WHAT HAPPENED? On January 4th, some of Bitstamp’s operational wallets were compromised, resulting in a loss of less than 19,000 BTC. This breach represents a small fraction of Bitstamp’s total bitcoin reserves, the overwhelming majority of which are held in secure offline cold storage systems. We can assure customers that any bitcoins held with us prior to temporary suspension of services on January 5th at 9am UTC are completely safe and will be fully honored. Upon learning of the breach, we immediately notified all customers that they should no longer make deposits to previously issued bitcoin deposit addresses. As an additional security measure, we suspended our systems to fully investigate the incident and actively engage with law enforcement officials.

https://www.bitstamp.net/article/bitstamp-is-open-for-business-better-than-ever/