Understanding Regional Filter Lists: Efficacy and Impact
Authors: Christian Böttger (Institute for Internet Security; Westphalian University of Applied Sciences), Nurullah Demir (Institute for Internet Security; Westphalian University of Applied Sciences), Jan Hörnemann (AWARE7 GmbH), Bhupendra Acharya (CISPA), Norbert Pohlmann (Institute for Internet Security; Westphalian University of Applied Sciences), Thorsten Holz (CISPA), Matteo Grosse-Kampmann (Rhine-Waal University of Applied Sciences and AWARE7 GmbH), Tobias Urban (Institute for Internet Security; Westphalian University of Applied Sciences)
Volume: 2025
Issue: 2
Pages: 309–325
DOI: https://doi.org/10.56553/popets-2025-0063
Abstract: Filter lists are used by various users, tools, and researchers to identify tracking technologies on the Web. These lists are created and maintained by dedicated communities. Aside from popular blocking lists (e.g., EasyList), the communities create region-specific blocklists that account for trackers and ads that are only common in these regions. The lists aim to keep the size of a general blocklist minimal while protecting users against region-specific trackers. In this paper, we perform a large-scale Web measurement study to understand how different region-specific filter lists (e.g., a blocklist specifically designed for French users) protect users when visiting websites. We define three privacy scenarios to understand when and how users benefit from these regional lists and what effect they have in practice. The results show that although the lists differ significantly, the number of rules they contain is unrelated to the number of blocked requests. We find that the lists' overall efficacy varies notably. Filter lists also do not meet the expectation that they increase user protection in the regions for which they were designed. Finally, we show that the majority of the rules on the lists were not used in our experiment and that only a fraction of the rules would provide comparable protection for users.
Keywords: user tracking, tracking protection, filter lists, web privacy, adblocker
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