According to the post, the attack was centered on web servers and the Tranquility cluster, "which houses Eve Online and Dust 514." Shortly after the discovery of the attack, the servers were taken offline. Although briefly reinstated, servers have once again been taken down while the company continues its investigation.
According to Eve Online's official Twitter account, servers are expected to remain offline for "several hours ... as this process should not be rushed."
We have contacted CCP for comment and will update accordingly.
Eve Online and Dust 514 merged servers in January of this year in order to create a single cross-platform universe. Users that participated in the Dust 514 beta were migrated from the Singularity server to the Tranquility server to help facilitate the merge.
Update: Servers are back online and, according to a recent update, CCP has secured the vulnerability that led to the attack. The developer also stressed that no customer data was compromised or accessible in any way, and that players will be compensated for the outage. Details will follow soon.