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Google has fixed a vulnerability in their Gmail web based email service which would have allowed Internet attackers to steal mail messages from users without being noticed.

The attack works by forcing a logged-in user to add a mail filter to their Gmail account, thereby allowing their mail to be forwarded to an external mail address controlled by the attacker. Because the Gmail service did not adequately verify the origin of such requests, it was possible for attackers to create their own web pages that used JavaScript to automatically make such requests on behalf of their victims. In essence, a Gmail user would visit one of these pages and have their account compromised without necessarily realising anything is awry. Only close inspection of the Filters tab in the Gmail Settings menu would reveal what had happened.

Proof of concept exploits used JavaScript to make a silent POST request to the Gmail service and add the attacker’s filter. With the results of the request hidden in an iframe, it is highly unlikely that a victim will have noticed that their Gmail account would have been compromised, particularly while they are browsing a completely different website. While this attack scenario would only be successful if the victim was logged in, many Gmail users remain constantly logged in throughout the day, thus increasing the likelihood of a successful attack.