Social networking website LinkedIn has said some of its members' passwords have been "compromised" after reports that more than six million passwords had been leaked onto the internet.
Hackers posted a file containing encrypted passwords onto a Russian web forum.
They have invited the hacking community to help with decryption.
LinkedIn, which has more than 150 million users, said the leaked passwords would no longer be valid.
Members would receive an email with instructions on how to reset them, the company said. Users would then receive a second email with further details about why the change was necessary, it added.
The news comes as LinkedIn was forced to update its mobile app after a privacy flaw was uncovered by security researchers.
Skycure Security said the the mobile app was sending unencrypted calendar entries to LinkedIn servers without users' knowledge.
The information included meeting notes, which often contain information such as dialling numbers and passcodes for conference calls.
In response LinkedIn said it would "no longer send data from the meeting notes section of your calendar".
The company stressed that the calendar function was an opt-in feature.
However, the researchers who uncovered the flaw said the transmission of the data to LinkedIn's servers was done without a "clear indication from the app to the user".
In a statement posted on the company's blog, LinkedIn's mobile product head Joff Redfern said a new "learn more" link would be added to the app so users have a clearer picture about how their information is being used and transmitted.
What to do - Security experts have advised users to change their passwords on LinkedIn.
Visit www.linkedin.com, and log-in with your details
Once logged-in, hover over your name in the top right-hand corner of the screen, and select 'Settings' from the menu
You may be asked to log-in again at this point
On the next screen, click the 'Account' button which is near the bottom of the page
Under the 'Email & Password' heading, you will find a link to change your password
If you use the same password on other sites, be sure to change those too.
LWDLIK - Oh man! I just joined LinkedIn two days ago :O(