LinkedIn users have been advised to change their passwords immediately after an unconfirmed report claimed that millions of 'unsalted' hashed passwords have turned up on a Russian hacker website.
According to the Norwegian Dagens IT site (translated through Google), a forum user recently posted nearly 6.5 million stolen LinkedIn user passwords to the site, asking for help from the community in breaking the relatively weak SHA-1 encryption hashes used to secure them.
Although not in a clear text state, approaching 300,000 of the weakest hashed passwords had already been cracked; these could be re-purposed to break into other popular websites frequented by the same users.
Some of LinkedIn's 160 million registered users will use the same weak passwords so the 6.5 million trove could represent a larger proportion of the user base than the numbers suggest.
LinkedIn has yet to comment on the reported breach beyond saying "Our team is currently looking into reports of stolen passwords. Stay tuned for more," via Twitter.
"Think of all the other accounts you have - e-mail, Gmail, Instant Messenger, Skype which use the same password [as LinkedIn]. Change all of them, too - ideally use different new passwords for each one," said IT blogger Alec Muffett on Computerworld UK.
Other security experts struck a wearier tone.
"LinkedIn seems to be one of those services where I never go to...except to change my password," said Mikko Hypponen of Finnish security form F-Secure.
If confirmed (and we should underline that the beach has not been confirmed) the news would be the second security controversy to hit LinkedIn in days after researchers alleged that the company's iPhone and iPad apps were transmitting users' calendar entries for a five-day period back to its servers in an unencrypted state.