Riot is currently looking into a League of Legends exploit which allows browser access to the game’s store and makes player accounts vulnerable to hackers.
This is what a Riot representative had to say regarding the matter:
“We’re getting this fixed right now, though we can’t speak to the specifics of the exploit or the explanations”
“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.”
The exploit can be seen in more detail in the video below: