In what is shaping up to be the largest breach yet, Yahoo disclosed on Wednesday that hackers gained unauthorized access to data associated with more than I billion user accounts.

This revelation is separate from an earlier breach reported in September in which more than 500 million users were affected by a security breach that occurred in late 2014.

According to the beleaguered company, “we believe an unauthorized third party, in August 2013, stole data associated with more than one billion user accounts. We have not been able to identify the intrusion associated with this theft. We believe this incident is likely distinct from the incident we disclosed on September 22, 2016.”

The stolen user account information includes names, email addresses, telephone numbers, dates of birth, hashed passwords (using MD5) and, in some cases, encrypted or unencrypted security questions and answer, Yahoo confirmed.

According to the company’s CISO, Bob Lord, investigation by forensic experts indicates that the stolen information did not include passwords in clear text, payment card data, or bank account information. Payment card data and bank account information are not stored in the system the company believes was affected, said Lord.

The company said it previously disclosed that outside forensic experts were investigating the creation of forged cookies that could allow an intruder to access users’ accounts without a password. Based on the ongoing investigation, Yahoo believes an unauthorized third party accessed their proprietary code to learn how to forge cookies.

The outside forensic experts have identified user accounts for which they believe forged cookies were taken or used. Yahoo is notifying the affected account holders, and said it has invalidated the forged cookies.

State-Sponsored Actor Blamed

Yahoo said it has connected some of this activity to the same state-sponsored actor believed to be responsible for the data theft the company disclosed on September 22, 2016.

The company is notifying potentially affected users and says it has taken steps to secure their accounts, including requiring users to change their passwords. It has also invalidated unencrypted security questions and answers so that they cannot be used to access an account.

With respect to the cookie forging activity, Yahoo said it invalidated the forged cookies and hardened its systems to secure them against similar attacks. The company has also implemented continuous monitoring to detect and prevent unauthorized access to user accounts.