OP Productions was forced to take down The War Z and its forums after discovering a hack of its systems.

“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,” the company reported in an open letter(Opens in a new window) to players.

While the scope of the attack has not yet been determined, OP Productions promised that no payment or billing information was exposed. The attackers, however, did get away with some loot, including email addresses, forum and game passwords, in-game character names, and IP addresses. Other information posted to the forum was likely accessed as well, OP Productions said.

“We do not collect the names or addresses of our gamers so that information was not impacted unless you posted it on the forum,” the letter read. “We are investigating whether additional information may have been obtained.”

OP said it encrypts all passwords, but those who opted for easy passwords might have them exposed by brute force attacks. As a result, the company is urging all players to immediately change their passwords, online via the website or by clicking the “Forgot Password” link on the game’s launch screen. The longer the password, the stronger the password — OP suggest using a combination of upper- and lowercase letters, numbers, and special characters to create a code no shorter than eight symbols, but ideally somewhere between 12 and 15.

“The security of your data is important to us and we want our players to be assured that we take this situation very seriously,” the company wrote. “We have taken steps to improve security to minimize the chance of this happening in the future and will continue to invest in improving security going forward.”

OP’s survival game(Opens in a new window) was released in December, but pulled from Valve’s digital shelves only a day later, Venture Beat reported(Opens in a new window), when players fussed over features missing from the $15 product or the overall quality of the zombie game. Two months ago, the publisher re-launched the title, which has remained hiccup-free, until this week’s hack.

The War Z team promised further updates as it makes progress in its investigation.