The Web's modular nature is responsible for its tremendous success as well as its chronic insecurity. The modularity allows to build websites by adding third-party as "lego-blocks" - making websites implicitly trust the embedded third-parties. The browsers impose only modest restrictions on third-party resource embedding, leaving users susceptible to be exploited by malicious third-parties that blatantly bypass these restrictions. My research aims to build privacy-enhancing tools that can be deployed in browsers to rein in third-parties.