SugarCoat: Programmatically Generating Privacy-Preserving, Web-Compatible Resource Replacements for Content Blocking

Content blocking systems today exempt thousands of privacy-harming scripts. They do this because blocking these scripts breaks the Web sites that rely on them. In this paper, we address this privacy/functionality trade-off with SugarCoat, a tool that allows filter list authors to automatically patch JavaScript scripts to restrict their access to sensitive data according to a custom privacy policy. We designed SugarCoat to generate resource replacements compatible with existing content blocking tools, including uBlock Origin and the Brave Browser, and evaluate our implementation by automatically replacing scripts exempted by the 6,000+ exception rules in the popular EasyList, EasyPrivacy, and uBlock Origin filter lists. Crawling a sample of pages from the Alexa 10k, we find that SugarCoat preserves the functionality of existing pages-our replacements result in Web-compatibility properties similar to exempting scripts-while providing privacy properties most similar to blocking those scripts. SugarCoat is intended for real-world practical deployment, to protect Web users from privacy harms current tools are unable to protect against. Our design choices emphasize compatibility with existing tools, policy flexibility, and extensibility. SugarCoat is open source and is being integrated into Brave's content blocking tools: an initial set of SugarCoat-generated resource replacements are already shipping to users in the Brave Browser.