Yahoo Inc (YHOO.O) warned on Wednesday that it had uncovered yet another massive cyber attack, saying data from more than 1 billion user accounts was compromised in August 2013, making it the largest breach in history. The number of affected accounts was double the number implicated in a 2014 breach that the internet company disclosed in September and blamed on hackers working on behalf of a government. News of that attack, which affected at least 500 million accounts, prompted Verizon Communication Inc (VZ.N) to say in October that it might withdraw from an agreement to buy Yahoo's core internet business for $4.83 billion. Following the latest disclosure, Verizon said, "we will review the impact of this new development before reaching any final conclusions." A Yahoo spokesman told Reuters…