Nobody yet knows how 5 million of dollars in bitcoins were stolen from bitstamp (reported on 5 Jan 2015).  One expert report by Ferrin can be found here, however it is not clear if anybody knows at all HOW these bitcoins have been stolen. The bitcoin address of the thief is also known.
Possibly bitcoin could have been stolen using one of advanced attacks described in our recent paper. However it could have been stolen by another more basic technique. Many bitcoin systems and also many bitcoin cryptographic schemes and standards and protocols are notorious for shoddy security.