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TCP Vulnerability Published
Bob Slidell writes "According to Yahoo!, there is a critical flaw in TCP that affects everyone and everything. The article is scant on details and long on fear, hopefully someone will post more details on this." The advisory has more information, and is long on details but only moderate on fear.
This just hit the misc@openbsd mail list:It sounds (again) like proactive security auditting saves the day!
Just unplug your PC from the internet and wash your hands of it.. the whole thing feels holier than swiss cheese :(
This kind man responsible for finding this vulnerability is going to present this exploit at the security conference in Vancouver this Thursday. He then predicts "hackers will understand how to begin launching attacks 'within five minutes of walking out of that meeting.'" The article talks about how the government has been "fortifying" its networks against this, does that means they quickly rewrote the tcp protocol? I would love to know.
...will turn out to be nothi* [Carrier Lost]
let's all just start again
...
TCP2
SMTP2
POP32
http://www.osvdb.org/displayvuln.php?osvdb_id=4030
...
TCP Reset Spoofing
OSVDB ID: 4030
Rating: TBD
Disclosure Date: Apr 20, 2004
Description:
The TCP stack implementation of numerous vendors contains a flaw that may allow a remote denial of service. The issue is triggered when spoofed TCP Reset packets are received by the targeted TCP stack, and will result in loss of availability for the the attacked TCP services.
I'm removing support for TCP right now. Give me UDP or give me death!
Looks like someone left ISEXPLOITABLEFLAG = TRUE in the code.
The Blaster Master Fighting for Truth, Justice, and Evil Pie since 1979
I'll just switch to UDP.
-- Repeat with me: "There is no right to profits".
Move along, little to see here.
John.
As a web designer, taking advantage of this could get me off work faster than a snow storm. I don't know if I'm afraid or enthused. ;)
to switch over to IPX
Yep, that's the issue. I submitted too, but :(.
Anyway, the way I read it you basically run the TCP attack against a BGP peering router, causing it to drop one or more of it's peering relationships. Do that enough and you can cause the routes being advertised by that router (and also TO that router from the peering connections you're breaking) to be 'dampened' - a protective mechanism in BGP to prevent a flapping route from making all the peers recalculate their routes nonstop.
It's kind of like one peer putting the other one's routes in "time-out" until he plays nice.
you can exploit slow start too. was published last year for some conference. The paper is called "The Shrew vs. the Mice and the Elephants" and is by Aleksandar Kuzmanovic and Edward W. Knightly
I write code.
Your computer is broadcasting an IP address!
Seriously though, it doesn't look all that bad. (Nor does it look all that hard to do, but still..)
www.gotontheinter.net
Updated vaguely once a whenever, maybe once a whenever-and-a-half.
Winston Zeddemore: Tell him about the Twinky.
-G
Service providers on the other hands, must protect their routers because the BGP protocol used to distribute Internet routes between them, massively uses TCP. And when routes go missing, it is hundreds if not thousands of routes to your favourite places that go unreacheable.
The problem in the case of BGP is made worse by dampening, i.e. keeping the flapping routes out of the routing table for a certain amount of time (up to several hours). BGP routes dampening is not always configured. A determined attacker with this knowlege would be able to knock large portions of the Internet offline for hours.
I happen to work for a large, nationwide backbone. We've known about this for about a week now. BGP, configured without an MD5 key (as is usually the case) is extremely vulnerable to this exploit. This is the reason for the top-secret effort in the past week to MD5 all peering sessions, both internal and external on most major networks worldwide. Without this, it's trivial to exploit, in fact we already have source code provided by the NCISS. Input a few IPs and BGP's TCP port number, and wham you take down a peering session. For those that don't understand what this means, prior to the security changes that have been implemented in the last week, the global internet was largely susceptible to this flaw in such a way that major portions could have been taken offline easily. A priority was put on this within the intra-NOC communications channels that exist that has never been seen before to lock this down before the public knew about it. We were embargoed by DHS to not release the information until tomorrow.
Funny, but it seems that empherial source ports for a TCP connection may be more secure in this case, since it increases the space that the attacker has to guess within.
Of course it is a pure "D'oh" that large TCP windows increase exposure to the older known weakness of TCP RST attacks (Steve Bellovin, wrote a paper on it in 1989).
