The company admits to the "coordinated attack" but refuses to say how many of the 237 millions her accounts are affected.
Η Yahoo is the second largest e-mail service around the world afterwards gmail of Google, according to comScore research company.
There are 273 million Yahoo email accounts around the world, including 81 million in the US.
Yahoo admitted to a blog post about the violation of that: "The information requested in the attack seems to be the names and emails from the affected accounts".
However, the company did not respond to e-mail or phone calls from MailOnline to clarify the extent of the violation.
Yahoo said it believes usernames and passwords accessς δεν συλλέχθηκαν από τα δικά της συστήματα, αλλά από μια βάση data by third parties.
However, it is not clear why a third party database has information about Yahoo accounts.
Yahoo said it is proceeding to reset the passwords for affected accounts and that 'Implemented additional measures' to prevent further attacks.
By intercepting real names and codes, hackers could send out false messages and carry out (which is the greatest risk) more serious violations involving bank transactions and trading on commercial sites.
In December, the service suffered a long-term interruption that prompted Yahoo CEO Marissa Mayer to apologize. The case is being investigated in co-operation with the US federal authorities.
