Hundreds of millions of email addresses and passwords have been posted online for anyone to download.
Nearly 800 million logins are in the huge dump which contains information from thousands of data breaches.
The stolen details are likely to be in use for years as hackers attempt to take over affected users accounts.
Cybersecurity expert Troy Hunt said a list of more than 2.6 billion records containing around 773 million unique email addresses and more than 21 million unique passwords was being shared on a “popular hacking forum”.
Mr Hunt said his initial analysis of the data, which has been dubbed Collection £1, found it had been compiled from more than 2,000 different data breaches and hacked databases or websites confirming some of his own personal information had also appeared in the lists.
he database did not appear to contain any more sensitive information – such personal finance information and credit card details, he said.
Mr Hunt claimed his research on the list suggested around 140 million of the email addresses had not appeared in previous breaches and were therefore newly exposed details.