Measuring and Applying Invalid SSL Certificates: The Silent Majority

SSL and TLS are used to secure the most commonly used Internet protocols. As a result, the ecosystem of SSL certificates has been thoroughly studied, leading to a broad understanding of the strengths and weaknesses of the certificates accepted by most web browsers. Prior work has naturally focused almost exclusively on "valid" certificates-those that standard browsers accept as well-formed and trusted-and has largely disregarded certificates that are otherwise "invalid." Surprisingly, however, this leaves the majority of certificates unexamined: we find that, on average, 65% of SSL certificates advertised in each IPv4 scan that we examine are actually invalid. In this paper, we demonstrate that despite their invalidity, much can be understood from these certificates. Specifically, we show why the web's SSL ecosystem is populated by so many invalid certificates, where they originate from, and how they impact security. Using a dataset of over 80M certificates, we determine that most invalid certificates originate from a few types of end-user devices, and possess dramatically different properties than their valid counterparts. We find that many of these devices periodically reissue their (invalid) certificates, and develop new techniques that allow us to track these reissues across scans. We present evidence that this technique allows us to uniquely track over 6.7M devices. Taken together, our results open up a heretofore largely-ignored portion of the SSL ecosystem to further study.