Millions of users of the social networking website LinkedIn have been told to reset their passwords after security information was stolen.
The site, which is aimed at professionals and has in excess of 161 million members in more than 200 countries, was compromised and members' details were posted online.
LinkedIn director Vicente Silveira said in a statement: "We can confirm that some of the passwords that were compromised correspond to LinkedIn accounts."
He said the company was investigating the security breach and added that those who were affected will notice their LinkedIn passwords will no longer be valid.
It is thought the passwords of more than 6.5 million people were stolen.
Mr Silveira said: "Members that have accounts associated with the compromised passwords will notice that their LinkedIn account password is no longer valid.
"These members will also receive an email from LinkedIn with instructions on how to reset their passwords."
Users were told they should never change their passwords by following a link sent on an email.
"These affected members will receive a second email from our customer support team providing a bit more context on this situation and why they are being asked to change their passwords," Mr Silveira added.
IT security and data protection firm Sophos said the leaked encrypted data does not include associated email addresses but warned that hackers will be working to crack the "unsalted" password hashes and "it is reasonable to assume that such information may be in the hands of the criminals".
Graham Cluley, senior technology consultant at Sophos, said: "It would seem sensible to suggest to all LinkedIn users that they change their passwords as soon as possible as a precautionary step."
Mr Silveira said LinkedIn had recently improved its security, which included the "hashing and salting" of current password databases.
California-based LinkedIn launched in 2003 and made its stock market debut in May 2011 in the hope of raising money for expansion.
LinkedIn gets more than two-thirds of its revenue from fees it charges companies, recruiting services and other people who want broader access to the profiles and other data on the company's website. The rest comes from advertising.
