A Risk-Based Look at Fixing the Certificate Authority Problem

2011-08-31 by . 4 comments

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Bogus SSL certificates are not a new problem and incidents can be found going back over a decade. With a fake certificate for Google.com in the wild, they’re in the news again today. My last two posts have touched on the SSL topic and Moxie Marlinspike’s Convergence.io software is being mentioned in these news articles. At the same time, Dan Kaminski has been pushing for DNSSEC as a replacement for the SSL CA system. Last night, Moxie and Dan had it out 140 characters at a time on Twitter over whether DNSSEC for key distribution was wise.

I’m going to do two things with the discussion: I’m going to cover the discussion I said should be had about replacing the CA system, and I’m going to try and show a risk-based assessment to justify that.

The Risks of the Current System

Risk: Any valid CA chain can create a valid certificate. Currently, there are a number of root certificate authorities. My laptop lists 175 root certificates, and these include names like Comodo, DigiNotar, GoDaddy and Verisign. Any of those 175 root certificates can sign a trusted chain for any certificate and my system will accept it, even if it has previously seen a different unexpired certificate before and even if that certificate has been issued by a different CA. A greater exposure for this risk comes from intermediate certificates. These exist where the CA has signed a certificate for another party that has flags authorizing it to sign other keys and maintain a trusted path. While Google’s real root CA is Equifax, another CA signed a valid certificate that would be accepted by users.

Risk mitigation: DNSSEC. DNSSEC limits the exposure we see above. In the DNSSEC system, *.google.com can only be valid if the signature chain is the DNS root, then the “com.” domain. If Google’s SSL key is distributed through DNSSEC, then we know that the key can only be inappropriate if the DNS root or top-level domain was compromised or is mis-behaving. Just as we trust the properties of an SSL key to secure data, we trust it to maintain a chain, and from that we can say that there is no other risk of a spoofed certificate path if the only certificate innately trusted is the one belonging to the DNS root. Courtesy of offline domain signing, this also means that host a root server does not provide the opportunity to provide malicious responses even by modifying the zone files (they would become invalid).

Risks After Moving to DNSSEC

Risk: We distrust that chain. We presume from China’s actions that China has a direct interest in being able to track all information flowing over the Internet for its users. While the Chinese government has no control over the “com.” domain, they do control the “cn.” domain. Visiting google.cn. does not mean that one aims to let the Chinese government view their data. Many countries have enough resources to perform attacks of collusion where they can alter both the name resolution under their control and the data path.

Risk mitigation: Multiple views of a single site. Convergence.io is a tool that allows one to view the encryption information as seen from different sites all over the world. Without Convergence.io, an attacker needs to compromise the DNS chain and a point between you and your intended site. With Convergence.io, the bar is raised again so that the attacker must compromise both the DNS chain and the point between your target website and the rest of the world.

Weighing the Risks

The two threats we have identified require gaining a valid certificate and compromising the information path to the server. To be a successful attack, both must occur together. In the current system, the bar for getting a validly signed yet inappropriately issued certificate is too low. The bar for performing a Man-in-the-Middle attack on the client side is also relatively low, especially over public wireless networks. DNSSEC raises the certificate bar, and Convergence raises the network bar (your attack will be detected unless you compromise the server side).

That seems ideal for the risks we identified, so what are we weighing? Well, we have to weigh the complexity of Convergence. A new layer is being added, and at the moment it requires an active participation by the user. This isn’t weighing a risk so much as weighing how much we think we can benefit. We also must remember that Convergence doesn’t raise the bar for one who can perform a MITM attack that is between the target server and the whole world.

Weighing DNSSEC, Moxie drives the following risk home for it in key distribution: “DNSSEC provides reduced trust agility. We have to trust VeriSign forever.” I argue that we must already trust the DNS system to point us to the right place, however. If we distrust that system, be it for name resolution or for key distribution, the action is still the same — the actors must be replaced, or the resolution done outside of it. The counter Moxie’s statement in implementing DNSSEC for SSL key distribution is we’ve made the problem an entirely social one. We now know that if we see a fake certificate in the wild, it is because somebody in the authorized chain has lost control of their systems or acted maliciously. We’ve reduced the exposure to only include those in the path — a failure with the “us.” domain doesn’t affect the “com.” domain.

So we’re left with the social risk that we have a bad actor who is now clearly identified. Convergence.io can help to detect that issue even if it only enjoys limited deployment. We are presently ham-strung by the fact that any of a broad number of CAs and ever-broader number of delegates can be responsible for issuing “certified lies,” and that still needs to be reduced.

Making a Decision

Convergence.io is a bolt-on. It does not require anybody else to change except the user. It does add complexity that all users may not be comfortable with, and that may limit its utility. I see no additional exposure risk (in the realm of SSL discussion, not in the realm of somebody changing the source code illicitly) from using it. To that end, Moxie has released a great tool and I see no reason to not be a proponent of the concept.

Moxie still argues that DNSSEC has a potential downside. The two-part question is thus: is reducing the number of valid certification paths for a site to one an improvement? When we remove the risk of an unwanted certification of the site, is the new risk that we can’t drop a certifier a credible increase in risk? Because we must already trust the would-be DSNSEC certifier to direct us to the correct site, the technical differences in my mind are moot.

To put into a practical example: China as a country can already issue a “valid” certificate for Google. They can control any resolution of google.cn. Whether the control the sole key channel for google.cn. or any valid key channel for google.cn., you can’t reach the “true” server and certificate / key combination without trusting China. The solution for that, whether it be the IP address or the keypair is to hold them accountable or find another channel. DNSSEC does not present an additional risk in that regard. It also removes a lot of ugliness related to parsing X.509 certificates and codes (which includes such storied history as the “*” certificate that worked on anything before browser vendors hard-coded it out). Looking at the risks presented and arguments from both sides, I think it’s time to start putting secure channel keys in the DNS stream.

Filed under Authentication Crypto Risk

4 Comments

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  • Although DNSSec is the ideal direction, the time and cost of implementing this will leave this vulnerability out in the open for way too long.

    A more immediate solution to this problem could be simply listing the valid CA thumbprints in a corresponding TXT record per website. This thinking is very similar to how SPF records are used to prevent email spoofing. In this context, we’re preventing CA spoofing and leaving the burden/responsibility of security on the network admin.

    For example, if a browser queries “www.google.com” I get an address. Then if SSL is being used, the browser issues a TXT query for “www.google.com”. That response will include a list of permitted Intermediate CAs.

  • Please disregard that last comment. According to the links below, DNS vulnerabilities and CA vulnerabilities have overlapping threat models. DNSSec is essential for “risky configurations” which include Wifi configurations, SoHo routers, or for whenever there is an active intermediary.

    http://security.stackexchange.com/questions/6827/h

    http://security.stackexchange.com/questions/6824/c

  • roryalsop says:

    Hi Chris – yes there isn’t a single cure, unfortunately, so I think we will see a number of partial solutions implemented… It’s not clean, but the options do manage the different risk profiles.

  • I have to say that for the last few of hours i have been hooked by the amazing posts on this site. Keep up the good work.

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