Greetings Summoner,
Keeping your information secure is very important to us. That's why we're sorry to inform you that some of the account information you provided us may have been accessed by hackers. Most critically, some players' email address, encrypted account password, and date of birth. No payment or billing information of any kind was accessed.
As a safety precaution, please change your account passwords by logging in to euw.leagueoflegends.com and visiting the "my account" page (linked in the upper-right hand corner of the page after login). If you use the same password for accounts on other services, you should change those passwords as well.
For your security, the URL above isn't clickable. If you need to access your account, type the URL in directly. On that note, please be extra vigilant about emails you receive, especially if those emails contain attachments or links.
We're keeping this note short, but have created a forum post with much more information and the latest official updates on this situation at euw.leagueoflegends.com/board within the announcements forum.
We apologize for the inconvenience, and appreciate your immediate attention to resetting your password.
The League of Legends Security Team