By Emily Flitter NEW YORK (Reuters) – A massive cyber attack from unknown sources that has been spamming bitcoin exchanges is highlighting some of the dangers people can encounter when they exchange cash for digital currencies like the bitcoin, experts said on Wednesday. The attack, which is technically known as a distributed denial of service attack, involved thousands of phantom transactions, forcing at least three of the online platforms that store bitcoins and trade them for traditional currencies to halt withdrawals of bitcoins until they can determine which transactions were real.