- Double Spend
Regarding virtual currency: What occurs when someone makes a transaction using a cryptocurrency and then makes a second purchase from someone else, using that same cryptocurrency, and convinces the rest of the network to confirm only one of the transactions by hashing it in a block. Double spending is not easy to do, but is a risk run by those accepting zero-confirmation transactions. Regarding Ethereum: A deliberate fork, where a user with a large amount of mining power sends a transaction (usually in exchange for fungible goods or services). The attacker then creates a block at the same level as the block containing the original transaction, but which does not contain the transaction; instead, the hacker often sends those same funds to him or herself. The attacker then starts mining on the block without the original transaction, making it appear as if it had never occurred. If the attacker has more than 50 percent of all mining power, the double spend is guaranteed to succeed eventually, at any block depth. Below 50 percent there is some probability of success, but it is usually only substantial at a depth up to about 2-5. For this reason, most cryptocurrency exchanges, gambling sites, and financial services wait until six blocks have been produced (“six confirmations”) before accepting a payment.