If you’re a registered account holder with Steam, we encourage you to review the information provided by Valve as soon as possible. We also recommend that you update your Battle.net password via Account Management (https://us.battle.net/account/management) should it be the same as or similar to the one you used for Steam or for the Steam forums. You may also wish to change your registered email address as a further security measure, or to change the password to your email account and other online accounts that might be at risk of compromise. If you are unable to update your Battle.net email address and/or password on your own, please contact our customer support staff for assistance: http://us.blizzard.com/support/webform.xml
For additional tips and suggestions for how to improve your Battle.net account security, such as by adding a Battle.net Authenticator to your account, check out our Account Security Awareness guide (http://us.battle.net/en/security/) and blog post (http://us.battle.net/wow/en/blog/2299938).