Irreversible Transactions

Traders and merchants who accept a payment immediately on seeing "0/unconfirmed" are exposed to the transaction being reversed. An attempt at fraud could work that the fraudster sends a transaction paying the merchant directly to the merchant, and sends a conflicting transaction spending the coin to himself to the rest of the network. It is likely that the second conflicting transaction will be mined into a block and accepted by bitcoin nodes as genuine.

Merchants can take precautions (e.g., disable incoming connections, only connect to well connected nodes) to lessen the risk of a race attack but the risk cannot be eliminated. Therefore, the cost/benefit of the risk needs to be considered when accepting payment on 0/unconfirmed when there is no recourse against the attacker.

The research paper Two Bitcoins at the Price of One finds that the protocol allows a high degree of success by an attacker in performing race attacks. The method studied in the research paper depends on access to the merchant's Bitcoin node which is why that even prior to this paper, recommendations for merchants include disabling incoming connections and to choose specific outgoing connections[1].

Another attack the trader or merchant is exposed to when accepting payment on 0/unconfirmed. The Finney attack is a fraudulent double-spend that requires the participation of a miner once a block has been mined[2]. The risk of a Finney attack cannot be eliminated regardless of the precautions taken by the merchant, but some miner hash power is required and a specific sequence of events must occur. Just like with the race attack, a trader or merchant should consider the cost / benefit when accepting payment on just one confirmation when there is no recourse against the attacker.

A Finney attack works as follows: Suppose the attacker is generating blocks occasionally. in each block he generates, he includes a transfer from address A to address B, both of which he controls. To cheat you, when he generates a block, he doesn't broadcast it. Instead, he opens your store web page and makes a payment to your address C with his address A. You may wait a few seconds for double-spends, not hear anything, and then transfer the goods. He broadcasts his block now, and his transaction will take precedence over yours.

Also referred to as a one-confirmation attack, is a combination of the race attack and the Finney attack such that a transaction that even has one confirmation can still be reversed. The same protective action for the race attack (no incoming connections, explicit outgoing connection to a well-connected node) significantly reduces the risk of this occurring.

It is worth noting that a successful attack costs the attacker one block - they need to 'sacrifice' a block by not broadcasting it, and instead relaying it only to the attacked node.

See on BitcoinTalk or further example of an attack scenario.

Also called alternative history attack. This attack has a chance to work even if the merchant waits for some confirmations, but requires relatively high hashrate and risk of significant expense in wasted electricity to the attacking miner.

The attacker submits to the merchant/network a transaction which pays the merchant, while privately mining an alternative blockchain fork in which a fraudulent double-spending transaction is included instead. After waiting for n confirmations, the merchant sends the product. If the attacker happened to find more than n blocks at this point, he releases his fork and regains his coins; otherwise, he can try to continue extending his fork with the hope of being able to catch up with the network. If he never manages to do this then the attack fails, the attacker has wasted a significant amount of electricity and the payment to the merchant will not be reversed.

Also referred to as a 51% attack or >50% attack. If the attacker controls more than half of the network hashrate, the previously-mentioned Alternative history attack has a probability of 100% to succeed. Since the attacker can generate blocks faster than the rest of the network, he can simply persevere with his private fork until it becomes longer than the branch built by the honest network, from whatever disadvantage.

No amount of confirmations can prevent this attack; however, waiting for confirmations does increase the aggregate resource cost of performing the attack, which could potentially make it unprofitable or delay it long enough for the circumstances to change or slower-acting synchronization methods to kick in. Bitcoin's security model relies on no single coalition of miners controlling more than half the mining power. A miner with more than 50% hash power is incentived to reduce their mining power and reframe from attacking in order for their mining equipment and bitcoin income to retain it's value.

In the past after large bitcoin thefts, it has been suggested that the theft victim attempts to bribe miners into reversing the confirmed transaction. This does not work because the thief can easily outbid with their own bribe. The thief gets a head-start as the transaction would already have confirmations. Every block that gets mined adds a block reward amount of bitcoins more that the attacker could keep while still paying more than the victim, as is every percentage of hashpower that doesn't go along with it. Such a reorg attempt, if successful, would also cause massive disruption in the bitcoin network and open the reorganizing miners and victim to litigation.

Examples:

• In August 2016 the Bitfinex exchange suffered a hack of 120000 bitcoins. Here is a reddit discussion involving bitcoin experts about bribing miners to reorg and why that's a bad idea: https://old.reddit.com/r/Bitcoin/comments/4vupa6/p2shinfo_shows_movement_out_of_multisig_wallets/d61qyaj/

• In May 2019 the Binance exchange was hacked for 7000 bitcoins and a similar suggestion was made: https://twitter.com/JeremyRubin/status/1125919526485254144