Personal data, such as user names, email addresses and passwords, may have been taken in the attack.
Credit card and personal financial information was stressed to have not been compromised.
"Over the past weekend, a hacker group attempted an unlawful intrusion of our websites to gain access to data," began a statement.
"We believe we have taken appropriate action to protect our data against these attacks. While no personal financial information or credit card data was obtained, the hackers may have gained access to some user names, email addresses and/or passwords.
"As we don't know what further plans the hackers may have, we suggest that you keep an eye out for suspicious emails and account activity."
It concluded: "We regret any inconvenience that these attacks on us cause for you. These attacks will be evaluated to determine if there are any additional protections we might take that would be prudent."
The Fallout and Brink publisher has advised users to change any passwords associated with its accounts.
Codemasters and Nintendo are the latest victims of online security breaches in the past week.
PlayStation Network went offline in April due to a severe breach in security that led to personal details and credit card information being stolen. Full access to the service was restored earlier this month.