NEW YORK - Yahoo announced on Tuesday that it has evidence that all of its user accounts have been affected by an August 2013 data theft.

The company first announced last December that more than one billion of the approximately three billion accounts existing in 2013 had likely been affected.

Yahoo, which has since been acquired by Verizon, said in a media release that it recently obtained new information and now believes, following an investigation with the assistance of outside forensic experts, that all Yahoo user accounts in existence in August 2013 were affected by the theft.

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 information that was stolen 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 announced that it is sending email notifications to the additional affected user accounts.

Yahoo also notified users via a notice on its website.

Users are being advised to take the following steps:

• 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 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.
• Consider using Yahoo Account Key, an authentication tool that eliminates the need to use a password on Yahoo altogether.

The company says it is continuing to work closely with law enforcement.

Additional information regarding this issue is available on the Yahoo 2013 Account Security Update FAQs page, https://yahoo.com/security-update.