Player passwords, email addresses, IP addresses, and any other information posted to the forums has been exposed. Payment information is not at risk, as all transactions take place through a third-party, OP Productions said in a statement.
"We are sorry to report that we have discovered that hackers gained access to our forum and game databases and the player data in those databases. We have launched a thorough investigation covering our entire system to determine the scope of the intrusion," the company said. "This investigation is ongoing and is our top priority. As part of the remediation and security enhancement process we will be taking the game and forums down temporarily."
It was not mentioned when the intrusion was first discovered.
OP Productions said all War Z game and forum passwords are encrypted, but that did not keep the hackers from accessing the data.
"There is a possibility that simple passwords can be obtained using brute force even if they are encrypted," reads a line from the statement. "Our research shows that many users are not using strong passwords."
The company urged all players to change their passwords immediately by visiting the War Z website or through the game launcher.
OP Productions is working with "outside experts and investigators" to help with the ongoing investigation and said it has committed "substantial resources" to that end.
"We are undertaking a full review and update of our servers and the services we use and adding additional security mechanisms," the company said. "In addition to this post, we are emailing all of our players just to make certain that everyone is informed and has been advised to change their passwords."
"This has been a humbling experience for us. While we all know that there is no guarantee of security on the Internet, our goal is to try our very best to protect your data. We sincerely apologize," the statement goes on. "We will update you on status as we make progress."
The War Z launched on Steam in December and was pulled just a day after release, after users reported numerous issues. Valve called its decision to originally publish the game a "mistake." The game was reinstated on Steam in late February.