In September 2014, a large dump of nearly 5M usernames and passwords was posted to a Russian Bitcoin forum. Whilst commonly reported as 5M "Gmail passwords", the dump also contained 123k yandex.ru addresses. Whilst the origin of the breach remains unclear, the breached credentials were confirmed by multiple source as correct, albeit a number of years old.

Compromised data: Email addresses, Passwords

Official site and data verification:
https://forum.btcsec.com/index.php?/topic/9426-gmail-meniai-parol/
http://web.archive.org/web/20140910[....]of_gmail_passwords_leaked_rus_most/

Download: (This archive only contains the leaked emails. The passwords were removed)
https://forum.bits.media/uploads/manual_09_2014/google_5000000.7z
http://www.mediafire.com/file/1hfk55ke73y0dx3/google_5000000.7z/file

Download: (This archive contains the emails and passwords w/ other leaks)
https://nitroflare.com/view/94DE7484455DB86/Leaked_Email_Project.rar