CAA Records and Site Security

The little green lock—now being deprecated by some browsers—provides some level of comfort for many users when entering personal information on a web site. You probably know the little green lock means the traffic between the host and the site is encrypted, but you might not stop to ask the fundamental question of all cryptography: using what key? The quality of an encrypted connection is no better than the quality and source of the keys used to encrypt the data carried across the connection. If the key is compromised, then entire encrypted session is useless.

So where does the key pair come from to encrypt the session between a host and a server? The session key used for symmetric cryptography on each session is obtained using the public key of the server (thus through asymmetric cryptography). How is the public key of the server obtained by the host? Here is where things get interesting.

The older way of doing things was for a list of domains who were trusted to provide a public key for a particular server was carried in HTTP. The host would open a session with a server, which would then provide a list of domains where its public key could be found in the opening HTTP packets. The host would then find one of those hosts, and hence the server’s public key. From there, the host could create the correct nonce and other information to form a session key with the server. If you are quick on the security side, you might note a problem with this solution: if the HTTP session itself is somehow hijacked early in the setup process, a man-in-the-middle could substitute its own host list for the one the server provides. Once this substitution is done, the MITM could set up perfectly valid encrypted sessions with both the host and the server, funneling traffic between them. The MITM now has full access to the unencrypted data flowing through the session, even though the traffic is encrypted as it flows over the rest of the ‘net.

To solve this problem, a new method for finding the server’s public key was designed around 2010. In this method, the host requests the Certificate Authority Authorization (CAA) record from the server’s DNS server. This record lists the domains who are authorized to provide a public key, or certificate, for the servers within a domain. Thus, if you purchase your certificates from BigCertProvider, you would list BigCertProvider’s domain in your CAA. The host can then find the correct DNS record, and retrieve the correct certificate from the DNS system. This cuts out the possibility of a MITM attacking the HTTP session during the initial setup phases. If DNSSEC is deployed, the DNS records should also be secured, preventing MITM attacks from that angle, as well.

The paper under review today examines the deployment of CAA records in the wild, to determine how widely CAAs are deployed and used.

Scheitle, Quirin, Taejoong Chung, Jens Hiller, Oliver Gasser, Johannes Naab, Roland van Rijswijk-Deij, Oliver Hohlfeld, et al. 2018. “A First Look at Certification Authority Authorization (CAA).” SIGCOMM Comput. Commun. Rev. 48 (2): 10–23.

In this paper, a group of researchers put the CAA system to the test to see just how reliable the information is. In their first test, they attempted to request certificates that would cause the issuer to issue invalid certificates in some way; they found that many certificate providers will, in fact, issue such invalid certificates for various reasons. For instance, in one case, they discovered a defect in the provider’s software that allowed their automated system to issue invalid certificates.

In their second test, they examined the results of DNS queries to determine if DNS operators were supporting and returning CAA certificates. They discovered that very few certificate authorities deploy security controls on CAA lookups, leaving open the possibility of the lookups themselves being hijacked. Finally, they examine the deployment of CAA in the wild by web site operators. They found CAA is not widely deployed, with CAA records covering around 40,000 domains. DNSSEC and CAA deployment generally overlap, pointing to a small section of the global ‘net that is concerned about the security of their web sites.

Overall, the results of this study were not heartening for the overall security of the ‘net. While the HTTP based mechanism of discovering a server’s certificate is being deprecated, not many domains have started deploying the CAA infrastructure to replace it—in fact, only a small number of DNS providers support users entering their CAA certificate into their domain records.