51% Attack

What Is a 51% Attack (51% Attack)?

A 51% attack refers to the majority control over block production and the ability to rewrite transaction history on a blockchain.

This is a majority control risk in blockchain consensus mechanisms. If a single entity obtains more than half of the key resources on a chain—meaning over 50% of the network’s computational power in Proof of Work (PoW) or more than half of the staked assets in Proof of Stake (PoS)—it can dominate block production, alter the order of recent blocks, and reverse transactions that have been broadcast but are not yet final.

Here, “computational power” refers to mining capacity, while “stake” denotes the amount of tokens participating in consensus. Malicious rewriting of transactions may lead to double-spending, where the same asset is spent more than once.

Why Does Understanding 51% Attacks Matter?

51% attacks directly impact the security of funds and the overall credibility of a blockchain network.

For regular users, the most immediate effect is transaction rollbacks on exchanges—assets that appear deposited may be reversed, disrupting financial planning. Merchants may also see received payments canceled, resulting in losses.

For projects and broader blockchain ecosystems, frequent chain reorganizations can undermine trust among developers and institutions. Exchanges may raise confirmation requirements or suspend deposits and trading for affected tokens, leading to decreased liquidity and increased price volatility.

How Does a 51% Attack Work?

A 51% attack is executed by privately building a longer chain and then replacing the public chain.

1. The attacker amasses majority resources and privately mines or proposes blocks without broadcasting them, creating a “hidden longer chain.”
2. Meanwhile, the attacker sends transactions on the public chain—such as depositing to an exchange and swapping assets. These transactions appear confirmed but are not yet final. Confirmation count refers to how many blocks have been added after the transaction; more confirmations mean lower risk of reversal.
3. Once the hidden chain surpasses the public chain in length or weight, the attacker publishes it. By consensus rules, the network accepts the longer or heavier chain, replacing previous transaction history—thus deposits or payments are rolled back, enabling double-spending.

In Proof of Stake, if a single party controls the majority stake, they can similarly dictate recent block order and finality. Although penalties (like slashing for malicious forks) reduce attack incentives, concentrated control remains a security risk.

Typical Manifestations of 51% Attacks in Crypto

Common signs include transaction reversals, abnormal chain reorganizations, and emergency platform responses.

On exchanges, deposits usually require a set number of confirmations. If abnormal reorganizations or concentrated block production are detected, platforms may temporarily increase confirmation requirements or suspend deposits/withdrawals to prevent double-spending. For example, Gate’s risk management increases confirmations for small PoW tokens upon reorg alerts and notifies users until stability returns.

Mining pools and block explorers may display “reorg” notices if recent blocks are replaced; sudden concentration of block production among few nodes is another red flag.

In DeFi scenarios, if a base layer blockchain undergoes reorganization, transaction order changes can disrupt loan liquidations, cross-chain bridge settlements, and protocol operations, possibly triggering emergency protection modes and freezing certain functions.

How Can 51% Attacks Be Mitigated?

Protection requires coordinated efforts across networks, platforms, and users.

• Projects should enhance decentralization: incentivize broader miner or validator participation, cap mining pool dominance, implement checkpoints or stronger finality to shorten rollback windows; in PoS systems, reinforce slashing penalties for malicious behavior.
• Exchanges can adopt robust deposit policies: set higher confirmations for small PoW chains, adjust dynamically; deploy reorg monitoring and computational power concentration alerts; enforce withdrawal restrictions such as “withdrawal to same address only” or delayed withdrawals to reduce double-spending risks. For instance, Gate increases confirmation thresholds during reorg warnings and alerts users about potential risks.
• Users should plan fund movements carefully: for large deposits or payments, choose chains with higher stability or wait for additional confirmations; avoid sensitive actions like cross-chain transfers or liquidations during periods of reorg news.
• Ecosystem collaboration: mining pools, explorers, and security teams should share intelligence and standardize alert thresholds; on chains with high mining pool concentration, encourage merged mining or diversified algorithms to distribute risk.

Recent Trends and Notable Data on 51% Attacks

Large blockchains have become more secure in 2025, but smaller chains still face significant risks.

