Thursday, July 10, 2008

DNS Insecurity

Imagine an attack in which the hacker controls all your Internet traffic, and is able to redirect your web site requests away from your requested destination to a spoofed web site that they control.

This scenario is called a Man-In-The-Middle (MITM) attack, and is achieved when a hacker is successful in "poisoning" or modifying the Domain Name Server cache.

Once a DNS cache is poisoned, it enables intelligent interception and redirection of web site requests to be managed from a point remote from the client (and the destination.) DNS poisoning is, in many ways, a case study in online criminal efficiency.

Next month, as everyone in the security industry now knows, Dan Kaminsky is going to step up to the mic at Black Hat and talk about something everyone already knows is a big problem - DNS insecurity.

So what is Kaminsky going to tell us? The fact that an out-of-sequence patch was issued by Microsoft two nights ago (a patch that apparently kicked users of Zone Alarm firewalls off the Internet) explains where the problem probably lies.

The Register (which refers, accurately, to DNS insecurity as "the mad woman in the attic" and a "peripheral, forgotten issue") added some color today, unearthing a 2005 paper from Ian Green which makes for some interesting reading. Here's a peek at his paper:

" the infamous Mitnick vs Shimomura attack and other subsequent attacks have shown, many weaknesses in network protocols are a result of poor implementation rather than weaknesses in the underlying protocol. In the Mitnick attack, 'IP source address spoofing and TCP sequence number prediction were used to gain initial access'."

Hmmm. Can you can tell what is coming next? Three pages later, post a few hours of research, Green writes, of his target research (the XP DNS Resolver):

"The DNS transaction ID always begins at 1 and is incremented by 1 for each subsequent DNS query; and... the UDP source port of the query (which becomes the UDP destination port of the response) remains static for the entirety of a session (from startup to shutdown)."

In other words, Green has followed Mitnick's advice and found exactly what was predicted: stupid levels of predictability. The DNS transaction ID, which is allowed to be a random number 16 bits long, has been implemented in such a way it can be easily guessed ("n" + 1).

In his paper, Green faults Microsoft's flawed implementation of DNS in XP ("ten years after the Mitnick attack"). The Register article uses this as the basis of a theory about what Kaminsky is going to talk about - a theory that was bolstered by MSFT's out-of-sequence patch this week.

Anyway, let's assume that's right. That leaves us Internet users with a problem. Mitnick first paved the way 13 years ago. Green's paper, which was published by the SANS Institute, came out three years ago, in 2005.

If it turns out this is what Kaminsky is going to talk about, why is everyone assuming the problem will be taken care of quickly?

The truth is, it won't. Only a minority of vulnerable users will hear about this and download and install the patch - leaving lots of room for those folks looking to pull off the perfect Internet crime - the MITM, or Man In The Middle attack.

Note: It would not be proper for me to sign off without pointing out that a solution exists for XP users: Every single DNS request made inside Authentium SafeCentral is handed off to our secure DNS service.

This ensures that even users with totally compromised machines get to where they want to go, without experiencing a MITM attack.

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