Marketing Companies Exploit a Flaw in Browsers’ Built-In Password Managers to Track Users

A group of researchers from Princeton’s Center for Information Technology Policy has discovered that at least two marketing companies, AdThink and OnAudience, that are exploiting an 11-year-old vulnerability in major browsers to track visitors.

The researchers discovered that the marketing firms have started exploiting the flaw in browsers’ built-in password managers that allow them to secretly steal email address. The gathered data allow them to target advertising across different browsers and devices.

password-manager tracking

Of course, the same flaw could be exploited by threat actors to steal saved login credential from browsers without requiring users interaction. Every browser (i.e. Google Chrome, Mozilla Firefox, Microsoft Edge, and Opera) implements a built-in password manager tool that allows users to save login information for automatic form-filling.

The researchers from Princeton’s Center for Information Technology Policy discovered that both AdThink and OnAudience are exploiting the built-in password managers to track visitors of around 1,110 of the Alexa top 1 million sites across the Internet.

“We found two scripts using this technique to extract email addresses from login managers on the websites which embed them. These addresses are then hashed and sent to one or more third-party servers. These scripts were present on 1110 of the Alexa top 1 million sites.” states the analysis of  the Princeton’s Center for Information Technology Policy.

The experts have found third-party tracking scripts on these websites that inject invisible login forms in the background of the webpage, the password managers are tricked into auto-filling the form using these data.

The scripts detect the username and send it to third-party servers after hashing with MD5, SHA1, and SHA256 algorithms, these hashed values are used as an identifier for a specific user. Typically tracker used the hashed email as user’s ID.

“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.” continue the researchers.

“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.”

browser password-manager tracking

 “Email addresses are unique and persistent, and thus the hash of an email address is an excellent tracking identifier,” the researchers said. “A user’s email address will almost never change—clearing cookies, using private browsing mode, or switching devices won’t prevent tracking.”

Third-party password managers like LastPass and 1Password are not exposed to this tracking technique because they avoid auto-filling invisible forms and anyway they require user interaction.

Users can test the tracking technique using a live demo page created by the researchers.

Below the list of sites embedding scripts that abuse login manager for tracking, it also includes the website of the founder of M5S Beppe Grillo (