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A researcher today released a proof-of-concept for a vulnerability he discovered in Google Gmail that lets an attacker change a Gmail user's password, wage a denial-of-service attack on the account, or even access other Gmail users' email.

The cross-site request forgery (CSRF) flaw -- which researcher Vicente Aguilera Diaz from Madrid-based Internet Security Auditors first reported to Google in August 2007 -- takes advantage of the way Gmail's "Change Password" function operates. "The only token for authenticat[ing] the user is a session cookie, and this cookie is sent automatically by the browser in every request," according to the vulnerability disclosure post.

An attacker can build a phony Web page that accepts requests for Gmail password changes, and then lets the attacker change the victims' passwords without their knowing and evading CAPTCHA restrictions.

Google maintains that the flaw is not a major one because such an attack wouldn't be easy to pull off.

This isn't the first CSRF flaw reported in Gmail: In October 2007, the US CERT issued an alert about a CSRF bug that let attackers create mail filters and send mail to arbitrary email accounts. Google patched the bug.

"Cross-site request forgery merely transmits unauthorized commands from a user the Website trusts," said today's Internet Security Auditors' post. CSRF basically exploits the trust a Website site has for a user. An attacker can force the user's browser to request a page or action without the user knowing or the Website recognizing that the request didn't come from the actual, legitimate user.

A Google spokesperson said the company hasn't heard of any such attacks in the wild, but is looking at ways to mitigate abuse of the CSRF flaw. "We've been aware of this report for some time, and we do not consider this case to be a significant vulnerability, since a successful exploit would require correctly guessing a user's password within the period that the user is visiting a potential attacker's site," the spokesperson said. "Despite the very low chance of guessing a password in this way, we will explore ways to further mitigate the issue. We always encourage users to choose strong passwords, and we have an indicator to help them do this."

Internet Security Auditors first reported the vulnerability to Google's security team in August 2007.

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