Private information from dozens of popular sites, including X, Dropbox, and LinkedIn, has been discovered on an unsecured page.
Over 280 million Twitter records were leaked in the data breach, experts say, while over 250 million LinkedIn records were affected.
Other popular sites across the world to be affected include MyFitnessPal, Canva, MySpace, Adobe, and Deezer.
Bob Dyachenko, who owns SecurityDiscovery.com, discovered the data break alongside researchers from Cybernews, who have suggested a malicious actor may be behind it.
It is thought the records come from a collection of previous breaches, with some duplicates among the 12 terabytes of data.
"'The dataset is extremely dangerous as threat actors could leverage the aggregated data for a wide range of attacks," researchers said.
The attacks could constitute identify theft, phishing schemes or targeted cyberattacks, they added.
Jake Moore, a global cybersecurity advisor for ESET told the Mail: "This is an absolutely huge breach of data.
"Cybercriminals cannot ever be underestimated with what they can achieve with even minimal information but if passwords have been taken the victims need to be aware of the consequences and must make the appropriate security updates."
Tenacent - 1.5bn
Weibo - 504m
MySpace - 360m
Twitter - 281m
Wattpad - 271m
Netease - 261m
Deezer - 258m
LinkedIn - 251m
AdultFriendFinder - 220m
Zynga - 217m
Luxottica - 206m
Evite - 179m
Zing - 164m
Adobe - 153m
MyFitnessPal - 151m
Canva - 143m
JD.com - 142m
Badoo - 127m
VK - 101m
Youku - 100m
To check if you have been affected, you can enter your email here.
Source: https://www.lbc.co.uk/news/biggest-ever-data-leak-26-billion-records-leaked-twitter-linkedin/