IDS mailing list archives
Active response... some thoughts.
From: "Abe L. Getchell" <abegetchell () qx net>
Date: Thu, 16 Jan 2003 13:37:11 -0500
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
Current thread:
- Active response... some thoughts. Abe L. Getchell (Jan 20)
- RE: Active response... some thoughts. Abe L. Getchell (Jan 23)
- Re: Active response... some thoughts. Martin Roesch (Jan 26)
- <Possible follow-ups>
- RE: Active response... some thoughts. Abe L. Getchell (Jan 26)
- RE: Active response... some thoughts. Ralph Los (Jan 26)
- RE: Active response... some thoughts. Christopher Lyon (Jan 26)
- RE: Active response... some thoughts. Alan Shimel (Jan 26)
- RE: Active response... some thoughts. Kohlenberg, Toby (Jan 28)
- RE: Active response... some thoughts. Garbrecht, Frederick (Jan 28)
- Message not available
- Re: Active response... some thoughts. Stone Cold (Jan 31)
- Message not available
- RE: Active response... some thoughts. Kohlenberg, Toby (Jan 28)