LinkedIn has turned to the FBI for help after 6.5 million of its users’ passwords were dumped online by hackers.

The business network said “a small subset” of the hashed data had been deduced and revealed, but the rest is “hard to decode”. Security biz Sophos estimated that as much as 60 per cent of the leaked list had been cracked.

It is relatively trivial to work out the original passwords from the unsalted SHA-1 hashes, and LinkedIn has tacitly reiterated that it is upping its database security by sprinkling in some cryptographic salt.

The social network for suits is still silent on what other information the hackers may have lifted. It gave a somewhat slippery statement to the effect that punters’ email addresses have not been revealed – as far as it knows – which doesn’t answer the question of whether or not that information was stolen.

“To the best of our knowledge, no email logins associated with the passwords have been published, nor have we received any verified reports of unauthorised access to any member’s account as a result of this event,” the company stated in a blog post.

Yesterday, members reported that they were being inundated with spam and phishing emails pretending to originate from LinkedIn, which would suggest that their email addresses had been stolen or that the hackers still had access to the network’s databases.

LinkedIn has yet to return today’s or yesterday’s requests from The Register for comment on the spam. The company said on its blog that users whose passwords were leaked had had their accounts locked down for now, but also said it was going to cancel other passwords as well.

“As a precautionary measure, we are disabling the passwords of any other members that we believe could potentially be affected,” it said, without giving the criteria for how LinkedIn will figure out which accounts might be in trouble.

Any members who need to come up with yet another new password will be told to do so by email, but there will be no links in the email to click – just the instructions of what to do next.

LinkedIn said it was still looking into things and was also helping law enforcement with its investigation of the breach.

Meanwhile, dating site eHarmony and music site Last.fm have also reported hack attacks in which user passwords were nicked. eHarmony users say they are being spammed as well, although again only passwords have been confirmed stolen by the site. ®