4.79M accounts from Bitcoin Security Forum Gmail Dump (forum.btcsec.com) were exposed in this breach. Besides email addresses, it included passwords, which makes it a high-risk breach.
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Email addressesPasswords
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.
Source: Have I Been Pwned (CC BY 4.0)