No attack necessary: the surprising dynamics of SSL trust relationships

Abstract

Much of the Internet's end-to-end security relies on the SSL/TLS protocol along with its underlying X.509 certificate infrastructure. However, the system remains quite brittle due to its liberal delegation of signing authority: a single compromised certification authority undermines trust globally. Several recent high-profile incidents have demonstrated this shortcoming convincingly. Over time, the security community has proposed a number of counter measures to increase the security of the certificate ecosystem; many of these efforts monitor for what they consider tell-tale signs of man-in-the-middle attacks. In this work we set out to understand to which degree benign changes to the certificate ecosystem share structural properties with attacks, based on a large-scale data set of more than 17 billion SSL sessions. We find that common intuition falls short in assessing the maliciousness of an unknown certificate, since their typical artifacts routinely occur in benign contexts as well. We also discuss what impact our observations have on proposals aiming to improve the security of the SSL ecosystem.