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Written by
Ciara Nightingale
Published on
March 3, 2024

Understanding Blockchain Double Spending Attacks - With examples

In this article, we explore how blockchain double spending attacks occur, with examples and ways to mitigate and avoid them.

Table of Contents

Double spending attacks occur when the same tokens are spent multiple times, undermining the trust and security of financial transactions. This issue is particularly prevalent in blockchain technology, where transactions are decentralized without a central authority managing them.

In this article, we explore, with examples, how double spending can occur, its origins, and the mechanisms employed to mitigate it and avoid it.

What are double spending "attacks" in Blockchain?

Double spending attacks in blockchain are exploits where the same token is used more than once.

If you're not familiar on what is a blockchain, Blockchains consist of a chain of blocks where each block contains the transactions. A newly created block is hashed and contains data, including when the block was added.

Transaction data is verified by nodes and added to the blockchain using the following steps:

  1. A user initiates a transaction (like sending over an asset), which broadcasts the transaction to the network for validation.
  2. Transactions are validated by the network participants (miners or validator nodes).
  3. Validated transactions are grouped into blocks by miners or validators.
  4. Consensus is reached (depending on the chain, the network's consensus mechanism varies) and participants agree on the block's validity and order of transactions.
  5. These blocks are added to the blockchain in a sequential order.
  6. Each block added to the blockchain confirms the transactions it contains.
  7. The number of confirmations for a transaction refers to the number of blocks added to the blockchain after the block containing that transaction.

Double spending can occur when a transaction is confirmed in one block but then a conflicting transaction spending the same token is inserted into a preceding block or the order of the blocks is manipulated. This creates a situation where nodes may mistakenly consider the earlier non-final block as finalized, enabling double-spending.

Remediating and avoiding blockchain double spending attacks is crucial for ensuring trust and adoption of blockchain systems as secure and reliable mediums of exchange.

Background: The Byzantine Generals’ Problem and BFT

The Byzantine Generals’ Problem, introduced by Leslie Lamport, Robert Shostak, and Marshall Pease in 1982, is a theoretical framework for understanding distributed consensus in a network of unreliable nodes without a central authority.

In the problem's scenario, a group of Byzantine generals surround a city and decide whether to attack or retreat. Some generals may be traitors, sending conflicting messages to different generals.

Just as the Byzantine Generals' Problem requires the generals to reach a consensus despite the presence of traitorous actors, the double-spending problem requires blockchain participants to reach a consensus on the validity of transactions despite the potential presence of malicious actors attempting to double-spend.

The problem arises when malicious actors attempt to manipulate the network by propagating conflicting transactions, leading to a double-spending scenario.

Image showing how sybil attacks works, specifically how the bizantine problem works
The Byzantine Generals’ problem and the double-spending problem

Byzantine Fault Tolerance (BFT) algorithms aim to address this problem by enabling a distributed network of nodes to reach a consensus despite faulty or malicious nodes. These algorithms, such as Practical Byzantine Fault Tolerance (PBFT), ensure that honest nodes can agree on the validity of transactions, even in the presence of Byzantine faults.

How Bitcoin Solves the Double Spending Problem

Bitcoin was launched in 2009 by an individual or group using the pseudonym Satoshi Nakamoto. It utilizes a proof-of-work (PoW) based consensus mechanism to address the double-spending problem with previous cryptocurrencies such as DigiCash, and achieve decentralized consensus. The PoW algorithm requires miners to solve computationally intensive mathematical puzzles to validate and add new blocks to the blockchain. This process makes it economically costly for attackers to rewrite transaction history or engage in double-spending attacks, as it would require controlling the majority of the network's computational power.

In blockchain networks, the double-spending problem is primarily resolved through consensus mechanisms based on Sybil resistance mechanisms such as Proof of Work (PoW), Proof of Stake (PoS), and Practical Byzantine Fault Tolerance (PBFT). These mechanisms establish a shared ledger where transactions are verified and added to the blockchain in a secure and immutable manner.

However, some types of double-spending attacks still exist.

Types of Double Spending Attacks in Blockchain

Double-spending attacks can manifest in different forms, including:

Double spending race Attacks

Occur when an attacker exploits the time delay in transaction propagation across a decentralized blockchain network.

  • The attacker initiates two conflicting transactions, each spending the same cryptocurrency on different goods or services.
  • The conflicting transactions are broadcasted to different subsets of nodes in the network, aiming to exploit variations in transaction confirmation times.
  • Inconsistencies in transaction confirmation may lead to a temporary blockchain fork.
  • The attacker aims to have one conflicting transaction confirmed and accepted by a subset of nodes, effectively nullifying the payment (e.g. since the same tokens are being spent).
  • The vendor accepts the pending transaction and sends the goods or services
  • Meanwhile, the network rejects the other pending transaction. However, the attacker has already benefited from the goods or services.
Image showing how blockchain double spending attacks race works
Race attack enabling double-spending

Double spending Finney attacks

Named after Bitcoin pioneer Hal Finney, these attacks involve:

