Hackers stole data associated with over one billion accounts in 2013.

Yahoo announced that hackers stole data from over one billion accounts in 2013. According to the company, the data may have included names, email IDs, phone numbers, hashed passwords, and "encrypted or unencrypted security questions and answers."

This attack is separate from the one Yahoo disclosed back in September, in which the company believed a "state-sponsored actor" compromised its servers to access user data from over 500 million accounts. However, it looks like the same hackers were able to make away with more data this time around.

From the official announcement on Tumblr:

As we previously disclosed in November, law enforcement provided us with data files that a third party claimed was Yahoo user data. We analyzed this data with the assistance of outside forensic experts and found that it appears to be Yahoo user data. Based on further analysis of this data by the forensic experts, 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.

For potentially affected accounts, the 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 also said that the hackers were able to forge the company's authentication "cookies," allowing them access to user accounts without the need for a password:

Based on the ongoing investigation, we believe an unauthorized 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 September 22, 2016.

If you have a Yahoo account, it's time you changed your password. Create a strong password, and ensure the password you use on the service isn't reused anywhere else. You should also enable two-factor authentication for your Yahoo account.