Kaminsky DNS Flaw + Domain Validated SSL = Phisher's Paradise
The recent news about a DNS flaw discovered by Dan Kaminsky has been highly publicized and the details of the flaw have even been released. The flaw effectively allows someone to redirect a vulnerable DNS server so that www.paypal.com may actually go the an attacker's IP address instead of PayPal's IP address. This would be complately masked to the user.
How does this affect SSL?
SSL Certificates are normally issued only to a domain name, not an IP address. If an attacker was able to get an SSL certificate for www.paypal.com, they could set up a phishing site with the SSL Certificate and a customer would see the lock icon and think that they were using PayPal's website. What would stop a phisher from getting an SSL Certificate for another company's domain name? It depends on the type of certificate.
SSL Certificate Types
Normal SSL certificates go through a validation process that not only verifies that the company that is getting the certificate owns the domain name but also verifies that they are a legally registered entity. An EV SSL Certificate, requires more detailed validation of the company and domain name as well as verification of the certificate requestor's authority.
Then there are domain validation SSL certificates (offered by companies like GoDaddy and GeoTrust/RapidSSL). These certificates go through a much less stringent verification process. Normally they only rely on a response to an email that is sent to the address in the WHOIS record or an administration address at the domain name.
The Problem With Domain Validated SSL Certificates
How can a phisher get a domain validated SSL certificate for www.paypal.com? Just use Kaminsky's DNS flaw to route www.paypal.com to their own mail server, respond to the verification email, and voila. Now use the same DNS flaw to route as many users as possbile to your version of www.paypal.com and start collecting login details. There are certainly difficulties with carrying out this process but the possibility is still there. This problem is avoided with EV SSL Certificates and (for the most-part) normal Organization validation SSL certificates because they also verify the company and the requestor's authority.
What does this mean for the end-user? It means end-users should verify that they are using patched DNS servers (like OpenDNS) and also be sure that they investigate browser error messages. For example, if someone signed their own certificate for www.paypal.com and used the DNS flaw to redirect a user to his phishing site, the user would see an error stating that the SSL certificate is not signed by a trusted authority.
Originally posted on Tue Aug 5, 2008