The professional social network LinkedIn said on Wednesday that it was probing a possible data breach after reports said more than 6.4 million passwords were stolen.
"Our team is currently looking into reports of stolen passwords. Stay tuned for more," the company said in a Twitter message.
Several security researchers reported the breach posted on a Russian hacker forum.
Graham Cluley of the British security firm Sophos said the posting "does contain, at least in part, LinkedIn passwords."
"Although the data which has been released so far does not include associated email addresses, it is reasonable to assume that such information may be in the hands of the criminals," Cluley said in a blog post.
As a result, Cluley said, "it would seem sensible to suggest to all LinkedIn users that they change their passwords as soon as possible as a precautionary step."
He said users should ensure the password you use is not used on any other websites, and hard to crack.
"If you were using the same passwords on other websites -- make sure to change them too. And never again use the same password on multiple websites," he said.