Mission Accomplished?: HTTPS Security After Diginotar

Driven by CA compromises and the risk of man-in-the-middle attacks, new security features have been added to TLS, HTTPS, and the web PKI over the past five years. These include Certificate Transparency (CT), for making the CA system auditable; HSTS and HPKP headers, to harden the HTTPS posture of a domain; the DNS-based extensions CAA and TLSA, for control over certificate issuance and pinning; and SCSV, for protocol downgrade protection. This paper presents the first large scale investigation of these improvements to the HTTPS ecosystem, explicitly accounting for their combined usage. In addition to collecting passive measurements at the Internet uplinks of large University networks on three continents, we perform the largest domain-based active Internet scan to date, covering 193M domains. Furthermore, we track the long-term deployment history of new TLS security features by leveraging passive observations dating back to 2012. We find that while deployment of new security features has picked up in general, only SCSV (49M domains) and CT (7M domains) have gained enough momentum to improve the overall security of HTTPS. Features with higher complexity, such as HPKP, are deployed scarcely and often incorrectly. Our empirical findings are placed in the context of risk, deployment effort, and benefit of these new technologies, and actionable steps for improvement are proposed. We cross-correlate use of features and find some techniques with significant correlation in deployment. We support reproducible research and publicly release data and code.