Over one billion Yahoo customers reportedly hacked
Breach could mean names, phone numbers and dates of birth have also been stolen
Yahoo has admitted that data from more than one billion accounts may have been hacked.
Although the attack happened three years ago, hackers might have targeted users' emails.
Yahoo has also revealed it is investigating a later, separate issue that might have made some accounts accessible without passwords.
"Your email account is the central hub of your entire online existence - if they own that they can ask for password resets on other accounts you have online as well," said security expert Graham Cluley to the BBC. "Don't just change your Yahoo password."
Anyone using their email account for work purposes - such as sending professional documents back and forth in attachments - could possibly become a target of industrial espionage.
It is now thought that data, including names, telephone numbers and dates of birth, will - or already has - become available to buyers on the dark net, although so far there has been no evidence of this.
Anyone connected with Yahoo is advised to change their passwords; some may not think of themselves as Yahoo users but the firm also provides some BT customers' email accounts and acquired Flickr in 2005.
Yahoo has insisted that it continuously enhances its safeguards and systems that detect and prevent unauthorised access to user account.
To read the report in full visit http://www.bbc.co.uk/news/technology-38327169