# Double-Spending in Cryptocurrency: Definition, Risks, and Prevention

## What Is Double-Spending?

Double-spending is spending the same cryptocurrency or blockchain token more than once. Cryptocurrency is a token representing value on a distributed ledger. Without proper safeguards, it's easy to alter a ledger and reclaim the spent amount.

Double-spending is not limited only to cryptocurrency; it is a problem in all blockchain designs. Here's more about this issue, how it's done, and some steps to take to keep from being a victim.

### Key Takeaways

*   Double-spending is a significant threat to blockchain networks, potentially allowing a token or cryptocurrency to be used more than once, undermining trust in digital transactions.  
    
*   Bitcoin and Ethereum address the double-spending problem through a combination of cryptographic techniques, distributed consensus, and consensus mechanisms like proof-of-work or proof-of-stake.  
    
*   A 51% attack is a major type of double-spending threat where an entity gains control over more than half of a network’s computational power, allowing them to alter transaction records.  
    
*   Race attacks and Finney attacks exploit unconfirmed transactions, highlighting the need for users to verify transaction confirmations before proceeding.  
    
*   While primarily a concern in cryptocurrency, double-spending can also occur in non-monetary settings, such as blockchain-based voting systems, posing ethical challenges rather than legal ones.  
    

## Why Double Spending Matters in Cryptocurrencies

Cryptocurrencies and [blockchain](https://www.investopedia.com/terms/b/blockchain.asp) had been under development for many years before Bitcoin was introduced. Cryptocurrencies didn't work until Bitcoin solved the issue of users altering ledgers to reclaim tokens.

This weakness in [digital money](https://www.investopedia.com/terms/d/digital-money.asp) systems is why third-party auditors are usually involved. These auditors must spend time, which equates to money, verifying transactions and amounts between parties. Successful systems require trust that auditors or ledger maintainers won't alter entries for personal gain.

## Strategies to Prevent Double Spending in Blockchain Networks

[Satoshi Nakamoto's](https://www.investopedia.com/terms/s/satoshi-nakamoto.asp) solution of timestamping and cryptographically chaining transactions solved double-spending. A large and fast distributed network is essential to prevent malicious actors from altering transactions. Bitcoin and Ethereum's scale prevents double-spending attacks, but individual caution is still necessary.

People who use or invest in cryptocurrency should ensure that they do not accept unconfirmed transactions. Due to awareness of these attempts, many wallet creators program wallets that do not allow unconfirmed transactions to be accepted. However, it's best to check and make sure your [wallet](https://www.investopedia.com/terms/b/bitcoin-wallet.asp) displays this information or prevents you from accepting one.

## Common Double Spending Attacks and Their Impact

The most significant double-spending risk for blockchains is a [51% attack](https://www.investopedia.com/terms/1/51-attack.asp), which can occur if an entity controls more than 50% of the hashing power or validation mechanisms on a network.

If this user—or users—assumes a majority of the network, the network's stake, or any other mechanism used, they will be able to dictate transaction consensus and control the award of currency. New or forked cryptocurrencies with smaller networks are susceptible to this attack. In cryptocurrency networks such as Bitcoin, this is very unlikely due to the number of network participants and the speed at which the network operates.

Ethereum uses a [staking mechanic](https://www.investopedia.com/how-to-stake-ethereum-7482623), where only those users who have locked large amounts of ether in smart contracts can become validators and propose blocks. To attempt this attack, a group or entity would need to control more than 50% of the staked ether on the network—a very costly effort because 32 ETH (about $95,200 at the May 15, 2024 price) is required to establish a node, and there is also a mechanism that burns the tokens of dishonest validators.

### Race Attack

Race attacks, also called unconfirmed transaction, are where a malicious user attempts to send two quick transactions, one to a recipient and one to the blockchain. The one to a recipient might transfer a token to them, but the transaction sent to the network would keep it in the sender's possession. This is an attempt to exploit network lag, with the sender's transaction establishing ownership being confirmed first.

This is easily prevented by not accepting unconfirmed transactions.

### Finney Attack

A finney attack, named after Hal Finney, the developer who pointed out the weakness, is a type of unconfirmed transaction attack. However, this attack requires a miner, who creates a block and sends an amount to two addresses they own. Another transaction is sent to another party in the same block. If the recipient accepts it before it is confirmed by the network, the sender can essentially return the amount sent and spend it again.

This attack is very rare on large blockchains but can be prevented by not accepting unconfirmed transactions or using a wallet that doesn't let you accept them.

### Sybil Attack

A Sybil attack is when multiple nodes are created in an attempt to gain influence over a network. This sounds like a 51% attack, but it is at a lower scale. Sybil attacks can be viewed as precursors to 51% attacks.

## Legal and Ethical Implications of Double Spending

Whether double-spending is illegal depends on the circumstances. With regards to a cryptocurrency with a market value, it is illegal because it is committing fraud.

However, double spending is not limited to cryptocurrency. For example, imagine a gaming community has created a blockchain to facilitate its voting processes, where one blockchain token represents one vote. The token has no monetary value and is not a cryptocurrency.

The gamers want to hold an event but are divided on where to do it. A vote seems the best way to resolve the division, so they vote. If someone initiated a double-spend attack and changed peoples' votes, those members' tokens would be spent twice, and the result wouldn't be representative of the true outcome of the vote. This is generally not illegal, but it is unethical within the gaming community and might violate their conditions of membership.

## What Is the Meaning of Double Spend?

Double spending is the act of using the same digital currency or token more than once.

## What Is an Example of a Double Spend Problem?

Double spending is altering a transaction on a blockchain or distributed ledger so that a spent token can be spent again.

## How Did Bitcoin Solve Double Spending?

Satoshi Nakamoto's Bitcoin solution was to use an encryption algorithm, distributed consensus, proof that work was done to validate transactions, and timestamps on a large, fast network.  

## The Bottom Line

A double-spending attack is an alteration in a blockchain's operation that allows the attacker to use a token more than once. It is most often thought to be a cryptocurrency problem, but it can happen on blockchains and distributed ledgers.

Users can prevent a double-spend attack by not accepting unconfirmed transactions. Smaller networks are prone to double-spending attacks, so it's best to use cryptocurrencies and blockchains with larger networks.

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