Rusted Anchors: A National Client-Side View of Hidden Root CAs in the Web PKI Ecosystem

HTTPS secures communications in the web and heavily relies on the Web PKI for authentication. In the Web PKI, Certificate Authorities (CAs) are organizations that provide trust and issue digital certificates. Web clients rely on public root stores maintained by operating systems or browsers, with hundreds of audited CAs as trust anchors. However, as reported by security incidents, hidden root CAs beyond the public root programs have been imported into local root stores, which allows adversaries to gain trust from web clients. In this paper, we provide the first client-side, nation-wide view of hidden root CAs in the Web PKI ecosystem. Through cooperation with a leading browser vendor, we analyze certificate chains in web visits, together with their verification statuses, from volunteer users in 5 months. In total, over 1.17 million hidden root certificates are captured and they cause a profound impact from the angle of web clients and traffic. Further, we identify around 5 thousand organizations that hold hidden root certificates, including fake root CAs that impersonate large trusted ones. Finally, we highlight that the implementation of hidden root CAs and certificates is highly flawed, and issues such as weak keys and signature algorithms are prevalent. Our findings uncover that the ecosystem of hidden root CAs is massive and dynamic, and shed light on the landscape of Web PKI security. Finally, we call for immediate efforts from the community to review the integrity of local root stores.