When a transaction is included in a block by a miner, it is considered to have one confirmation. Each subsequently found block adds another confirmation. Only sufficiently confirmed transactions are considered ‘final.’ Due to the possibilities of attacks, it might be possible to reverse a transaction that only has one or two confirmations. But, after six confirmations, a transaction is generally considered to be irreversible, although depending on the size of the transaction, it might be unviable to double spend it much earlier. Unconfirmed transactions are generally thought to be insecure, although attacks are rare.