Yahoo’s public confession entitled “Important Security Information for Yahoo Users” about the August 2013 theft of “data associated with more than one billion user accounts” including “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” follows:

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.

Separately, we previously disclosed that our 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, 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.

Yahoo’s also encourages “ users to visit our Safety Center page for recommendations on how to stay secure online:”

  • 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.

However since the theft occurred in 2013 it’s hard to believe that this advice will solve many user problems.