A post from Riot Games to a thread on reddit confirms that accounts in League of Legends are being exploited by third-parties (thanks PC Gamer). They do not confirm theories about what's going on in the MOBA, but they do state they plan on setting everything right again. Word is:

We're getting this fixed right now, though we can't speak to the specifics of the exploit or the explanations fellow Redditors have been offering. What we can say is that we can see everyone who was hit by an attack, and we'll be returning all RP/IP that was lost.

Since the store was involved, we also want to reassure you that this didn't expose any personal information like credit card numbers. Your data is safe.