The administration of LinkedIn said to be investigating claims from a Russian hacker who said to have got access to 6,458,020 accounts of the website. There is a possibility the claims are fake, but several Twitter accounts claimed to have received a sheet of working LinkedIn passwords.
Mikko Hypponen, Chief Research Officer at F-Secure, told The Verge that the list is "a real collection", and he believes some kind of exploit was found on LinkedIn's user interface, but right now, he does not have much information about it.
Passwords are hard to crack if they are not dictionary words or simple number combinations, and even so, it take a few minutes to crack one, so the hacker must have had access to LinkedIn's database. The website hasn't confirmed the leak, but they recommend everyone to change passwords, just in case.