Hackers exploited a vulnerability in the way in which Parity, an Ethereum wallet, implemented multi-sig wallets, stealing some 153,000 worth of ether, Yesterday. Which was valued around $32 million at the time of theft. Since, then the value has gone down significantly.
A multi-sig wallet is a wallet that requires more than one signature for an action to be performed on its contents. Users that had regular wallets on Parity were not in danger (incidentally, the author of this text has until recently held some ether in a Parity wallet, but not a multi-sig one).
On Tuesday, an initial coin offering (ICO) of an Ethereum-based startup called CoinDash went south as hackers managed to change the wallet address on the project's web page, Stealing away more than $10 million worth of ether.
And in June 2016, hackers exploited a vulnerability in the code of DAO, another Ethereum-based project, stealing some 3.6 million ether, which today would be worth more than $700 million.
Source: Mashable.com