This is separate from the hack disclosed in September this year, when the company said that 500 million accounts had been compromised.
In a post, Yahoo! chief information and security officer Bob Lord wrote that stolen user account information may have included names, email addresses, telephone numbers, dates of birth, hashed passwords (using MD5) and, in some cases, encrypted or unencrypted security questions and answers.
"The investigation 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."
Yahoo! was acquired by Verizon in July for US$4.83 billion.
Lord said that with regard to the September disclosure, outside forensic experts had been investigating the creation of forged cookies that could allow an intruder to access users’ accounts without a password.
"Based on the ongoing investigation, we believe an unauthorised third party accessed our 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.
"We are notifying the affected account holders, and have invalidated the forged cookies. We have connected some of this activity to the same state-sponsored actor believed to be responsible for the data theft the company disclosed on 22 September," he said.