Yahoo warns users of 'potential malicious activity' on accounts
Yahoo has issued a new warning to users of "potential malicious activity" on their accounts between 2015 and 2016.
It comes after it emerged in December that between 2013 and 2014 more than one billion users' data was stolen, including email addresses, birth dates and answers to security questions.
Yahoo has confirmed users are being notified their accounts had been "potentially compromised", but refused to say home many people were affected.
A warning message sent to users on Wednesday read: "Based on the ongoing investigation, we believe a forged cookie may have been used in 2015 or 2016 to access your account."
Some users posted their warning messages on Twitter.
A statement from Yahoo said it had "invalidated the forged cookies so they cannot be used".