Over the past year, Bitcoin’s total network hashrate reached record highs (hundreds of EH/s in Q3), making attacks prohibitively expensive. In contrast, some small PoW chains operate at just tens to hundreds of TH/s; recent rental market data shows computational power can cost as little as $0.2–$0.5 per TH/s per hour. This means controlling a majority hashrate for one hour could cost under $10,000—posing real threats during periods of low liquidity.

Incident reports from 2024 show that most notable 51% attacks targeted low-cap PoW chains; this year, reported cases have decreased thanks to improved monitoring and response strategies by exchanges and mining pools. However, risks vary by token—users should monitor real-time mining pool dashboards and exchange announcements.

For Proof of Stake networks in 2025, security discussions focus on the “majority stake versus finality” dilemma: while majority stake can influence short-term block ordering, strong slashing penalties and social recovery mechanisms greatly increase the long-term cost of attacks. Recently, many chains have integrated extra security modules such as rapid finality and additional validation to minimize rollback windows.

How Does a 51% Attack Differ from a Sybil Attack?

The two attacks target different resources and have distinct goals.

A 51% attack relies on majority computational power or stake to rewrite recent ledger history at the consensus layer. A Sybil attack manipulates network propagation or voting by creating fake identities or controlling multiple nodes—it does not require substantial computational power or stake, focusing instead on identity-level manipulation.

Understanding this distinction helps choose appropriate defenses: for 51% attacks, enhance confirmations and decentralization; for Sybil attacks, introduce identity costs and reputation mechanisms.

Related Terms

• 51% Attack: When a single miner or mining pool controls over half of a network’s computational power, enabling malicious actions like double-spending.
• Proof of Work (PoW): A consensus mechanism where miners solve complex mathematical problems to validate transactions and generate new blocks.
• Mining Pool: A collaborative group where miners combine resources to increase block production probability and share rewards.
• Double-Spending Attack: An attempt by an attacker to spend the same funds twice—undermining blockchain transaction security.
• Computational Power: The number of calculations a miner can perform per unit time; directly impacts mining rewards and network security.

FAQ

Are Small Blockchain Projects More Vulnerable to 51% Attacks Than Large Ones?

Yes—smaller projects face higher risks because their computational power is more distributed and cheaper for attackers to control. In contrast, Bitcoin’s high concentration in large mining pools means an attack would cost billions of dollars—making it practically unfeasible. Projects can improve security by increasing node counts and optimizing consensus mechanisms.

If I Hold a Coin Subject to a 51% Attack, Is My Asset Directly Threatened?

The risk depends on attack type. Double-spending attacks can steal coins directly but rarely target individual wallets; chain reorganizations may reverse transactions or cause price drops. The best protection is to store assets on major exchanges like Gate rather than self-custody wallets since exchanges employ multi-layer verification mechanisms.

Is PoW or PoS More Prone to 51% Attacks?

PoW systems are generally more susceptible—controlling 51% of computational power is all that’s needed to launch an attack. While PoS can theoretically be attacked if someone acquires over 50% stake, doing so is extremely costly and would drive up token prices. In practice, most recorded 51% attacks target small PoW coins; PoS projects see such incidents far less frequently.

Does a Higher Number of Confirmations Mean Greater Security?

Usually yes—the more confirmations a transaction has, the further it is from potential rollback. Six confirmations are often recommended for final settlement; for large amounts or smaller tokens, waiting for more may be prudent. Exchanges like Gate set sufficient confirmation thresholds to safeguard user assets.

What Measures Can Development Teams Take to Prevent 51% Attacks?

Common approaches include: adopting hybrid consensus mechanisms (e.g., PoW+PoS) to raise attack costs; increasing node counts to distribute computational power; implementing checkpointing to prevent long-range attacks; adjusting difficulty more frequently to make attacks harder to sustain; forming emergency response teams to monitor unusual hashrate shifts and issue timely alerts.

References & Further Reading

• https://www.investopedia.com/terms/1/51-attack.asp
• https://www.dci.mit.edu/projects/51-percent-attacks