On the Complexity of the Web's PKI: Evaluating Certificate Validation of Mobile Browsers

Digital certificates are frequently used to secure communications between users and web servers. Critical to the Web's PKI is the secure validation of digital certificates. Nonetheless, certificate validation itself is complex and error-prone. Moreover, it is also undermined by particular constraints of mobile browsers. However, these issues have long been overlooked. In this article, we undertook the first systematic and large-scale study of the certificate validation mechanism within popular mobile browsers to highlight the necessity of reassessing it among all released browsers. To this end, we first compile a comprehensive test suite to identify security flaws in certificate validation from various aspects. By designing and implementing a generic, automated testing pipeline, we effectively evaluate 30 popular browsers on two mobile OS versions and compare them with five representative desktop browsers. We found the latest mobile browsers Accept as many as 33.2% invalid certificates and Reject merely 5.4% invalid ones on average, leaving the majority of them to be decided by users who usually have little expertise. Our findings shed light on the severity and inconsistency of certificate validation flaws across mobile browsers, which are likely to expose users to MITM attacks, spoofing attacks, and so forth.