Yahoo disclosed today that it has discovered a breach of more than one billion user accounts that occurred in August 2013. This attack is distinct from the 500 million accounts hacked in late 2014 that Yahoo told us about in September.

Yahoo’s chief information security officer Bob Lord said Yahoo was alerted to the attack by law enforcement which provided the internet company with data files that a third party claimed was Yahoo user data.

“We believe an unauthorised 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,” Lord wrote.

The most recent admission is likely to to complicate matters for Yahoo which is in the process of selling its core business to Verizon for US$4.83 billion. Verizon said it may seek to renegotiate terms after the first hack was disclosed in September.

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 stored in a different system to the one targeted in the attack.

Yahoo has notified affected users and has taken steps to secure their accounts, including requiring users to change their passwords.

Separately, Yahoo’s outside forensic experts are investigating the creation of forged cookies that could allow an intruder to access users’ accounts without a password.

Lord explained, “Based on the ongoing investigation, we believe an unauthorised 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.”

As Statista points out in the chart below, Yahoo now holds the title for largest and second largest data breaches to date:

Infographic: Latest Yahoo Hack Is the Largest Data Breach To Date | Statista
You will find more statistics at Statista

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