A First Look at Related Website Sets

TL;DR

This work evaluates Google's Related Website Sets, a proposal to relax privacy protections between related sites by sharing unpartitioned storage. Through a user study (n=30) and an analysis of the RWS list governance, the authors show that users struggle to correctly identify relatedness, with substantial privacy-harming misclassifications (e.g., $36.8\%$ of related pairs misidentified) and frequent incorrect evaluations overall. Branding cues and domain-name signals drive user judgments, while automated similarity metrics ($e.g.$, Levenshtein distance with median ~$7$, HTML similarity median ~$0.04$) provide weak predictive power. Governance analysis reveals a growing but error-prone submission workflow (114 PRs; $58.8\%$ rejected; median 5 days to process successful PRs), underscoring the need for clearer UI signals and tooling. Overall, the findings suggest that relying on user-perceived relatedness as a privacy boundary can harm users, motivating explicit UX cues and further research into privacy-preserving alternatives that maintain compatibility.

Abstract

We present the first measurement of the user-effect and privacy impact of "Related Website Sets," a recent proposal to reduce browser privacy protections between two sites if those sites are related to each other. An assumption (both explicitly and implicitly) underpinning the Related Website Sets proposal is that users can accurately determine if two sites are related via the same entity. In this work, we probe this assumption via measurements and a user study of 30 participants, to assess the ability of Web users to determine if two sites are (according to the Related Website Sets feature) related to each other. We find that this is largely not the case. Our findings indicate that 42 (36.8%) of the user determinations in our study are incorrect in privacy-harming ways, where users think that sites are not related, but would be treated as related (and so due less privacy protections) by the Related Website Sets feature. Additionally, 22 (73.3%) of participants made at least one incorrect evaluation during the study. We also characterise the Related Website Sets list, its composition over time, and its governance.

Figures (9)

• Figure 1: Website relatedness survey results matrix; percentages and heatmap color are within Expected response.
• Figure 2: Website relatedness survey timing distributions; for pairs within the RWS (same set) category, split by response.
• Figure 3: CDFs of the Levenshtein edit distance between service/associated sites and their primary domain, per the Related Website Set list at 26 March 2024.
• Figure 4: CDFs of HTML similarity scores of set primaries and their service/associated sites, per the RWS list at 26 March 2024.
• Figure 5: Cumulative count of PRs that propose a new set, split by final state.