This was an incident that happened in 2014 and supposedly led to the theft of over 1.2 billion credentials on the internet including a huge amount of usernames, passwords and email addresses.
The security breach was reported by an internet business called Hold Security - They reported that a Russian organization had somehow managed to obtain this huge amount of data using SQL injection - Actual details of how they achieved it, and how Hold Security obtained this information was not disclosed. This coupled with the fact that immediately after the report, Hold Security promoted their own password checking service meant that many were skeptical about the supposed incident and many believe it to be a money making scam.
Despite this people were urged to change passwords to their accounts to improve their online security.
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