make it to either end of the connection in time...
this was before the days in Snort where the precaching I mentioned above
was implemented.  I haven't been lucky (?) enough to be in a situation
again where I've needed to test this ability in a high-volume production
environment.

Thanks,
Abe

--
Abe L. Getchell
Security Engineer
abegetchell () qx net


> -----Original Message-----
> From: Ron Gula [mailto:ronald.gula () verizon net] 
> Sent: Monday, January 20, 2003 11:01 PM
> To: focus-ids () securityfocus com
> Subject: Re: Active response... some thoughts.
>
>
> I have not looked at this problem in a while, but why not 
> just send your reset
> packets to the target host and not the attacker? That way 
> nothing comes
> from the NIDS to the attacker directly.
>
> Ron Gula, CTO
> Tenable Network Security
>
> At 01:37 PM 1/16/2003 -0500, Abe L. Getchell wrote:
> > 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]--[R
> outer]--[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