Security researchers have uncovered how marketing companies have started exploiting an 11-year-old bug in browsers' built-in password managers, which allow them to secretly steal your email address for targeted advertising across different browsers and devices.

The major concern is that the same loophole could allow malicious actors to steal your saved usernames and passwords from browsers without requiring your interaction.

Every modern browser—Google Chrome, Mozilla Firefox, Opera or Microsoft Edge—today comes with a built-in easy-to-use password manager tool that allows you to save your login information for automatic form-filling.

These browser-based password managers are designed for convenience, as they automatically detect login form on a webpage and fill-in the saved credentials accordingly.

However, a team of researchers from Princeton's Center for Information Technology Policy has discovered that at least two marketing companies, AdThink and OnAudience, are actively exploiting such built-in password managers to track visitors of around 1,110 of the Alexa top 1 million sites across the Internet.

Third-party tracking scripts found by researchers on these websites inject invisible login forms in the background of the webpage, tricking browser-based password managers into auto-filling the form using the saved user's information.

"Login form auto filling 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," the researchers say.

"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."

Since these scripts are primarily designed for user-tracking, they detect the username and send it to third-party servers after hashing with MD5, SHA1 and SHA256 algorithms, which could then be used as a persistent ID for a specific user to track him/her from page to page.

"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."

Although the researchers have spotted marketing firms scooping up your usernames using such tracking scripts, there is no technical measure to prevent these scripts from collecting your passwords the same way.

However, most third-party password managers, like LastPass and 1Password, are not prone to this attack, since they avoid auto-filling invisible forms and require user interaction as well.

Researchers have also created a demo page, where you can test if your browser's password manager also leaks your username and password to invisible forms.

The simplest way to prevent such attacks is to disable the autofill function on your browser.