Close to 6.5 million passwords for the online professional network LinkedIn have been leaked by hackers.
LinkedIn has said it is “looking into” theft of passwords from the site, which has more than 150 million users worldwide. Reports suggest that personal email addresses could be included in the stolen data, which will cause severe problems for users who employ the same password over a number of sites online.
"We can confirm that some of the passwords that were compromised correspond to LinkedIn accounts," said LinkedIn director Vicente Silveira. "We are continuing to investigate this situation.
"We sincerely apologize for the inconvenience this has caused our members," he added. "We take the security of our members very seriously."
According to Norweigan newspaper Dagens IT, the encrypted passwords were posted onto a Russian hacker site on Monday with a callout for readers to try and crack them. Reports suggest that 300,000 of the 6.5 million passwords have already been hacked.
Users of the online dating site eHarmony.com have also been hit by the breach in security, it is thought that 1.5m of the site's 20m users' details have been leaked.
Sophos senior security consultant Carole Theriault told Yahoo! News, "With the situation as it is we are really advising people to go in and change their passwords as soon as possible. Don't wait around, go in now. There is the likelihood that email addresses attached to these accounts have also been leaked.
"Make sure that passwords for site profiles are all different as you can be vulnerable with just one password," she added. "Personally I use password managers to encrypt logins for each site as I have more than 100 in total.
"Above all make sure that passwords are complicated with 12-13 characters or so."