3.2.1.4. TCP RST/FIN/FIN-ACK
Well, that means our home PCs aren't likely to get exploited by this. However, if our ISP's router gets exploited, we're knocked offline and our PC isn't as useful as it used to be.
The threat here is a DOS aimed at a few things that we don't want to see go down.
... since TFA goes on to say the vulnerability explicitly affects "long-lived" TCP connections, not the POP3 / HTTP / SMTP that Joe User relies on. However, for businesses and security wonks this is a potentially big deal.
i'm posting this over NetBEUI Protocol ;)
*sight*
For more information about what TCP resets are and why they can be harmful, see RFC3360.
Please correct me if I got my facts wrong.
The article is being presented at CanSecWest, and is called "Slipping in the Window" by Paul A. Watson. I have two friends at CanSecWest, I've asked them to attend and report back what the feeling is.
NANOG members are talking about it, and several regional Tier-1 players have already issued customer notifications.
This exploit goes up against TCP connections that have been established for long periods of time. i.e. not web connections. The most prevalent would be BGP peer connections, which can be up for days on end easily. Without having read details, or the paper itself, by forging packets of BGP peers with adjusted window sizes, you can cause a router to reset (possibly hang, depending on IOS or JunoOS version, not sure about this) it's BGP peer connection. If you were doing eBGP and had your own AS, a directed attack against your gateway routers could force flapping, which would cause route dampening, and lead to denial of service.
What you need to do, is contact your ISP if you are an enterprise network admin, and establish MD5 authentication on your BGP sessions. Check with Cisco or Juniper and find out if your code will drop non-MD5 BGP packets directed at it. An ACL won't do, the attacker would forge the src-ip of a known peer.
This is a completely non-trivial attack to coordinate. You need to know the IP address of the BGP peer of a customer, or the route reflector, and then get the IP address right in an attempt to bypass ACLs and get the BGP session to hang. eBGP multihop means that IP could be any number of routers, and unless you have inside info, you don't know what it is.
Potentially, looking glasses could be used to mount attacks at NAPs or other peering points, but again, I think the major players will be ready for it very shortly, and will spend most of today (if they are any good) coordinating with legal teams to slam the shit out of any forged sessions they see, and start cooperating to run traces with other providers.
If I could editorialize one moment, none of this would be an issue if providers took better care to implement anti-spoofing techniques. Forged src-ip addresses are the bane of security. Most of these attacks don't care about 2-way communcations, they just want to reset connections. Spoofed src-ip lets them do that. Rant off.
Is that you master?
L. Skywalker
Real quote.
God Bless America. Why? Did it sneeze?
In a quickly following press release, Bill Gates adds:
Wow. That uninterrupted block of text hit so hard it set off my browser's airbag!
Slashdot still doesnâ(TM)t support Unicode after it was added to the HTML standard in 1997.
I am a lonely man living on the Galapagos Island. I use TCP/IP over carrier pigeon to communicate with a Nigerian who has promised my great wealth in exchange for securing funds in the First Galapagos Bank, of which I am owner/ceo/clerk, and janitor.
/obscure humor (Does this make me a Galapagos Spammer?)
I suspect someone is interupting my data stream and keeping the replies and account numbers he has been sending me in regards to my money. This vulnerability proves my theory. I am in desperate need!! How can I prevent this!!
Anyone willing to help I will share my wealth with.
The NISCC articles explains things clearly. Nobody needs any more information. Besides, this "new" attack was already known. People just didn't realize how easy it is.
Here's a link to US-CERT's TA04-111A on this topic.
What is new here is that you don't have to know the completely specified sequence number, but only something close enough to fit within a window. This reduces the search space and makes the attack more pragmatic.
As they state, there is a simple solution: TCP MD5 Signature Option with BGP. Any ISP worth their salt will already be doing it. The rest will learn the hard way.
.
This has been supported in Cisco IOS way back since ~1998 in IOS 11.2
Read the BGP "Bible": Internet Routing Architectures or look at any best-practices guides which will state that TCP MD5 sigs should always be used with BGP.
Or search CCO:
router bgp 109
neighbor 145.2.2.2 password v61ne0qkel33&
It's just a single line that has to be added to both peer sides.
The issue described in this advisory is the practicability of resetting an established TCP connection by sending suitable TCP packets with the RST (Reset) or SYN (Synchronise) flags set.
