Domain Name Encryption Is Not Enough: Privacy Leakage via IP-based Website Fingerprinting
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Domain name encryptions (DoTH and ESNI) have been proposed to improve security and privacy while browsing the web. Although the security benefit is clear, the positive impact on user privacy is still questionable. Given that the mapping between domains and their hosting IPs can be easily obtained, the websites a user visits can still be inferred by a network-level observer based on the destination IPs of user connections. However, content delivery networks, DNS-based load balancing, co-hosting of different websites on the same server, and IP churn, all contribute towards making domain-IP mappings unstable, and prevent straightforward IP-based browsing tracking for the majority of websites. We show that this instability is not a roadblock for browsing tracking (assuming a universal DoTH and ESNI deployment), by introducing an IP-based fingerprinting technique that allows a network-level observer to identify the website a user visits with high accuracy, based solely on the IP address information obtained from the encrypted traffic. Our technique exploits the complex structure of most websites, which load resources from several domains besides their own primary domain. We extract the domains contacted while browsing 220K websites to construct domain-based fingerprints. Each domain-based fingerprint is then converted to an IP-based fingerprint by periodically performing DNS lookups. Using the generated fingerprints, we could successfully identify 91 IPs. We also evaluated the fingerprints' robustness over time, and demonstrate that they are still effective at identifying 70 two months. We conclude by discussing strategies for website owners and hosting providers to hinder IP-based website fingerprinting and maximize the privacy benefits offered by domain name encryption.
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