A worm in the new Twitter has affected thousands of users including celebrities like Sarah Brown. The bug only affects people using Twitter on the Twitter.com website, not those on third party apps like TweetDeck.
The bug makes users inadvertently post malicious links which open up new windows showing either porn or dubious software. The particularly dangerous bit of the bug is that you don’t even need to click on the links for the windows to be opened up, it’s enough to simply roll the mouse over them. The attacking software is known as OneMouseOver and redirects users to hardcore Japanese porn.
Advice is to use Twitter clients not the main webstite until the flaw is cleared.