A group of experts immediately started working on the issue. While the game went back live for a brief moment, the folks at CCP Games realized that it was more important to exhaustively scan the entire infrastructure before bringing the game online. Therefore, the game was taken back offline for a number of hours.
What we can now confirm is that a person was able to utilize a vulnerability in one of the back-end services that support the operation of the Tranquility server. This vulnerability has now been secured and thoroughly tested.
We would like to stress that at no time was customer data compromised or accessible in any way.
The team is very thankful for their players' patience, and is trying to compensate them as soon as possible.
Source: Official game forum and Twitter