Zane from Bitfinex has confirmed the worst, USD 60,000,000 worth of BTC has been stolen from customer accounts.

This thief is of the same magnitude as the DAO hack & Mt. GOX. It would be a catastrophic loss for not just Bitfinex but it's many customers, whos savings are now gone. We are still awaiting official confirmation from Bitfinex at the time of writing.

To recap the Bitfinex announced that customers accounts were breached and that they were going offline. Traders on Twitter noticed a large sudden drop in Bitcoins held in multi-sig. The amount of drop equal roughly ~120,000 BTC which is very close to the number reported by Zane on Reddit.

If this holds true then it would mean that the Bitgo multi-sig technology failed to secure it's users funds. Multi-sigs work by distributing the keys for an individual wallet to multi holders. For example in an 2 of 3 multi-sig, two of the three key holders must sign off on the transaction before it is broadcasted over the network. It seems in this particular case the hackers were able to locate an exploit in the Bitgo architecture thereby acting as the legitimate key holding party of each compromised wallet. This is very worrying as there are a large amount of Bitcoin still held in Bitgo multi-sig wallets.