Yahoo has revealed that more than one billion user accounts may have been affected in a breach dating back to 2013.

The company said the attack was separate from the 2014 breach disclosed in September, when Yahoo revealed 500 million accounts had been accessed.

Names, phone numbers, passwords and email addresses are understood to have been compromised in this latest attack, but not bank and payment information.

Account users have been urged to change their passwords and security settings.

James Maude, senior security engineer at Avecto said continued failings by the company was deeply concerning:

“One in 6 people globally have now had their data breached thanks to Yahoo. With a breach on such an unprecedented scale, users should be concerned about how a behemoth of the internet failed to notice this for such a long period of time. This is especially concerning as recent reports have shown that around this time Yahoo was busy undermining their own security by installing backdoors in their own infrastructure for government agencies. There is the worrying possibility that this undisclosed backdoor served as cover for the data breaches as employees deliberately ignored or hid these back channels.

“Initial reports suggest that the attackers manipulated cookies, which are normally used to authenticate or track users, however in this case the attackers changed them to bypass logins without requiring a password. Using this technique attackers could have logged into account at will and monitored them for great lengths of time. With such negligence, questions must be asked as to what was going on at Yahoo to allow this to happen.

“For the consumer, it is time to consider the impact of this breach and evaluate what data they stored on their Yahoo account. Users of Yahoo mail services should be the most vigilant as their email account maybe the gateway to access many online services, from banking sites to dating websites, containing highly personal information. It is important to reset passwords that may be at risk and consider what other accounts were linked to Yahoo services, such as Flickr, that may store private family photos.

“Time and time again we see organizations failing to notice suspicious activity occurring in their environment and on their endpoints as they are reliant on failing detection solutions that simply can’t spot unique targeted attacks. As Yahoo were acquired by Verizon who annually publish the industry leading report on data breaches, they may seek to renegotiate the terms of the acquisitions following these hugely damaging breaches.”