51% Attack

What Is a 51% Attack?

A 51% attack refers to a scenario where an individual or group controls more than half of the blockchain network's record-keeping power, allowing them to dominate the most recent version of the ledger, rewrite transactions, and perform double-spending. Imagine the blockchain as a shared ledger maintained by multiple parties—whoever holds the majority "voting power" can ensure their version prevails.

In Proof of Work (PoW) networks, this "record-keeping power" is known as computational power or hash rate—the speed at which mining machines attempt solutions. In Proof of Stake (PoS) networks, it corresponds to "staking power," determined by the amount and influence of tokens held and staked. Controlling the majority means your chain can surpass others'.

Why Do 51% Attacks Occur?

51% attacks typically happen when record-keeping power is highly concentrated, network security budgets are insufficient, or when hash rate can be temporarily rented. The main motivation is economic gain—for instance, profiting by double-spending the same coins.

Common factors increasing risk include: excessive mining pool concentration granting immense hash rate to a few entities; low total hash rate on smaller chains, making it easier for external attackers to surpass the network using cloud or rented computing power; and aggressive transaction confirmation strategies, where merchants release goods after minimal confirmations, exposing themselves to chain reorganizations.

How Does a 51% Attack Work?

The attack relies on the "longest chain rule." In PoW, the network recognizes the longest chain—i.e., the one with the most accumulated work—as valid. If an attacker controls over half of the network’s hash rate, they can privately mine a longer "secret chain." Once it surpasses the public chain, they release it, causing the network to accept this new chain as canonical.

A "chain reorganization" occurs when recent pages of the ledger (blocks) are replaced with an alternative version. The "number of confirmations" refers to how deeply a block is recognized by the network; more confirmations mean lower risk of reorganization.

A typical attack flow: the attacker makes a payment to a merchant on the public chain, receives goods after a few confirmations, while simultaneously mining a private chain that omits this payment. When their private chain becomes longer and is broadcasted, the network switches to it, invalidating the original payment. The attacker keeps both the goods and their coins—this is double-spending.

What Are the Consequences of a 51% Attack?

The direct result is double-spending, causing losses for recipients. Other impacts include transaction rollbacks, diminished user trust, increased confirmation requirements by nodes and exchanges, and short-term rises in network usage costs. Asset prices and liquidity may suffer, project reputation can be damaged, and developers or ecosystem participants may leave.

For exchanges and merchants, responses often include raising deposit and payment confirmation thresholds or temporarily suspending withdrawals or deposits for affected chains. Cross-chain bridges may halt services involving compromised networks to prevent malicious rollbacks.

How Does a 51% Attack Differ Between Proof of Work and Proof of Stake?

In Proof of Work systems, a 51% attack depends on hash rate. Gaining majority control requires deploying mining hardware or renting computational power, with primary costs being hardware and electricity. Superior hash rate enables faster block production and reorganizations.

In Proof of Stake systems, an attack requires controlling over half of staked voting power. This demands acquiring large amounts of tokens and entails the risk of slashing penalties. Many PoS chains feature "finality," meaning blocks become irreversible after a certain voting threshold, with malicious actors facing token destruction or other penalties. Overall, PoS attacks hinge more on token economics and governance than on energy or hardware.

Real-World Examples of 51% Attacks

Public reports indicate that Ethereum Classic suffered multiple 51% attacks in 2019 and 2020. In August 2020, a deep reorganization led to transaction rollbacks and service suspensions. Bitcoin Gold also faced reported attacks in 2018 and 2020, causing losses for exchanges and merchants. These cases show that smaller PoW chains are more vulnerable to hash rate concentration and external rental attacks.

As of 2025, leading PoW networks like Bitcoin have not experienced successful deep 51% attacks thanks to their massive hash rate, wide miner distribution, and significant economic scale. However, mining pool centralization remains an ongoing concern that requires vigilant governance.

How Can Individual Users and Merchants Reduce 51% Attack Risks?

The core defense strategies involve increasing confirmation requirements, choosing safer networks, and using risk monitoring tools.