  • A miner withholds a block containing a legitimate transaction for purchasing goods or services.
  • The attacker receives the goods or services assuming the transaction will be confirmed later.
  • Subsequently, the attacker broadcasts the withheld block to the network, which includes a manipulated conflicting transaction.
  • This conflicting transaction redirects the cryptocurrency to another address controlled by the attacker.
  • Although this conflicting transaction invalidates the original legitimate transaction, the attacker, having already received the goods or services, also retains the cryptocurrency used for the purchase since they redirected it to themselves.
Image showing how blockchain double spending attacks work
Finney attack enabling double-spending

51% Attacks

If a node controls the majority of the network's hashing power for Proof-of-Work (PoW) or the cryptocurrency or stake for Proof-of-Stake (PoS) blockchains, it could potentially mine blocks faster than the rest of the network and have control over the confirmation process. This allows it to rewrite transaction history and double-spend tokens. This type of attack is primarily an issue for PoW as PoS blockchains (more on this shortly) often have mechanisms in place to deter such attacks, such as slashing penalties for validators engaging in malicious behaviour.

Image showing how blockchain double spending 51% attacks work
51% attack enabling double-spending

How to avoid Double Spending - Mitigation strategies

Blockchain networks continuously evolve their security protocols and consensus mechanisms to mitigate double-spending risks.

Some effective mitigation strategies include:

Consensus Mechanisms

Blockchain networks employ various consensus mechanisms based on Proof of Work (PoW), Proof of Stake (PoS), Practical Byzantine Fault Tolerance (PBFT), or other Sybil resistance mechanisms to achieve agreement among distributed nodes. These mechanisms ensure that most nodes agree on the validity and order of transactions, making it economically or computationally infeasible to execute double spends.

Proof of Work (PoW)

Consensus is achieved through mining. Miners compete to solve complex mathematical puzzles, and the first one to solve the puzzle adds a new block of validated transactions to the blockchain. This process requires substantial computational power; the fastest miner to solve the puzzle typically adds the block.

In Proof of Stake (PoS)

Validators are chosen to propose and validate blocks based on factors like the amount of cryptocurrency they hold or have staked as collateral. Validators take turns proposing blocks, and the majority of validators must agree on the validity of a block before it is added to the blockchain. The chosen validator adds the block to the blockchain. Validators are economically incentivized to act honestly, as they risk losing their staked tokens if they validate invalid transactions.

Confirmation Mechanisms

Generally, blockchain systems implement confirmation mechanisms, where transactions become progressively more secure as they receive more network confirmation. Confirmations represent the number of blocks added to the blockchain after the block containing the specific transaction. As transactions accrue more confirmations, they gain increased security, making reversing or altering them increasingly difficult.

Transaction Finality

Some blockchain platforms offer features like transaction finality, which is irreversible once a transaction is confirmed. Ethereum's transition to Ethereum 2.0, which adopts the Proof of Stake-based consensus mechanism, aims to provide faster finality than the current Proof of Work system.

Network Monitoring

Continuous network monitoring for suspicious activities or abnormal behavior can help detect and mitigate double spending attempts promptly. Nodes can implement algorithms or methods to detect potentially fraudulent transactions, and consensus rules to identify and reject potentially fraudulent transactions.

Confirm if UXTO Has Been Spent

Unspent Transaction Output (UTXO) represents outputs of previous transactions that have not been spent. Before a new transaction is processed, the blockchain network verifies that the UTXO being referenced has not already been spent, ensuring it has only been used once and preventing double-spending attempts.

Education and Awareness

Educating users and stakeholders about the risks of double spending and the importance of verifying transaction details before confirming transactions with smart contract development and security courses such like these ones. Promoting best practices for securely managing cryptocurrency assets can help prevent unauthorized transactions and mitigate the risk of double spending.

Security and Community Vigilance

Implementing security best practices, including encryption and authentication mechanisms, helps prevent unauthorized tampering with data, therefore mitigating potential attacks like double-spending. Additionally, active participation in security audits and bug bounties enhances overall security and aids in identifying and resolving potential double-spending vulnerabilities in smart contracts.

Preventing Replay Attacks

Replay attacks can be used to execute double-spending by retransmitting valid transactions using the same funds. Preventing replay attacks helps mitigate the risk of replay attacks and, therefore, reduces the risk of double spending.To learn about replay attacks and how to mitigate them, refer to the Comprehensive Guide to Replay Attacks article.

Example Double Spending Exploits and Audit Findings

  • Ethereum Classic and Bitcoin Gold: 51% double-spending attacks on Bitcoin Gold and Ethereum Classic allowed the attackers to double spend $70k and  $1.68M respectively in 2020.
  • Flexa: An audit conducted by Trail of Bits discovered a vulnerability in the protocol that had the potential to lead to a double-spending attack. While technically a replay attack, this finding demonstrates how vulnerabilities in smart contracts can lead to double-spending. The PendingDepositRefund event was missing a nonce parameter which might have allowed a third-party tool to not know which deposit was refunded.


The double-spending problem poses a significant challenge to blockchain networks, threatening the integrity and trustworthiness of transactions and therefore preventing mainstream adoption.

Through Byzantine Fault Tolerance algorithms and consensus mechanisms based on Sybil resistance mechanisms such as PoW and PoS, blockchain networks strive to achieve consensus and prevent double-spending attacks.

Getting your protocol audited significantly decreases the probability of an attack happening.

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