------------------------------------------------ Explanation of the 'spank' attack -- a new breed stream/raped ------------------------------------------------ By: lst (yardley@uiuc.edu) This is a tad different than the previous release. Stream/Raped mearly flooded the host with ack's (or no flags) and came from random ips with random sequence numbers and/or ack numbers. The difference now is that this not only does the previous stuff, but also directly attacks from and to multicast addresses as well. Just as before, rate limiting should be done to counteract its effect (the same idea as ICMP_BANDLIM). The multicast handling should also be checked to verify that it is behaving properly. The attacker specifies the port[s] that they want to send the attack to, depending on what ports are selected, you will have different net results. If the port is an open port, then you will possibly have a longer kernel path to follow before the drop. Therefore, a smart attacker will hit open ports, but havoc can also come about from random ports due to states and processing. In the best case scenario, you will experience only the lag of the flood and the lag of the processing (currently) and then be fine when the attacker stops, In the worst case, you lockup, kill the network, and possibly have to reboot. Once you patch it, you deal with a lot less processing time (the drops are handled without the RST flag when appropriate--bandlim type idea). In other words, you go to the drop routine instead of dropwithrst silencing your response, which decreases your processing time, the hit on your network, and the effect of the flood (once a threshold is reached, all those bad packets are silently dropped and the attack has less of a net effect). The filters that were presented at the beginning of this email will block all multicast packets that come out (and in) the tcp stack I have been getting mailed a lot about this. Here is why I said the previous statement. Receiving a packet with no flags is considered an illegal packet (obviously) and is often dumped, however, as we have seen in the past, illegal packets often wreak havoc and often go untested. There is very little that "raped.c" or "stream.c" actually showed as problems in the TCP/IP stacks. The true problem lies more in the effects of the response (caused by the attack). This is the same concept as the SYN floods of yesteryear, and the same type of thing will be done to handle it. The main difference is that it will be on a simpler note because there isn't much need for a "cookie" based system. One should just throttle the response of the reset packets which in turn will help stop the storm that you generate and in general, harden the tcp/ip stack to behave the way it is supposed to. The main effect of this attack is that you are shooting back RST+ACK's at all the spoofed hosts. Obviously, a lot of these hosts will not exist and you will get ICMP unreaches (as an example) bounced back at you. There are other possibilities as well, but unreach would be the most common (redirects might be common as well although i did not spend the time to analyze that). The ones that don't respond back may send you some packets back as well (depending on if the port was valid or not and what their firewall rules are). This type of attack is complicated by the multicasts, and the effect is amplified as well. All in all, it becomes very nasty very quick. Basically, this causes a nice little storm of packets, in the ideal case. Note that I said ideal case in the previous paragraph. This is not always the observed behavior. It all depends on what is on the subnet, what type of packets are recieved, what rules and filters you have setup, and even the duration of the flood. It has been pointed out several times that the machine will go back to normal once the attack is stopped, which is exactly why something like ICMP_BANDLIM will work. I have also been asked a lot about what this "bug" affects. I have seen it have effects on *BSD, Linux, Solaris, and Win* as far as OS's go. It has also seemed to affect some hubs, switches, routers, or gateways since entire subnets have "disappeared" briefly after the attack. The multicast attack seems to be more deadly to teh network than the previous attack and its affects get amplified and even carried over to the rest of the network (bypassing secluded network bounds). I don't have more specifics on the systems affected because of the difficulty in testing it (and keeping the network up) since I do not have local access to the networks that I tested on, and remote access gets real ugly real fast. Another possibility that has been suggested as to why some machines die is that the machine's route table is being blown up by the spoofed packets. Each spoofed packet has a different source address which means that a temporary route table entry is being created for each one. These entries take time to timeout. Use 'vmstat -m' and check the 'routetbl' field while the attack is going on. Route table entries can be controlled somewhat under freebsd with: [root@solid]::[~] sysctl -a | fgrep .rt net.inet.ip.rtexpire: 3600 net.inet.ip.rtminexpire: 10 net.inet.ip.rtmaxcache: 128 You can do the following, to help if the route table is at least part of the problem: sysctl -w net.inet.ip.rtexpire=2 sysctl -w net.inet.ip.rtminexpire=2 Things that will help: 1. Drop all multicast packets (ingress and egress) that are addressed to the tcp stack because multicasts are not valid for tcp. 2. Extend bandwidth limiting to include RST's, ACK's and anything else that you feel could affect the stability of the machine. 3. Don't look for listening sockets if the packet is not a syn I hope that this helps, or explains a little more at least. --------------------------------------------------- Temporary remedy --------------------------------------------------- If you use ipfilter, this MAY help you, but the issue is quite a bit different than the previous issue. -- start rule set -- block in quick proto tcp from any to any head 100 block in quick proto tcp from 224.0.0.0/28 to any group 100 pass in quick proto tcp from any to any flags S keep state group 100 pass out proto tcp from any to any flags S keep state pass in all -- end rule set -- optionally, a rule like the following could be inserted to handle outgoing packets (if they send from the firewall somehow) but you have bigger problems than the attack if that is the case. -- start additional rule -- block out proto tcp from any to 224.0.0.0/28 -- end additional rule -- That will help you "stop" the attack (actually it will just help minimize the affects), although it will still use some CPU though Note: If you use IPFW, there is no immediate way to solve this problem due to the fact that it is a stateless firewall. If you are getting attacked, then temporarily use ipfilter (or any other state based firewall) to stop it. Otherwise, wait for vendor patches or read more about the explanation for other possible workarounds. FreeBSD "unofficial patch" by Don Lewis: http://solid.ncsa.uiuc.edu/~liquid/patch/don_lewis_tcp.diff ----------------------- Conclusion ----------------------- This bug was found in testing. It seems a bit more lethal than the previous and should be addressed as such. Patches should be available now, but I do not follow all the platforms. -------------------- References -------------------- This was done independantly, although some of the analysis and reverse engineering of concept was done by other people. As a result, I would like to give credit where credit is due. The following people contributed in some way or another: Brett Glass <brett@lariat.org> Alfred Perlstein <bright@wintelcom.net> Warner Losh <imp@village.org> Darren Reed <avalon@coombs.anu.edu.au> Don Lewis <Don.Lewis@tsc.tdk.com> Also, I would like to send shouts out to w00w00 (http://www.w00w00.org) ------------------- Attached ------------------- These programs are for the sake of full disclosure, don't abuse them. Spank was written with libnet, so you will need to obtain that as well. You can find that at http://www.packetfactory.net/libnet For an "unofficial" patch: http://www.w00w00.org/files/spank/don_lewis_tcp.diff For spank.c: http://www.w00w00.org/files/spank/spank.c