OzCoin Hack

On 19 April 2013 an unknown attacker broke into the server of the OzCoin Bitcoin mining pool. The attacker defaced the pool's website and database and stole 923 BTC (about $135,000 at the time) from the pool's Bitcoin wallet. OzCoin shut down its web and database server, and staff advised miners on the pool's Bitcointalk thread to switch to another pool for the time being. How the attacker got into the server has not been published.

In less than a day, part of the stolen bitcoin passed through StrongCoin, a web wallet service. StrongCoin seized about 569 BTC and returned it to OzCoin, leaving about 354.06 BTC unrecovered. StrongCoin said it spotted the funds by tracing them on the blockchain. The seizure was controversial because StrongCoin had told users that no one, including StrongCoin, could spend their coins, and freezing these funds showed it could.