There is a big thread on this on reddit for those who want more info:

https://www.reddit.com/r/BitcoinMarkets/comments/4vtv1m/bitfinex_down_due_to_bitcoin_security_breach/

P.S. The following thread from 11 months ago says they used defunct code from a website that had already been hacked:

https://www.reddit.com/r/Bitcoin/comments/3igv0r/bitfinex_pretty_sure_we_are_dealing_with_amateurs/

>Bitfinex was released within some weeks of the Bitcoinica source code leak.
>The codebase was copied from a defunct, thrice robbed company.
>That code is not capable of keeping the database consistent at peak loads.
>That code is not scalable. That code was not, and is not correct.
>Not correct according to specification. (Which doesn't exist.)