Tech giant, Yahoo, have confirmed that all its three billion user accounts were affected by 2013 hacking attack.
The company, which is now under the control of telecom giant Verizon, said an investigation had shown the breach went much further than originally thought.
Yahoo was taken over by Verizon earlier this year.
Yahoo said that while its latest announcement did not represent a new "security issue" it was sending emails to all the "additional affected user accounts".
In Tuesday's statement Verizon's chief information security officer Chandra McMahon said: "Verizon is committed to the highest standards of accountability and transparency, and we proactively work to ensure the safety and security of our users and networks in an evolving landscape of online threats.
"Our investment in Yahoo is allowing that team to continue to take significant steps to enhance their security, as well as benefit from Verizon's experience and resources."