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Un intento de Blog de Sebastián D. Criado – 09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0

URGENT: The cat is out of the bag. DNS Problem.

Posted by Sebastián Criado en Lunes, julio 21, 2008 23:55

Señores, llego el día. al parecer y según lo que indica Dan Kaminsky, el momento de actualizar los dns IS NOW

El titulo del post “13>0” parece hacer referencia al día de publicación (el 8 de Julio) y 13 días después, el 21 de Julio.
Además de esto, lo que dice el post, es bastante.

Patch.  Today.  Now. Yes, stay late.  Yes, forward to OpenDNS if you have to.  (They’re ready for your traffic.)  Thank you to the many of you who already have.

Y es que al parecer los datos sobre el error de los DNS ya están dando vueltas por lo que se espera un sploit en breve.

Así que como indica el mismo Dan, si es necesario, hagan forward a OpenDNS.

Todo parece provenir del hecho que se publicaron los detalles del ataque antes de lo que se hubiera querido gracias a que en el blog de Matasano se habían publicado los mismos ( A este momento la nota original del blog no está más disponible.

La nota era la siguiente:

Reliable DNS Forgery in 2008: Kaminsky’s Discovery
from Matasano Security by ecopeland

0. The cat is out of the bag. Yes, Halvar Flake figured out the flaw Dan Kaminsky will announce at Black Hat.

1. Pretend for the moment that you know only the basic function of DNS — that it translates WWW.VICTIM.COM into The code that does this is called a resolver. Each time the resolver contacts the DNS to translate names to addresses, it creates a packet called a query. The exchange of packets is called a transaction. Since the number of packets flying about on the internet requires scientific notation to express, you can imagine there has to be some way of not mixing them up.

Bob goes to to a deli, to get a sandwich. Bob walks up to the counter, takes a pointy ticket from a round red dispenser. The ticket has a number on it. This will be Bob’s unique identifier for his sandwich acquisition transaction. Note that the number will probably be used twice — once when he is called to the counter to place his order and again when he’s called back to get his sandwich. If you’re wondering, Bob likes ham on rye with no onions.

If you’ve got this, you have the concept of transaction IDs, which are numbers assigned to keep different transactions in order. Conveniently, the first sixteen bits of a DNS packet is just such a unique identifier. It’s called a query id (QID). And with the efficiency of the deli, the QID is used for multiple transactions.

2. Until very recently, there were two basic classes of DNS vulnerabilities. One of them involves mucking about with the QID in DNS packets and the other requires you to know the Deep Magic.First, QIDs.

Bob’s a resolver and Alice is a content DNS server. Bob asks Alice for the address of WWW.VICTIM.COM. The answer is Mallory would like the answer to be

It is a (now not) secret shame of mine that for a great deal of my career, creating and sending packets was, to me, Deep Magic. Then it became part of my job, and I learned that it is surprisingly trivial. So put aside the idea that forging IP packets is the hard part of poisoning DNS. If I’m Mallory and I’m attacking Bob, how can he distinguish my packets from Alice’s? Because I can’t see the QID in his request, and the QID in my response won’t match. The QID is the only thing protecting the DNS from Mallory (me).

QID attacks began in the olden days, when BIND simply incremented the QID with every query response. If you can remember 1995, here’s a workable DNS attack. Think fast: 9372 + 1. Did you get 9372, or even miss and get 9373? You win, Alice loses. Mallory sends a constant stream of DNS responses for WWW.VICTIM.COM. All are quietly discarded —- until Mallory gets Bob to query for WWW.VICTIM.COM. If Mallory’s response gets to your computer before the legitimate response arrives from your ISP’s name server, you will be redirected where Mallory tells you you’re going.

Obvious fix: you want the QID be randomly generated. Now Alice and Mallory are in a race. Alice sees Bob’s request and knows the QID. Mallory has to guess it. The first one to land a packet with the correct QID wins. Randomized QIDs give Alice a big advantage in this race.

