The trove of data, which upon analysis appears to date to 2008 and 2009, holds a massive 360,213,024 records reportedly featuring email addresses, usernames and passwords.
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According to an initial assessment of the data, passwords were stored in SHA1 with "no salting".
Based on fresh analysis from independent security researcher Troy Hunt, who operates breach notification website HaveIBeenPwned.com, the hacked MySpace data is from "mid-2008 to early 2009".
He added: "It may be that the incident occurred after 2008, but I doubt it was much later as they were still getting registrations and they would have been very heavily Gmail biased by that time."
At the time, Tim Vanderhook, chief-executive of MySpace parent company Viant, said the company often relies on its vast collection of email addresses to tailor communications with users.
Now, it seems, this feature may have to be used to send a significant amount of breach notification letters.