The packets need to have source and destination IP addresses that match the established connection as well as the same source and destination TCP ports.
The fact that TCP sessions can be reset by sending suitable RST and SYN packets is a design feature of TCP according to RFC 793, but a reset attack is only possible at all because the source IP address and TCP port can be forged or "spoofed".
Although denial of service using crafted TCP packets is a well known weakness of TCP, until recently it was believed that a successful denial of service attack was not achievable in practice. The reason for this is that the receiving TCP implementation checks the sequence number of the RST or SYN packet, which is a 32 bit number, giving a probability of 1/232 of guessing the sequence number correctly (assuming a random distribution).
The discoverer of the practicability of the RST attack was Paul A. Watson, who describes his research in his paper "Slipping In The Window: TCP Reset Attacks", presented at the CanSecWest 2004 conference. He noticed that the probability of guessing an acceptable sequence number is much higher than 1/232 because the receiving TCP implementation will accept any sequence number in a certain range (or "window") of the expected sequence number. The window makes TCP reset attacks practicable.
Any application protocol which relies on long term TCP connections and for which the source and destination IP addresses and TCP ports are known or can be easily guessed will be vulnerable to at least denial of service attacks
Saying Java is nice because it works on all OS's is like saying that anal sex is nice because it works on all genders.
It is indeed a problem with TCP but, it is in no way new. Many people have relied on this "vulnerabillity" for years. This is the technique that many firewalls implement in order to terminate undesirable sessions.
This vulnerabillity impacts the applications that are unable to handle session interruptions. BGP is such an appilication but, its impact on BGP is critical because this attack could seriously impeed BGP and since the internet backbone relies on BGP such an attack could covceivably shut down the internet. This is a known vulnerabillity in BGP, as you said, and it is surprising to me that BGP has not been modified to prevent this from happening.
By and large most applications will be able to deal with this and only minor annoyances such as DoS will occur.
As for earlier posts about OpenBSD being immune, I am very eager to hear how that is possible. No matter what the implementation, it is still possible to reset TCP sessions. This means that OpenBSD is still potentially vulnerable to DoS attacks.
Provided that BGP is modified to make it more fault tolerant, I can't see this problem with TCP being anything more than a short-lived niusance. I'm sure however that many people will try to use this as leverage to force IPv6 migration however, I believe that it too is vulnerable.
I read the paper, which is nothing more than a rehash of what every security person already knows about: tcp sequence guessing.
.02c
This issue erupted on the freebsd irc chat, and I had to kill it by posting this linkage: http://lcamtuf.coredump.cx/newtcp/
As you can see TCP sequncing is fairly random in most of the IP/TCP stacks out there in commercial use. The reasearch paper mentioned in the article only applies to the OS's in the above link that dont' have near true 32bit randomness.
The feasability of overcoming realtime 32 sequence guessing is insane, however non-zero. just my
It isn't a lie if you belive it.
There is a new Internet draft addressing this issue.
A recently published I-D (here) claims 200 seconds is sufficient time for a broadband sub to successfully attack a TCP session, provided their ISP doesn't use egress filtering (and way too few do so).
This is rather serious. Whether you personally aren't susceptible is irrelevant if your upstreams are.
Note to netadmins and sysadmins: block packets with source IPs that are not valid for the interface they came from! Geesh!
In Aug 1998, RFC 2385 came out with protection of BGP with MD5 signatures. Using MD5 sigs will defeat this attack.
This is a well known issue with well known solutions. If the infrastructure is at risk it is because ISPs haven't been doing their job and following best practices.
-weld
There is a new vulnerability that will cause every GM vehicle and cause your children to cry. Vandals can place 1 domestic house cat into the fan and cause the fan to stop and under some cases, cause the vehicle to overheat. This was previously written off as house cats are usually soft ans squishy and have little effect on the powerful fan but Joe Shmoe PHD realised that many house cats have colars that are pretty tough for the fan to digest. Car experts say this is a serious problem and will be dealt with in a serious manner. Suggested work around is to keep your cat tied in the house, and to drive a bicycle instead.
Saying Java is nice because it works on all OS's is like saying that anal sex is nice because it works on all genders.
Maybe I missed something in the advisory, but this sounds like a good old TCP reset attack...
You did.
The news here is that you no longer require the sniffer, because you don't have to know the exact sequence number. Now you just have to be kind of close. How close apparently varies in size amongst vendor implementations.
