Wednesday, July 30, 2008

The Impact of Dan’s DNS Debacle on Internet Risk

Blogger: Pete Lindstrom

On July 8th, Dan Kaminsky of IOActive announced a major DNS “vulnerability” in conjunction with a number of major DNS vendors. The announcement was off the charts in fanfare and attention, but what was the real impact on risk?

First, it is worth noting that this “bug” is more properly classified as a new attack technique invented by Dan. It combines two vulnerabilities that have been well-known for some time – the ability to guess non-random transaction IDs and the use of Additional RRs to insert new entries into the DNS cache. A fix against either of these vulnerabilities also negates the attack itself.

The fundamental question that determines the risk impact revolves around whether it is reasonable to expect fewer or more incidents that use this technique when comparing the period prior to disclosure -- or, more properly, before the date of Dan’s invention of the technique (this also assumes prior art) – with the period after invention/disclosure and into the future. If the disclosure reduces the number of those incidents, then risk is reduced; if the disclosure increases the number of those incidents, then risk is increased.

With that litmus test as our guideline, it is useful to break down the functional elements of risk and look at the impact on threats, vulnerabilities, and consequences (we will cover consequences, then vulnerabilities, and finally threat).

Though the consequences are the same before and after disclosure, it is worth discussing the impact here, given that the implication was that the “entire web” could be taken down. The nature of the attack requires the following:

  1. An attacker must convince/trick a user into making a DNS request for a domain that doesn’t already exist in their DNS server’s cache. The expectation here is that s/he can be easily tricked into doing this.
  2. Then, the attacker must simultaneously attack the DNS server by guessing the transaction ID. According to Kaminsky, the request/attack phase can be done reliably in about 10 seconds.
  3. The attack is DNS server-specific. Only users on the same DNS server are affected.
  4. Propagation: once the cache is poisoned, anyone requesting that domain will be routed to a malicious server.

Without combining this attack with other attack techniques, there can be three results:

  1. Spoofing of a single website for multiple, perhaps many, users using the same DNS server. Presumably, this would be followed by more traditional phishing and malware attacks.
  2. Denial-of-service by rerouting traffic from a legitimate site thereby taking potential customers or “eyeballs” away.
  3. Denial-of-service be rerouting traffic from a legitimate high volume site to a legitimate low-volume site thereby overloading the servers on the low-volume site.

Because of the point-to-point (user-to-website) nature of the attack, to do something that constitutes “taking over the entire web” is infeasible by a longshot.

The bottom line analysis for the effect on risk due to a change in consequences from pre-invention to post-invention: no change, and therefore no impact.

These vulnerabilities have existed for years, and there have been workarounds for years. Along with this announcement, new patches were introduced in all major DNS server solutions. It is reasonable to assume that many DNS server implementations have been patched, though public accounts have suggested that number is in the 66%-75% range.

Bottom line analysis: the vulnerability level has been reduced, probably significantly, and the affect is positive for risk reduction. If 100% of DNS servers were patched, then overall risk would be reduced for this attack (assuming that there were actual attacks using this technique in the past.)

The real question regarding risk impact comes in the arena of the less-controllable manipulation of threat. The general threat equation revolves around an attacker’s willingness to attack, based on his/her own cost/benefit analysis that compares the cost to attack to the expected benefits, tempered by the potential for being caught and penalized.

Cost to attack – prior to disclosing the invention, there were likely few, if any attackers with “prior art” that mirrored this technique. It is anybody’s guess how many potential attackers might have figured it out eventually, but they would have had to come from the pool of folks with enough expertise to do so – I am going to guess 500,000 people.

After the disclosure, the hints provided in the press release, the podcast, the sorted stories, and the blog entries made it much easier to figure out. Let’s guess that 5 million people could execute the attack. With automated tools, that number goes up to 50 million.

These numbers are estimates that illustrate the nature of the exercise. You are welcome to fill in your own estimates and come to your own conclusions.

Bottom line analysis: a significant increase in threat and corresponding risk.

Net Effect
The risk manager's challenge is to weigh the decrease in vulnerable systems compared with the corresponding increase in threat, within the context of number of incidents and anticipated future incidents. Given the sheer size differential, it is difficult to conceive of a situation where risk is not increased.

Sometimes it "feels" like someone is taking action for the greater good, when that action actually creates a negative impact for all. For example, it is common for people to believe that raising prices of scarce resources during times of trouble (e.g. gasoline in the hurricane Katrina aftermath) is unconscionable even though a majority of economists recognize that raising prices actually provides for the greater public good. Vulnerability discovery and disclosure, and attack inventions, might feel like the right thing to do, but the net result is almost always a negative impact.

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