What Is a 51% Attack, and How Does It Work?

In the world of crypto — abundant with hacks, social engineering, malicious software, and even fraudulent on-chain transfer requests — one threat stands out for its scale, targeting an entire blockchain ecosystem: the 51% attack.

A 51% attack on a blockchain network refers to a type of exploit when an individual or a group controls over half — at least 51% — of network consensus. Usually, this kind of attack refers to blockchains that use proof-of-work (PoW) consensus where miners validate transactions, like Bitcoin (BTC) or Ethereum Classic (ETC). In the case of a PoW chain, a 51% attack entails controlling the majority of mining power, letting the attacker manipulate the chain.

The possibility of such an attack is tied to the very nature of crypto — decentralization. Unlike a traditional financial system, like a bank, where a central authority controls and verifies all transactions, blockchains rely on decentralized networks of miners or validators to maintain consensus, a system where the majority agrees on which transactions are valid.

Decentralization is what makes cryptocurrencies innovative, yet it also opens the door to the risk of concentrated control, if one group gains more dominance than it should.

Can a 51% Attack Happen on a PoS Network?

In theory, a 51% attack can happen on proof-of-stake (PoS) networks. For networks like Ethereum, which transitioned from proof-of-work to proof-of-stake consensus, a 51% attack would require a bad actor to control more than half of the network’s staked tokens, instead of mining power.

However, while technically possible, such an attack would be much harder and riskier to pull off, given that malicious validators can have their staked tokens slashed, making the attack financially self-destructive.

What Happens During a 51% Attack?

With majority control of the network, attackers can prevent new transactions from gaining confirmations, halt payments, and even reverse transactions. Big networks like Bitcoin are safer because no single group can easily control most of their computing power, given the prohibitive cost and mining difficulty.

But smaller chains are much more vulnerable, as mining power can be relatively cheaply rented for short periods through services like NiceHash, significantly lowering the cost of executing such an attack.

Data from Crypto51, which tracks the current cost of 51% attacks across different networks, shows that carrying out a 24-hour attack on Ethereum Classic would cost more than $144,000, while doing the same on Bitcoin would cost nearly $53 million, as of October 2025.

Privacy chain Monero was the victim of several network “reorganization” attacks — also known as reorgs — in August and September 2025, leading to speculations that the network suffered a 51% attack. In the incident, a mining pool tied to Qubic, a Layer 1 chain, grabbed more than half the hash power and pushed through a longer chain, undoing about 18 blocks and sending roughly 117 to 118 transactions back to the mempool so payments were delayed.

And these attacks aren’t rare. A report from MIT’s Digital Current Initiative pointed out that between 2019 and 2020 there were over 40 network reorganizations that were six or more blocks deep on coins such as BTG, HANA, VTC, XVG, EXP and LCC.

What Is a Double-Spend Attack?

During a 51% attack, crypto exchanges often get the signal first, as attackers can try to double-spend tokens — spending what appears to be the same assets more than once. In this process, the attacker sends tokens from the chain under their control to an exchange, while manipulating the chain to erase that first payment, thus duplicating the tokens.

Ethereum Classic — a hard fork of Ethereum that remained PoW — suffered multiple 51% attacks in 2019 and 2020, and some exchanges lost funds in double-spend cases.

Here’s how it works: hackers send coins to an external party — like an exchange, where they can swap it for another asset to cash out — while secretly building a different version of the exploited blockchain, where that particular transaction is skipped. If the attacker’s new chain ends up being longer, the network switches over to it, in effect cancelling the first payment and letting the hacker use those same coins again.

As the team behind Ethereum Classic explained in a blog post, if the new alternative blockchain created by an attacker has “more accumulated work than the main chain,” or more activity, then “all nodes in the ETC network will abandon the main chain they were working on and immediately adopt the new alternative chain built by the attacker.”

In effect, the attackers in a 51% attack can walk away with both the assets on a new chain under their control, and whatever value they were able to get from swapping the double-spent assets on an exchange. As The Defiant reported earlier, the attacks on Ethereum Classic attacks caused some exchanges, like Coinbase, to slow down confirmations in an effort to keep the attacker from cashing out, making users wait more than 10 hours for their ETC deposits.

Hacken, a cybersecurity firm, reminded in its explainer that the infamous Bitcoin Gold attack of May 2018 enabled the attackers to double-spend roughly $18 million.

Not Always 51%

The history of the 51% attack goes even further back. Although early digital cash systems like DigiCash and E-gold were designed to enable secure online payments without relying on traditional banks, they were still vulnerable to attacks if only around 33% of their nodes turned malicious, a vulnerability known now as the Byzantine Generals Problem.

Bitcoin’s introduction of the Nakamoto Consensus mechanism using proof-of-work raised the threshold for such attacks to 51%, tying security directly to computational strength, rather than node count.

Ethereum Classic has also grown stronger over time. After Ethereum switched to proof-of-stake in 2022, some of its old mining power shifted to ETC, boosting its hashrate from 24 TH/s to over 150 TH/s and making it the largest proof-of-work chain supporting smart contracts.

With more hashing power, better confirmations, and extra protections, attacks like double-spends are now much harder to pull off on the network.

While 51% remain a risk for blockchain networks — especially those with lower total hash rate or more concentrated mining power — in practice, their success depends on multiple factors, and larger networks are increasingly resilient. As the crypto ecosystem more broadly matures and grows, detection and response to these kinds of attacks is is also improving.