Yahoo account holders are advised to change their passwords. Right Now.

Around 1 billion Yahoo email accounts were breached back in 2013. Yahoo finally admitted that a hack which occurred in August of that year affected that many accounts.

The hack not only involved the normal account information such as user names and email addresses but also more personal data such as phone numbers, dates of birth and passwords. According to reports, the thieves were also able to steal sensitive pieces of information such as security questions and their answers.

The news will greatly affect the supposed purchase of the company by Verizon Communications Inc. Yahoo announced back in July that Verizon will buy the company for $4.83 billion. The deal was expected to be consummated early next year but a couple of incidents could change everything.

Back in September, Yahoo disclosed that around 500 million users were compromised by a massive hack in 2014. Back then, it was considered the biggest theft of personal account information. It turns out, that was mincemeat as compared to the August 2103 hack which is now being labeled as the "biggest cyber attack of all time".

The September disclosure had Verizon thinking that it might change the terms of the deal. This newly discovered hacking incident may affect the terms even further or force Verizon to nix the deal entirely.

Yahoo believes the 2013 hack was perpetrated by a different person or group than the one behind the 2014 attack. The company also discovered that the hackers are no longer in Yahoo's corporate network.

The company has already started contacting the affected account holders.

Yahoo said that they "are notifying potentially affected users and have taken steps to secure their accounts, including requiring users to change their passwords."

The Federal Bureau of Investigation has already contacted Yahoo and helping out in the investigation which is believed to be perpetuated by an unauthorized third party.