The flaw is a "medium-sized problem" with the Buzz for Mobile Web site, said Robert Hansen, CEO of SecTheory, who first reported the issue.
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This type of Web programming error, called a cross-site scripting flaw, lets the attacker put his own scripting code into Web pages that belong to trusted Web sites such as Google.com. It is a fairly common flaw but one that can have major consequences when exploited on widely used Web sites.
The attacker "can force you to say things you don't want to say, to follow people," he said. "Whatever Google Buzz allows you to do, it allows him to do to you."
Because attackers can use the flaw to put their content on the Google.com domain, they could also create phishing attacks against Google users, Hansen said.
"If they left this unpatched, it could be horrible for any user of the site," he said. "It could easily be used to convince people that they're typing something into a valid Google sign-on page when they're really not."
The bug was discovered by a hacker known as TrainReq, who e-mailed Hansen details of the flaw without explanation. TrainReq is best known for posting photos stolen from pop star Miley Cyrus' e-mail account to the Internet.
Reached Tuesday afternoon Pacific Time, a Google spokesman confirmed that the company was working to fix the issue and predicted that it would be finished within a few hours.
"We're aware of a vulnerability that could affect users of Google Buzz for mobile, and we are now pushing a fix," spokesman Jay Nancarrow said via e-mail. "We have no indication that the vulnerability is being actively abused."
Launched last week, Google Buzz was blasted by some for automatically publishing lists of users' Gmail contacts with little notification. The company is making some changes this week to help alleviate those concerns.
However, the security flaw underscores another important issue, said Hansen, a vocal Google critic. "Google really can't be trusted with sensitive information because they can't protect their own applications."