But there’s a bunch more problems here:

* If you convince Bob to ask Alice the same question 1000 times all at once, and Bob uses a different QID for each packet, you made the race 1000 times easier for Mallory to win.

* If Bob uses a crappy random number generator, Mallory can get Bob to ask for names she controls, like WWW.EVIL.COM, and watch how the QIDs bounce around; eventually, she’ll break the RNG and be able to predict its outputs.

* 16 bits just isn’t big enough to provide real security at the traffic rates we deal with in 2008.

Your computer’s resolver is probably a stub. Which means it won’t really save the response. You don’t want it to. The stub asks a real DNS server, probably run by your ISP. That server doesn’t know everything. It can’t, and shouldn’t, because the whole idea of DNS is to compensate for the organic and shifting nature of internet naming and addressing. Frequently, that server has to go ask another, and so on. The cool kids call this “recursion”.

Responses carry another value, too, called a time to live (TTL). This number tells your name server how long to cache the answer. Why? Because they deal with zillions of queries. Whoever wins the race between Alice and Mallory, their answer gets cached. All subsequent responses will be dropped. All future requests for that same data, within the TTL, come from that answer. This is good for whoever wins the race. If Alice wins, it means Mallory can’t poison the cache for that name. If Mallory wins, the next 10,000 or so people that ask that cache where WWW.VICTIM.COM is go to

3. Then there’s that other set of DNS vulnerabilities. These require you to pay attention in class. They haven’t really been talked about since 1997. And they’re hard to find, because you have to understand how DNS works. In other words, you have to be completely crazy. Lazlo Hollyfeld crazy. I’m speaking of course of RRset poisoning.

DNS has a complicated architecture. Not only that, but not all name servers run the same code. So not all of them implement DNS in exactly the same way. And not only that, but not all name servers are configured properly.

I just described a QID attack that poisons the name server’s cache. This attack requires speed, agility and luck, because if the “real” answer happens to arrive before your spoofed one, you’re locked out. Fortunately for those of you that have a time machine, some versions of DNS provide you with another way to poison the name server’s cache anyway. To explain it, I will have to explain more about the format of a DNS packet.

DNS packets are variable in length and consist of a header, some flags and resource records (RRs). RRs are where the goods ride around. There are up to three sets of RRs in a DNS packet, along with the original query. These are:

* Answer RR’s, which contain the answer to whatever question you asked (such as the A record that says WWW.VICTIM.COM is

* Authority RR’s, which tell resolvers which name servers to refer to to get the complete answer for a question

* Additional RR’s, sometimes called “glue”, which contain any additional information needed to make the response effective.

A word about the Additional RR’s. Think about an NS record, like the one that COM’s name server uses to tell us that, to find out where WWW.VICTIM.COM is, you have to ask NS1.VICTIM.COM. That’s good to know, but it’s not going to help you unless you know where to find NS1.VICTIM.COM. Names are not addresses. This is a chicken and egg problem. The answer is, you provide both the NS record pointing VICTIM.COM to NS1.VICTIM.COM, and the A record pointing NS1.VICTIM.COM to

Now, let’s party like it’s 1995.

Download the source code for a DNS implementation and hack it up such that every time it sends out a response, it also sends out a little bit of evil — an extra Additional RR with bad information. Then let’s set up an evil server with it, and register it as EVIL.COM. Now get a bunch of web pages up with IMG tags pointing to names hosted at that server.

Bob innocently loads up a page with the malicious tags which coerces his browser resolve that name. Bob asks Alice to resolve that name. Here comes recursion: eventually the query arrives at our evil server. Which sends back a response with an unexpected (evil) Additional RR.

If Alice’s cache honors the unexpected record, it’s 1995 —- buy CSCO! —- and you just poisoned their cache. Worse, it will replace the “real” data already in the cache with the fake data. You asked where WWW.EVIL.COM was (or rather, the image tags did). But Alice also “found out” where WWW.VICTIM.COM was: Every resolver that points to that name server will now gladly forward you to the website of the beast.