I think the point is that it lets you perform this attack without being in the middle with a sniffer. The author figured out a way to significantly reduce the search space for the sequence number. Quoting from the article, "[Watson] noticed that the probability of guessing an acceptable sequence number is much higher than 1/2^32 because the receiving TCP implementation will accept any sequence number in a certain range (or 'window') of the expected sequence number. The window makes TCP reset attacks practicable."
Previously you had to have a packet sniffer in the middle to sniff the sequence and spoof it. Watson has pointed out a technique that allows you to guess the sequence in practical number of tries for connections that are long-lived (a few days).
Note: that's 1/2^32 (substitute your own favorite exponential syntax), not one in two hundred thirty-two.
Oh, say does that Star-Spangled Banner entwine / The myrtle of Venus with Bacchus's vine?
This came up on the linux-kernel mailing list two years ago during a thread on TCP MD5 stuff to avoid this very problem. Why is it just now making a splash?
This post from Alan Cox covers it nicely. Also see the rest of the thread.
Besides the fact that their little kitty bones could get into the works and actually stop the fan.
I'd say this is a real threat. We need to protect our SUV's from the mobs of 1337 haxor kitten terrorists! I propose bombing __insert country here__, under the guise of giving them democracy and freedom, and simultaniously pass some laws at home which take away some of our freedom.
Huh?
Guessing a port and IP is fairly easy. Guessing the sequence number is not. This is why making sure that TCP initial sequence numbers are random is important.
This is old news.
For the insanely paranoid use a hardware entropy generator (TRNG) for choosing ISN's.
There are all sorts of attacks against network protocols when poor random number sources are used. This is but one example...
Title: Juniper NetScreen Advisory 58784
t ao/co ntact.html
Date: 21 April 2004
Version: 1
Impact:
A design flaw in the RFC specification of TCP may allow a blind attacker to successfully close a TCP connection.
Affected Products:
Juniper NetScreen Firewalls (all versions)
NISCC Reference: Vul/236929
http://www.uniras.gov.uk/vuls/2004/236929/tcp.htm
Max Risk: Medium
Summary:
A blind attacker with limited knowledge of a TCP connection may be able to successfully brute force the TCP Sequence number space and thereby cause a connection endpoint or firewall stateful filter to process a spoofed RST packet and close the connection.
Details:
The TCP Sequence number is one of the mechanisms that TCP uses to prevent a third party from inserting forged packets into the data-stream between two other hosts. While such an attack has always been known to be theoretically possible against TCP, it is was believed that the range of over four billion possible TCP Sequence numbers was large enough to prevent a successful attack. However, recently such an attack has been proven to be feasible in certain situations.
Specifically, if two hosts are known to talk to each other on a regular basis and/or for long periods of time over known port(s) then it may be possible for an attacker to brute force the TCP Sequence number space and successfully inject a forged packet into the connection and possibly disrupt communications. Certain connections, such as BGP4, which are long lived between two devices are especially vulnerable.
While all TCP connections are potentially vulnerable, NetScreen believes that NetScreen firewalls running BGP4 or with TCP Syn-Check enabled are likely to be vulnerable in practice. Other protocols such as SSH, HTTP and SMTP which usually have shorter connection times are less vulnerable.
Recommended Actions:
NetScreen firewall customers should do one or more of the following:
1) Configure Anti-Spoof protection as appropriate.
2) Use secure protocols such as ssh, HTTPS, BGP4 w/ MD5 Authentication
and IPSec which are more resistant to attack.
3) Limit management to dedicated and/or internal interfaces
4) Upgrade to ScreenOS 5.0r6 which enhances the stateful firewall
functionality to protect devices on the network.
Patch Availability:
ScreenOS version 5.0r6 is currently available for Juniper NetScreen firewalls.
How to get ScreenOS:
Customers with a valid product warranty or a support contract may download the software from the Juniper NetScreen CSO web portal:
http://www.juniper.net/support/
For all other customers, including those with expired support contracts, please call your regional Juniper NetScreen TAC center at one of the numbers listed in:
http://www.juniper.net/support/nscn_support/
Select option 2 from the telephone menu and be sure to select the correct product from the phone tree. Once connected with an engineer state that you are calling in regards to a Security Advisory and provide the title of this notice as evidence of your entitlement to the specified release.
