Updated Micro-blogging site Twitter suffers from a potentially devastating vulnerability that forces logged-in users to post messages of an attacker's choice simply by clicking on a link. It could be used to spawn a self-replicating worm.

The XSS, or cross-site scripting, error was discovered by Secure Sciences Corp researchers Lance James and Eric Wastl, who have fashioned this link to demonstrate their finding. Clicking on it while logged in to Twitter causes users to immediately broadcast an innocuous message to all of their followers, as this dummy account shows.

More than 15 hours after this story was first published, the gaping hole remained. A Twitter representative has yet to return our email.

Of course, it would be just as easy to craft links that do considerably more damage. Tweets are limited to just 140 characters, making it almost mandatory to use shortened URLs that obscure their final destination. While it's possible to preview the link before visiting, many Twitter users have grown so accustomed to them they click on them directly.

"With a technology such as twitter, I could use it to infect massive amounts of twitter readers/users, say with malware or steal their accounts, etc.," James wrote to The Register by instant messenger. "Because it's a serious hack, I was being nice and put a disclaimer up, but it could (have) been as bad as the samy is my hero stuff + more."

A white-hat hacker, James was speaking hypothetically, of course.

James, who is the author of Phishing Exposed, was referring to the notorious Samy worm, the self-replicating XSS exploit that in 2005 added more than 1 million MySpace friends to a user named Samy. In the process, the social networking site was knocked out of commission.

As the user base of Twitter has skyrocketed, so too have attempts to exploit the site. Hackers have waged cat-and-mouse attacks on the site using so-called clickjacking exploits that, like the XSS vulnerability exposed by James and Wastl, forced logged-in users to tweet simply by clicking on an innocent-looking button. Twitter has been quick to patch the vulnerabilities, but the hackers have been known to launch new attacks that work around the countermeasures.

Now, it's clear Twitter has new potentially serious vulnerabilities to stay on top of. We'd expect this vulnerability to be fixed in fairly short order, but like the clickjacking tit-for-tat, we expect this isn't the last you'll hear of XSS attacks on the site. ®