1. Set Sufficient Confirmation Numbers: The more confirmations a transaction has, the safer it is. For example, on Gate, BTC deposits usually require at least six confirmations; merchants can set even higher confirmation counts (dozens) for smaller chains.
2. Use Tiered Approaches Based on Transaction Value: Small payments can use fewer confirmations; large transactions should have more confirmations or utilize escrow/clearing windows to avoid immediate settlement.
3. Choose Secure Chains for Settlement: Avoid large transactions on chains with low total hash rate or recent abnormal reorganizations.
4. Use On-Chain Monitoring Tools: Monitor abnormal hash rate changes, cross-pool hash migrations, or deep reorganizations; suspend credits or increase confirmations in response to alerts.
5. Ensure Fund Safety: For cross-chain transfers and exchange deposits, pay attention to platform announcements and risk-control updates; split deposits if necessary to diversify risk.

How Can Projects and Mining Pools Prevent 51% Attacks?

Supply-side measures focus on decentralization and increasing attack costs.

1. Reduce Mining Pool Concentration: Encourage smaller pools, limit any single pool’s block share, transparently disclose block distribution to mitigate centralization risks.
2. Increase Total Hash Rate & Merge Mining: Use merge mining with larger networks so attackers must overcome a much higher total hash rate.
3. Introduce Finality or Checkpoints: Set unchangeable milestone blocks or restrict reorganization depth to prevent long-range reorganizations enabling double-spends.
4. Adjust Economic Incentives: Increase block rewards or fee sharing to attract more honest miners and expand security budgets.
5. Improve Node Software Reorg Policies: Implement abnormal reorg detection and alerts, delay large credits, temporarily reduce block weights for suspicious chains.

Key Takeaways on 51% Attacks

A 51% attack stems from control over majority record-keeping power, exploiting consensus rules where “the longest chain wins.” Risks are highest for smaller PoW chains due to hash rate concentration and rentable computing power. Effective defenses include increasing confirmations, choosing secure networks, monitoring anomalies, and refining consensus/economic models. For users and merchants, combining confirmation count management with value-based tiering and watching platform risk controls (e.g., Gate’s deposit confirmations) can effectively reduce rollback and double-spending risks. For projects and mining pools, increasing decentralization, adopting finality and checkpoints, merge mining, and economic incentives can significantly raise attack costs and strengthen network resilience.

FAQ

Do 51% Attacks Actually Happen? Any Real Cases?

Yes, 51% attacks have occurred in practice. Historically, Ethereum Classic (ETC) suffered multiple such attacks in 2020 as attackers used overwhelming hash rate to roll back transactions. Smaller blockchains are more vulnerable due to dispersed hash rate. In contrast, major chains like Bitcoin are much harder to attack because legitimate mining pools control most hash rate, making such attacks prohibitively expensive.

As an Ordinary Trader, Does a 51% Attack Directly Threaten My Assets?

Direct threats are relatively limited but still warrant caution. If you conduct large transactions or hold assets on smaller blockchains, attackers could potentially roll back transactions to steal funds. It is recommended to use major platforms like Gate for trading top public-chain assets and always wait for sufficient block confirmations (typically six or more for large transfers) before considering funds final—this greatly reduces rollback risks.

Why Aren’t All Blockchains Vulnerable to 51% Attacks?

A blockchain’s resistance to 51% attacks depends on its hash rate/stake distribution. Bitcoin and Ethereum are highly resistant due to broad participation in mining/staking and widely distributed hash rates; attacking these would require astronomical investment. In contrast, smaller chains with fewer participants are more exposed. Chains using Proof of Stake (PoS) are generally harder to attack than those using Proof of Work (PoW), as attackers would need to acquire vast amounts of tokens—risking enormous economic losses if caught.

Will My Wallet Funds Disappear After a 51% Attack?

Funds will not vanish outright but could be reassigned through transaction history rewrites. The essence of a 51% attack is altering transaction records so previous transfers might be erased—returning assets to the attacker’s wallet. Your private key remains yours; however, blockchain records could be rolled back. When self-custodying assets via hardware wallets (with uncompromised private keys), you remain relatively secure; when using centralized exchanges, always select reputable platforms.

Does Proof of Stake Prevent 51% Attacks?

Proof of Stake (PoS) offers stronger resistance against 51% attacks. In PoS systems, attackers must control over half the total tokens staked—a massive economic commitment. Attacking would severely devalue their own holdings, making it economically irrational. In contrast, PoW attackers can simply rent mining power at comparatively lower costs—this is the key difference between the two mechanisms.