Up to 300,000 LinkedIn user passwords may have been cracked after a hash containing some 6.5 million passwords was posted to a hacker forum.
Engineering directorVicente Silveira said in a blog post that compromised accounts have had passwords reset.
The compromised passwords were stolen before Linkedin had a chance to implement stronger security measures, including salting.
The passwords were encrypted with the SHA-1 hash function and were unsalted and therefore easier to crack with rainbow tables.
About half of the passwords were cracked as of 7am AEST, according to US Security consultancy and crypto boffins KoreLogic who said in a tweet that 3,427,202 passwords were cracked.
The team warned prior to news breaking of the leak that "something big was brewing" and for users to change passwords on major sites.
News broke after a user 'dwdm' dumped a reported 6,458,020 encrypted LinkedIn passwords on crypto forum insidepro.
Affected users can check if they may be affected by running thier passwords on website LeakedIn, but are advised that a positive hit may mean their password was in use by other users.
Linked has more than 161 million members.