Privacy-Preserving Browser-Side Scripting With Bflow

Some web sites provide interactive extensions using browser scripts, often without inspecting the scripts to verify that they are benign and bug-free. Others handle users' confidential data and display it via the browser. Such new features contribute to the power of online services, but their combination would allow attackers to steal confidential data. This paper presents BFlow, a security system that uses information flow control to allow the combination while preventing attacks on data confidentiality.BFlow allows untrusted JavaScript to compute with, render, and store confidential data, while preventing leaks of that data. BFlow tracks confidential data as it flows within the browser, between scripts on a page and between scripts and web servers. Using these observations and assistance from participating web servers, BFlow prevents scripts that have seen confidential data from leaking it, all without disrupting the JavaScript communication techniques used in complex web pages. To achieve these ends, BFlow augments browsers with a new "protection zone" abstraction.We have implemented a BFlow browser reference monitor and server support. To evaluate BFlow's confidentiality protection and flexibility, we have built a BFlow-protected blog that supports Blogger's third party JavaScript extensions. BFlow is compatible with every legitimate Blogger extension that we have found, yet it prevents malicious extensions from leaking confidential data.