Twitter was overrun with posts on Tuesday morning that used a programming flaw to play pranks, distribute porn and spread worms to unsuspecting users.
The problem appeared to be confined to Twitter’s old Web interface, and does not affect the new interface that Twitter is gradually rolling out or to mobile applications. Security experts said that a JavaScript command in the offending posts included a command, “onmouseover,” that caused messages to pop up and Web sites to open automatically when a mouse hovered over it. The script in some cases also caused a user to forward the offending link, spreading it virally to their followers and the rest of Twitter.