Some 250,000 user accounts have been hacked, says Twitter. The company launched an investigation (which is still ongoing) after identifying unauthorized access attempts to Twitter user data.
Information that the attackers may have had access to include usernames, email addresses, session tokens and passwords that are encrypted or salted (salted means these passwords likely have additional random text added, as an additional layer of security).
Bob Lord (Director of Information Security, Twitter): This attack was not the work of amateurs, and we do not believe it was an isolated incident. The attackers were extremely sophisticated, and we believe other companies and organizations have also been recently similarly attacked.
Twitter has sent out emails to the users whose accounts have been compromised. The microblogging site is also encouraging its users to disable Java on their browsers, and follow good password hygiene.