"On Wednesday 19th July, 2017 a bug found in the multi-signature wallet ("multi-sig") code used as part of Parity Wallet software was exploited by parties unknown. As of the time of writing, three wallet accounts holding large balances of ETH have been compromised and the balances moved into accounts held by the attacker."
https://blog.ethcore.io/the-multi-sig-hack-a-postmortem/