If you are on the League of Legend's EU-West or EU East and Nordic servers you should have, by now, received an email from Riot regarding a potnetial security breach of your account. It would appear that hackers managed to gain access to Riot's Database and compromise account details.
If you play on the North American servers then you were not affected.
Among the information hackers got away with was email addresses, Summoner names, Encrypted account passwords, date of birth and - for a very small number of users - First and Last Name. Although the stolen passwords were encrypted, Riot is advising that all summoners check their account and change their passwords as a standard safety precaution.
Riot has confirmed that absolutely no billing information, paypal or credit card details were breached.
It is not yet clear how many players are affected by the breach and whether or not Riot is sending emails to all players or only those whose details were stolen. As at time of writing I have received no such email from Riot so guessing I'm good but I will still be changing my password anyway and Riot is advising the same.
To Riot's credit, they informed the playerbase immediately and took immediate remedial action using outside 'security experts' to plug the vulnerability - Riot receiving a lot of love for being straight up and honest about the situation and posting information as soon as they could, so kudos to Riot.
On the back of this is a very timely reminder about account security and choosing sensible passwords. Riot conducted an after action review and discovered that 11 passwords were shared by some 10,000 individual players and that double digit percentage of players had the same password as at least one other player.
It's also a very good reminder as to why you should not use the same password for all your online games or your email - especially if that email is your 'gaming' email, something I know a lot of players do so that their regular email is not filled with game related stuff.
You can view the full forum post here which includes what happened, what Riot have done about it and what you are advised to do yourself.