Twitter hack opens popups

NEW YORK -- A new way to cause mischief quickly spread through short-messaging service Twitter on Tuesday morning before the site could fix the problem, as mysterious "tweets" of blocked-out text propagated themselves and caused pop-up windows to open.

Shortly before 9 a.m. Twitter said on its "safety" feed on the site that the attack had been shut down. It also said it does not believe that any user information was compromised, rather, the "vast majority" of the breaches were pranks or promotions.

The hack had been extra nefarious because the tweets activated without being clicked on -- it was enough for Web surfers to move their mouse cursors over them. But it only affected visitors to Twitter.com. Various third-party programs used to send and read tweets, such as Tweetdeck, were unaffected.

The popups could, though didn't necessarily, contain malicious code that could take over poorly protected computers. The White House's official Twitter feed -- followed by 1.8 million users -- was among those affected, though the offending message was quickly taken down.

Fittingly for Twitter, which limits messages to just 140 characters, the virus may have been among the shortest on record. According to security software maker F-Secure Corp., the shortest virus so far was just 22 characters long.

Twitter said in a blog post it was notified of the security breach at 5:54 a.m. Eastern time. The problem was caused by something called "cross-site scripting." This allowed users to run JavaScript programs on others' computers, turning tweets different colors or causing the pop-up boxes to appear. Some users, Twitter added, took things a step further and included code that got people's accounts to re-tweet the messages without their knowledge.

"It was like a massive snowball fight that got out of control," said Ray Dickenson, chief technology officer at computer security firm SafeCentral.

But while the effects of Tuesday's mischief were very visible -- such as the pop-ups -- and playful, Dickenson said that he was worried because JavaScript can quietly do more malicious things, like sending people to sites that can infect computers.

Security breaches had been common in Twitter's early days, but the company has since worked to beef up its vigilance and the problems have become less common. Tuesday's hack coincided with Twitter's ongoing rollout of a redesign of its website, which tries to streamline users' Twitter feeds and make it easier to see photos and videos directly on the site, without having to click on a link to YouTube or Flickr.

Twitter said it discovered and fixed this problem last month, and that a recent site update unrelated to the redesign was responsible for its return.