The Tor Project has released Tor Browser 10.0.18 to fix numerous bugs, including a vulnerability that allows sites to track users by fingerprinting the applications installed on their devices.
To track users, a tracking profile is created for a user by attempting to open various application URL handlers, such as zoommtg://, and checking if the browser launches a prompt, like the one for Zoom below..
If the application's prompt is displayed, it can be assumed that the application is installed on the device. By checking for numerous URL handlers, the vulnerability can create an ID based on the unique configuration of installed apps on the user's device.
This vulnerability is especially concerning for Tor users who use the browser to protect their identity and IP address from being logged with sites. As this vulnerability tracks users across browsers, it could allow web sites, and even law enforcement, to track a user's real IP address when they switch to a non-anonymizing browser, such as Google Chrome.