As with any new software installation, NetScreen customers planning to upgrade to any version of ScreenOS should carefully read the release notes and other relevant documentation before beginning any upgrade.
Wrong. Very wrong. Are you anaware of this field called "banking"? What about "financial trading"? These firms have huge portfolios and run and re-run various models on them. At night, the systems have to run various "end-of-day" scripts and reports, which take CPU-hours to complete. Most of this things run on Unix...
There is also on-the-fly image manipulation, and the scene-rendering done by fleets of Unix machines. The more CPUs each such Unix machine can fit, the better.
Then come databases -- depending on the queries (with joins and orderings), DB-servers can be CPU bound and appreciate multiple processors when available.
What about PVM? What was it -- and similar packages both free and commercial -- written for? What about this proverbial "beowulf clusters"? Of course, it is much better to have several CPUs inside the box, rather than in separate machines.
Until which time, the OpenBSD zealots will continue to deny the issue exists or is of any importance. I see...
In Soviet Washington the swamp drains you.
If you read the article it'll explain it for you, but basically:
Routers from different AS's talk to each other using BGP4. This protocol uses TCP.
The "exploit" uses spoofed packets, so the router will process them as if it came from their neighbour.
So yes, while the router is mostly "shoveling packets", it learns the direction in which to shovel these packets via the exploitable method.
If you want to disagree, feel free to let all the major tier 1 ISPs who have been busily implementing MD5 authentication on their BGP sessions know their wrong.
Suicide terrorist kitties?
Al-Kitty?
Yes, that was corny, and no, I couldn't resist.
vi ~/.emacs
Actually, this is just a variation on an old attack with sequence number guessing. Some OSes have a very poor generator for these numbers (even though the vulnerability is known for ages) and it is possible to exploit it. Probably the most famous attack of this sort was Mitnick's break-in into Shimomura's machine ten years ago.
In practice, these attacks are quite tricky to perform, because they require good timing and quite a bit of skills, so they are quite rare. It is far easier to just exploit some common buffer overflow than this.
The impact could be not only DOS (by resetting the connection), but you can do also packet injection with potential for total system compromise. However, if the protocol used is encrypted (in Shimomura's case it wasn't, if I am not mistaken, Mitnick attacked rsh or rlogin), then the DOS is probably the worst thing that could happen to you. The encryption will reject the bogus packets, if properly implemented.
So, keep calm - this was here since at least 80-ties and there isn't much that you can do against it, except to use encryption. You can only hope, that your system vendor will decide to make their TCP stack less vulnerable (not likely to happen, since many systems still ship with the crappy sequence number generator!). Full fix is not possible anyway, unless you want to break the protocol.
Regards, JanThis is sort of on topic with your post. Here's a discussion and visualizations of various OS's TCP sequence numbers:
Strange Attractors and TCP/IP Sequence Number Analysis
Your hybrid is not saving the environment. Its purpose is to make you feel good about buying something.
There is RFC 2385 titled Protection of BGP Sessions via the TCP MD5 Signature Option. In the first paragraph of its Introduction section it says -
The primary motivation for this option is to allow BGP to protect itself against the introduction of spoofed TCP segments into the connection stream. Of particular concern are TCP resets.
The date of publishing - August 1998, which makes it about 6 years old.
However the proposed option was primarily meant as a quick BGP fixup, which should've got depricated as soon as IPsec got RFC'ed and deployed. It did went standard few months later, but IPsec implementations enjoyed full share of cross-vendor compatibility problems.
With time Authenticated Header (AH) IPsec protocol didn't see much use or acceptance and IPsec slowly evolved (and keeps evolving) into confidentiality rather than authentication layer.
Besides it's an IP security after all, while the RST spoofing is a TCP problem. And one can quite rightfully percieve authenticating TCP with IPsec as an overkill.
Anyhow, long story short - it's a known and rather old problem with handful of existing and equally unattractive solutions. Perhaps this time around it will be addressed better due to the amount of the publicity it's aimed to get.
3.243F6A8885A308D313
From the advisory:
We told you not to deploy NAT because (among other reasons) it would break IPsec authenticated header (AH) mode. You did it anyway and told us we were pedantic academic pinheads.
You deserve what you get.
--
jhw
In practice, these attacks are quite tricky to perform, because they require good timing and quite a bit of skills
Well, they require a packet with the right sequence number to hit in the right time period.
