Peeking through the window: Fingerprinting Browser Extensions through Page-Visible Execution Traces and Interactions

Browser extensions are third-party add-ons that provide myriads of features to their users while browsing on the Web. Extensions often interact with the websites a user visits and perform various operations such as DOM-based manipulation, script injections, and so on. However, this also enables nefarious websites to track their visitors by fingerprinting extensions. Researchers in the past have shown that extensions are susceptible to fingerprinting based on the resources they include, the styles they deploy, or the DOMbased modifications they perform. Fortunately, the current extension ecosystem contains safeguards against many such known issues through appropriate defense mechanisms. We present the first study to investigate the fingerprinting characteristics of extension-injected code in pages' JavaScript namespace and through other observable side-effects like changed cookies. Doing so, we find that many extensions inject JavaScript that pollutes the applications' global namespace by registering variables. It also enables the attacker application to monitor the execution of the injected code by overwriting the JavaScript APIs and capturing execution traces through the stacktrace, the set of APIs invoked, etc. Further, extensions also store data on the client side and perform event-driven functionalities that aid in attribution. Through our tests, we find 2,747 Chrome and 572 Firefox extensions to be susceptible to fingerprinting. Unfortunately, none of the existing defense mechanisms prevent extensions from being fingerprinted through our proposed vectors. Therefore, we also suggest potential measures for developers and browser vendors to safeguard the extension ecosystem against such fingerprinting attempts. CCS CONCEPTS • Security and privacy → Web application security.