Double spending is mentioned by Satoshi Nakamoto in the Bitcoin whitepaper’s abstract.

• While other networks have fallen victim to double spending attacks, Bitcoin has remained safe thus far.
• Define double-spending and explain how Bitcoin prevents this problem.

Despite being included in the Bitcoin whitepaper’s abstract, this topic receives little attention outside of Bitcoin’s core community. This is because double-spend attacks are extremely unusual in the cryptocurrency industry (Bitcoin has never experienced one), but they are nonetheless important to Satoshi Nakamoto and the Bitcoin’s fundamental ideals, so they are worth being aware of.

The Double-spending “Problem”

As early as the third phrase of the Bitcoin whitepaper, Satoshi Nakamoto mentioned double spending and how his vision for Bitcoin would prevent it.

We suggest a peer-to-peer network system to address the issue of double spending.

But what, exactly, is this “double spending problem”? As a potential vulnerability in digital currencies and electronic payment systems, “double spending” occurs when the same currency unit is spent twice. Even while this is impossible with physical cash, it is conceivable for a user to duplicate their digital currency and use it in many purchases before the network has updated and validated the transactions.

Satoshi Nakamoto eventually created Bitcoin to resist such attacks by implementing a proof-of-work consensus mechanism, which its forerunners, the myriad attempts at digital cash that came and went in the 1990s and early 2000s, lacked.

Bitcoin’s Anti-Attack Mechanisms

A transaction is broadcast to the network for validation and addition to the blockchain once it has been initiated. Validated transactions included in a block cannot be removed or reversed once the block has been created. The network will automatically reject any conflicting or fraudulent transactions attempting to spend the same funds twice due to the process’s decentralized nature and the time it takes to complete.

However, double-spend attacks can happen if a network doesn’t have enough hash power from miners. Suppose this is the case, a bad actor can ‘double spend’ cryptocurrencies by overwhelming the network of miners, undoing recent transactions, and sending the same coins again. Such an attack may have been feasible when Bitcoin was first starting out, but it would now require a huge amount of mining power to even be attempted.

They’ve Already Happened on Other Networks

Although double-spend attacks are currently impossible on the Bitcoin blockchain, they have been successfully implemented on other blockchains. The year 2019 saw attacks on Bitcoin Cash, 2020 saw two on Ethereum Classic, and 2021 saw attacks on Bitcoin SV. The limited number of verifiers in the network made these assaults possible. The fewer nodes there are that verify a transaction, the greater the risk that one or more of them will be compromised.

Bitmart, a cryptocurrency exchange, sought a court order to have some of the counterfeit BSV coins seized after the perpetrators submitted some of the coins to the exchange. BSV said that BitMart was hypocritical about the issue, and that the coin’s value had dropped and been falling because of double-spend assaults.