Yahoo! reported that one billion accounts have been breached. User data that has been compromised include names, email addresses, telephone numbers, dates of birth and MD5 hashed passwords. Some of the accounts also have encrypted or unencrypted security questions and answers.
Several million of the user data and email addresses belonged to military and civilian government employees from dozens of nations.
User data that consists of plaintext passwords, payment card data, and bank accounts were not compromised.
Yahoo! said that it has notified affected users and "has taken steps to secure their accounts, including requiring users to change their passwords."
The company further explained that the hack is distinct from the 500 million account data breach in September 2016. However, it also said that the hack was somehow connected to some of the activity by "the same state-sponsored actor believed to be responsible" for the previously announced incident.
The data is for sale on the dark web's market. At the end of October 2016, the stolen Yahoo! data was for sale for as much as $200,000. Later the bids come much lower at $20,000 to $50,000 since the data is much less valuable now that Yahoo! has changed the passwords.
Yahoo!'s full statements reads:
As Yahoo previously disclosed in November, law enforcement provided the company with data files that a third party claimed was Yahoo user data. The company 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, Yahoo believes an unauthorized third party, in August 2013, stole data associated with more than one billion user accounts. The company has not been able to identify the intrusion associated with this theft. Yahoo believes this incident is likely distinct from the incident the company 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 is notifying potentially affected users and has taken steps to secure their accounts, including requiring users to change their passwords. Yahoo has also invalidated unencrypted security questions and answers so that they cannot be used to access an account.
Separately, Yahoo previously disclosed that its 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, the company believes an unauthorized third party accessed the company’s 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. Yahoo is notifying the affected account holders, and has invalidated the forged cookies. The company has 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 encourages users to review all of their online accounts for suspicious activity and to change their passwords and security questions and answers for any other accounts on which they use the same or similar information used for their Yahoo account. The company further recommends that users avoid clicking links or downloading attachments from suspicious emails and that they be cautious of unsolicited communications that ask for personal information. Additionally, Yahoo recommends using Yahoo Account Key, a simple authentication tool that eliminates the need to use a password on Yahoo altogether.