We show how a long-known vulnerability in browsers’ built-in password managers is abused by third-party scripts for tracking on more than a thousand sites. post 
All major browsers have built-in login managers that save and automatically fill in username and password data to make the login experience more seamless. The set of heuristics used to determine which login forms will be autofilled varies by browser, but the basic requirement is that a username and password field be available.
Login form autofilling in general doesn’t require user interaction; all of the major browsers will autofill the username (often an email address) immediately, regardless of the visibility of the form. Chrome doesn’t autofill the password field until the user clicks or touches anywhere on the page. Other browsers we tested don’t require user interaction to autofill password fields.
Thus, third-party javascript can retrieve the saved credentials by creating a form with the username and password fields, which will then be autofilled by the login manager.
It is clear that the Same-Origin Policy is a poor fit for trust relationships on the web today, and that other security defenses would help. But there is another dilemma for browser vendors: should they defend against this and other similar vulnerabilities, or view it as the publisher’s fault for embedding the third party at all?
Publishers can isolate login forms by putting them on a separate subdomain, which prevents autofill from working on non-login pages.
Users can install ad blockers or tracking protection extensions to prevent tracking by invasive third-party scripts.
The “writeonly form fields” idea can be a promising direction to secure login forms in general. post