Biblio

Modern web browsers have accrued an incredibly broad set of features since being invented for hypermedia dissemination in 1990. Many of these features benefit users by enabling new types of web applications. However, some features also bring risk to users' privacy and security, whether through implementation error, unexpected composition, or unintended use. Currently there is no general methodology for weighing these costs and benefits. Restricting access to only the features which are necessary for delivering desired functionality on a given website would allow users to enforce the principle of lease privilege on use of the myriad APIs present in the modern web browser. However, security benefits gained by increasing restrictions must be balanced against the risk of breaking existing websites. This work addresses this problem with a methodology for weighing the costs and benefits of giving websites default access to each browser feature. We model the benefit as the number of websites that require the feature for some user-visible benefit, and the cost as the number of CVEs, lines of code, and academic attacks related to the functionality. We then apply this methodology to 74 Web API standards implemented in modern browsers. We find that allowing websites default access to large parts of the Web API poses significant security and privacy risks, with little corresponding benefit. We also introduce a configurable browser extension that allows users to selectively restrict access to low-benefit, high-risk features on a per site basis. We evaluated our extension with two hardened browser configurations, and found that blocking 15 of the 74 standards avoids 52.0% of code paths related to previous CVEs, and 50.0% of implementation code identified by our metric, without affecting the functionality of 94.7% of measured websites.

Modern web browsers are incredibly complex, with millions of lines of code and over one thousand JavaScript functions and properties available to website authors. This work investigates how these browser features are used on the modern, open web. We find that JavaScript features differ wildly in popularity, with over 50% of provided features never used on the web's 10,000 most popular sites according to Alexa We also look at how popular ad and tracking blockers change the features used by sites, and identify a set of approximately 10% of features that are disproportionately blocked (prevented from executing by these extensions at least 90% of the time they are used). We additionally find that in the presence of these blockers, over 83% of available features are executed on less than 1% of the most popular 10,000 websites. We further measure other aspects of browser feature usage on the web, including how many features websites use, how the length of time a browser feature has been in the browser relates to its usage on the web, and how many security vulnerabilities have been associated with related browser features.