51% Attack

What is a 51% attack?

A 51% attack happens when an individual or group gains control of more than half of a blockchain network’s validation power (hashing power for proof-of-work or stake in proof-of-stake). With majority control they can:

• Prevent or delay new transactions from being confirmed.
• Reverse transactions they previously made, enabling double-spending.
• Potentially block other miners/validators from participating (a form of denial-of-service).

This attack undermines the consensus rules that make a blockchain trustworthy. It is difficult and costly on large, well-distributed networks but represents a real threat to smaller or poorly secured chains.

How a 51% attack works

• Blockchains rely on majority consensus: the longest (or heaviest) chain accepted by most participants is treated as canonical.
• An attacker with majority power can privately build an alternate chain that excludes or changes transactions.
• If the attacker’s chain overtakes the honest chain, the network will adopt it, allowing reversed transactions and double-spends.
• Changing deep historical blocks is progressively harder; practical attacks target recent blocks and unconfirmed transactions.

Costs and practical obstacles

Executing a successful 51% attack requires substantial resources and precise timing:

• Hardware and energy: For proof-of-work chains, attackers need vast hashing capacity—usually thousands of high-end ASICs—plus power and infrastructure, which is prohibitively expensive for large networks.
• Rental services: Cloud-based hashing marketplaces let attackers rent large amounts of hashpower for a limited time, lowering the barrier for attacking smaller networks.
• Timing: Attackers must outpace honest miners/validators to get their alternate chain accepted. Even with majority power, introducing the altered chain at the right moment is technically challenging.
• Economic disincentives on proof-of-stake: On proof-of-stake networks an attacker would need to control a majority of staked tokens. Protocol defenses (e.g., slashing) and social/community responses make such attacks costly and self-destructive for attackers.

Consequences of a successful attack

• Double-spending: The attacker can reverse their own transactions and spend the same funds twice.
• Transaction censorship: Honest users’ transactions can be blocked or delayed.
• Network disruption: Confidence in the chain drops, exchanges may halt withdrawals, and token value can collapse.
• Loss of long-term trust: Even a temporary takeover can have lasting reputational and economic damage.

Which networks are most at risk?

• Smaller chains with low total hashpower or low total staked value are the most vulnerable because attackers can acquire or rent sufficient resources at a manageable cost.
• GPU-mined or lightly secured networks are particularly exposed because rented hashpower can more easily match their capacity.
• Major, long-established networks with high distributed hashpower or large amounts of staked assets are considerably harder to attack.

Examples: Several smaller cryptocurrencies have experienced chain reorganizations and 51% attacks in the past, while major networks remain largely secure due to scale and decentralization.

Bitcoin and Ethereum: how safe are they?

• Bitcoin (proof-of-work): Its enormous total hashpower and wide distribution of miners make a coordinated 51% attack extremely expensive and operationally difficult. However, concentration of mining in a few large pools is a point of ongoing concern if collusion were to occur.
• Ethereum (proof-of-stake): After its transition to proof-of-stake, an attacker would need to control a majority of the staked ETH. Protocol mechanisms (such as slashing) and the huge economic cost of acquiring such a stake make an attack highly impractical.

Has it happened before?

Yes—smaller or newer blockchains with limited security have suffered successful 51% attacks and large chain reorganizations. These incidents typically targeted lower-hashrate or poorly distributed networks.

Key takeaways

• A 51% attack gives an attacker temporary control to reverse transactions and censor activity, but it does not allow them to create coins out of thin air or break the underlying cryptography.
• The risk is highest for small or poorly decentralized networks; large networks are protected mainly by the cost and logistics required to gain majority control.
• Cloud hash rentals and concentrated mining power increase theoretical risk, so decentralization of validators and robust economic defenses remain important safeguards.