Yahoo has announced that “more than one billion user accounts” may have been stolen by hackers during an attack that took place in August 2013, according to a press release.

This is a separate hack than the one that Yahoo announced back in September, in which as many as 500 million user accounts were compromise.

The company also warned attackers have figured out a way to log into targeted Yahoo accounts without even supplying the victim’s password.

“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,”

Yahoo’s chief information security officer Bob Lord said in a statement the company published Wednesday afternoon. “We have not been able to identify the intrusion associated with this theft.”

The statement says that 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.”

The attackers in this case apparently found a way to forge these authentication cookies, which would have granted them to access targeted accounts without needing to supply the account’s password.

In addition, a forged cookie could have allowed the attackers to remain logged into the hacked accounts for weeks or indefinitely.

Yahoo’s statement said the company is in the process of notifying the affected account holders, and that it has invalidated the forged cookies.

What can users do to protect their account?

We encourage our users to visit our Safety Center page for recommendations on how to stay secure online. Some important recommendations we’re re-emphasizing today include the following:

• Change your passwords and security questions and answers for any other accounts on which you used the same or similar information used for your Yahoo account;
• Review all of your accounts for suspicious activity;
• Be cautious of any unsolicited communications that ask for your personal information or refer you to a web page asking for personal information;
• Avoid clicking on links or downloading attachments from suspicious emails; and
• Consider using Yahoo Account Key, a simple authentication tool that eliminates the need to use a password on Yahoo altogether.