IDS mailing list archives
Re: Active response... some thoughts.
From: Martin Roesch <roesch () sourcefire com>
Date: Sat, 25 Jan 2003 23:27:36 -0500
-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1Off the top of my head I can think of a quick and effective fix for this problem. Since we keep state on TCP connections with stream4 these days, it should be pretty trivial to record the TTL of packets transiting the segment that the NIDS is on and use that TTL for any response packets sent. This way it looks "natural" and we don't have to worry about TTL "fuzzing" caused by a PRNG.
If stream4 is turned off, then we're back to the PRNG route. Additionally, if you're worried about "missing" the TTL because you're resetting on the SYN packet, you need to remember that you shouldn't ever do an active response on the SYN packet with Snort! If you've got a rule that is meant to enforce policy via session kills you should never key on the SYN packet, if you "miss" on the first packet you don't get another chance. Traditionally we like to wait for the session to become established and then let the detection engine kill the connection:
alert tcp $HOME_NET $SOME_PORT -> any any (flags: A; resp: rst_all; ...)This way if the session kill misses on the first attempt, it'll keep trying until the session is dead.
If we were feeling really adventurous we could implement a persistent "kill this session tag" that lasts until the session is gone, that way you can start trying to kill the session on the SYN packet if desired without having to rely on continuous detects by the rules engine. This could be done pretty easily by extending the tagging mechanism or implementing a similar mechanism in a separate subsystem.
-Marty On Thursday, January 16, 2003, at 01:37 PM, Abe L. Getchell wrote:
- -- Martin Roesch - Founder/CTO Sourcefire Inc. - (410) 290-1616Greetings 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 IPheader of the TCP reset or ICMP error message he receives in response toan attack. For example, let's say we have the following network setup:[Server]--[Router]--[Router]--[IDS]--[Firewall]--[Router]--[Router]-- [Attacker] The attacker is trying to break into the server and the sensorhas a signature that resets the connection when it sees the exploit he'strying 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 hisconnection 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 thatan 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 255and hence, that's what it was when the reset left the NIC of the IDS, hecan 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, andhe now knows where he should focus his attack if he wants to disable thesensor 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 thatwe can up with was to randomize the TTL, though if an attacker would seea 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 someoff-the-wall number like 213 or so. The attacker wouldn't know what hopto 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. Ofcourse, 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
Sourcefire: Enterprise-class Intrusion detection built on Snort roesch () sourcefire com - http://www.sourcefire.com Snort: Open Source Network IDS - http://www.snort.org -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.1 (Darwin) iD8DBQE+M2O8qj0FAQQ3KOARAkG5AJ4ptaHSPCCgyWyIRavbpcKjAX125gCdE5S4 lhaIFET7TwSp/pZbrufKBlQ= =DU/S -----END PGP SIGNATURE-----
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)
- RE: Active response... some thoughts. mb_lima (Jan 28)
- Re: Active response... some thoughts. Paul Palmer (Jan 31)