4. It’s not 1995. It’s 2008. There are fixes for the attacks I have described.

Fix 1: The QID race is fixed with random IDs, and by using a strong random number generator and being careful with the state you keep for queries. 16 bit query IDs are still too short, which fills us with dread. There are hacks to get around this. For instance, DJBDNS randomizes the source port on requests as well, and thus won’t honor responses unless they come from someone who guesses the ~16 bit source port. This brings us close to 32 bits, which is much harder to guess.

Fix 2: The RR set poisoning attack is fixed by bailiwick checking, which is a quirky way of saying that resolvers simply remember that if they’re asking where WWW.VICTIM.COM is, they’re not interested in caching a new address for WWW.GOOGLE.COM in the same transaction.

Remember how these fixes work. They’re very important.

And so we arrive at the present day.

5. Let’s try again to convince Bob that WWW.VICTIM.COM is

This time though, instead of getting Bob to look up WWW.VICTIM.COM and then beating Alice in the race, or getting Bob to look up WWW.EVIL.COM and slipping strychnine into his ham sandwich, we’re going to be clever (sneaky).

Get Bob to look up AAAAA.VICTIM.COM. Race Alice. Alice’s answer is NXDOMAIN, because there’s no such name as AAAAA.VICTIM.COM. Mallory has an answer. We’ll come back to it. Alice has an advantage in the race, and so she likely beats Mallory. NXDOMAIN for AAAAA.VICTIM.COM.

Alice’s advantage is not insurmountable. Mallory repeats with AAAAB.VICTIM.COM. Then AAAAC.VICTIM.COM. And so on. Sometime, perhaps around CXOPQ.VICTIM.COM, Mallory wins! Bob believes CXOPQ.VICTIM.COM is!

Poisoning CXOPQ.VICTIM.COM is not super valuable to Mallory. But Mallory has another trick up her sleeve. Because her response didn’t just say CXOPQ.VICTIM.COM was It also contained Additional RRs pointing WWW.VICTIM.COM to Those records are in-bailiwick: Bob is in fact interested in VICTIM.COM for this query. Mallory has combined attack #1 with attack #2, defeating fix #1 and fix #2. Mallory can conduct this attack in less than 10 seconds on a fast Internet link.

Y aquí viene un problema, al día de hoy, existen varios servidores DNS de proveedores de internet en argentina que no han actualizado y estos son los resultados:

Ciudad Internet:

Your name server, at, appears vulnerable to DNS Cache Poisoning.

All requests came from the following source port: 57639

Do not be concerned at this time. IT administrators have only recently been apprised of this issue, and should have some time to safely evaluate and deploy a fix.

Requests seen for TXID=13136 TXID=63683 TXID=50088 TXID=11407 TXID=28809

Your name server, at, appears vulnerable to DNS Cache Poisoning.

All requests came from the following source port: 4585

Do not be concerned at this time. IT administrators have only recently been apprised of this issue, and should have some time to safely evaluate and deploy a fix.

Requests seen for TXID=25657 TXID=28474 TXID=4147 TXID=24751 TXID=65215


Your name server, at, appears vulnerable to DNS Cache Poisoning.

All requests came from the following source port: 4005

Do not be concerned at this time. IT administrators have only recently been apprised of this issue, and should have some time to safely evaluate and deploy a fix.

Requests seen for TXID=54829 TXID=15379 TXID=28658 TXID=33537 TXID=59331


Your name server, at, appears vulnerable to DNS Cache Poisoning.

All requests came from the following source port: 32770

Do not be concerned at this time. IT administrators have only recently been apprised of this issue, and should have some time to safely evaluate and deploy a fix.

Requests seen for TXID=9551 TXID=40133 TXID=24597 TXID=43028 TXID=32818

Hagan la prueba con su proveedor 😉

Una respuesta to “URGENT: The cat is out of the bag. DNS Problem.”

  1. […] o símplemente un hacker, te doy toda la información publicada al respecto, date un paseo por Criado Indomable y me lo […]


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