Since there's a window of accepted sequence numbers, it really only requires a shitload of packets with likely numbers. Send enough good guesses and one will hit at the right time.
Like a race exploit, I don't think this requires 'good timing', I think it requires enough attempts to reduce the odds - many will fail, but one may succeed.
Code or be coded.
Yep, sure, however, if you flood the network with your packets, it will be probably noticed soon. Also in order to flood the network, you need a fast connection to the attacked host, reducing the problem to the immediate neighborhood of the attacked host. That's why this attack is not very practical to perform and the impact on most normal mortals is low (except for the BGP issue, which could take out your upstream router)
The problem with BGP, which is mentioned in the original article, is that it has a particularly bad failure mode - when a BGP session goes down, it is very expensive to restart it. To top it off, BGP uses very long transmission windows (because it transfers lot of data and that is more efficient with longer windows) and that makes it easier to squeeze-in the spoofed packet. This is quite messy affair, when it happens.
However, if somebody attacks your favorite web site in this way, I doubt that you are even going to notice it, since http sessions are stateless and comparably cheap to establish.
So, I think this is just a big scare for nothing, it is mainly BGP issue for now. It affects just people who know how to fix it (and are fixing it right now) and the machines involved are relatively few. Unlike some Windows RPC exploit which unleashed a massive world-wide worm in few minutes.
Time flies like an arrow, fruit flies like a banana.
"Al-Kitty?"
You're not mangling your Arabic-to-English transilteration enough. It would probably look more like "al Qiddy"
Routers don't usually do a lot of TCP themselves, except SSH or telnet access for management, but the one big exception is the BGP protocol that routers use to connect to each other, primarily at the interfaces between carriers. The BGP sessions stay up for a long time, and killing them tells the router that it's no longer talking to its neighbor so it should go find a different route to get to that network, which is really really annoying. On the other hand, BGP has options to do more checking on BGP messages before accepting them, and carriers that do spoof-proofing to prevent their customers from forging packets have less risk, and carriers that block packets addressed to their routers accept from approved destinations are much safer.
Some customer connections to ISPs use BGP, especially if the customer uses multiple ISPs, though most are static - these could be subject to spoofing by somebody inside the customer's network, if they're not careful. (The customer could be an ISP, or they could be a regular customer with an employee who has next month's interesting virus.) So customers who don't want to get thrown off the net should probably be careful to do good firewalls to keep their own users from spoofing their connections.
Bill Stewart
New Fast-Compression-only CPR http://preview.tinyurl.com/dy575ks
And putting Al-Kitty through said fan will result in Al-Gore.
The following pseudo command will send an exploit packet:
linverse@KOS-MOS:~/blackhat/hping2-linv erse$ hping2 [victim.ipaddy] --spoof
[server.ipaddy] --rst --baseport [server.port] --destport [rand()%65536] --setseq
[rand()%((2^32)-1)] --sign "TCP RST Attack"
Each packet sent in this manor has a 1/2^32 chance of succeeding. You need to guess two (effectively) 16 bit numbers: the victim's unknown open port, and the tcp sequence number (within ~16 of its 32 bits, depending on the windowsize) This requires something on the order of 4 billion packets to have a reasonable expectation of closing the connection. There is really no way of knowing if or when a given connection is closed, unless you're taking down routers with it, or some othe observable scenario.
I modified hping2 to spam a victim with these packets. Attempting to reset a local connection with a remote machine takes approximately 20 hours to send the requisite ~4 billion packets. Were these packets to actually travel over a network, it should slow things down significantly.
If one had an army of zombies, then obviously the 20 hour figure can become a 20 minute, or even 20 second figure. But flooding a computer with 4 billion packets in 20 seconds will likely be at least as destructive as the actual payload, except perhaps in the case of major routers communicating over BGP.
Interestingly enough, however, one can still cause massive damage without flooding a single router. Instead, have each of your thousands of zombies try to take down arbitrary routers (perhaps each one sends packets randomly to each known major router). This algorithm allows one to make huge amounts of guesses without saturating the connection to any given router.
If my calculations are correct, then 10000 zombies armed with my exploit and a list of major routers could take them down at a rate of one per 7.2 seconds. At this point, we're talking about serious problems.
Has anybody else done any field testing / analysis?