Definition
51% Attack is a scenario in which a single entity controls more than half of a Proof-of-Work network’s hashrate, giving it enough power to outpace every other miner and rewrite recent history on the blockchain. With a sustained majority of mining power, that entity can reorganize the chain, censor transactions, and double-spend coins it has already moved.
Also known as: majority attack, majority hashrate attack.
How a majority attack works
Bitcoin’s security rests on the longest-chain rule: honest nodes accept whichever valid chain has accumulated the most proof-of-work. An attacker with the majority of network hashrate can mine a private chain faster than the rest of the world combined. By withholding that chain and then releasing it, the attacker forces a reorg in which honest blocks become orphaned and the attacker’s version becomes canonical.
The practical payoff is the double-spend. The attacker pays someone in Bitcoin, waits for the merchant to see confirmations, then publishes a longer private chain that omits the original payment. A 51% attacker can also censor transactions by refusing to include them in blocks, and can selfishly mine to claim a disproportionate share of the block reward. What it cannot do is steal coins from arbitrary wallets, spend coins it never controlled, or alter old, deeply buried blocks — those would require forging digital signatures or redoing all the work since.
Why Bitcoin has held up
For Bitcoin, a sustained 51% attack is extraordinarily expensive. With the global network secured by a hashrate measured in hundreds of exahash per second, an attacker would need to acquire and power an equivalent fleet of ASICs — a capital and energy outlay that dwarfs anything a rational attacker could expect to recoup, since the very act of attacking would crater the value of the coins they hold. The difficulty adjustment also pushes back: as honest hashrate returns, the attacker must keep spending to maintain a lead. This is why majority attacks are a constant threat to small Proof-of-Work coins with cheap, rentable hashrate, but remain theoretical for Bitcoin itself.
The real risk: pool centralization
The more grounded concern is not one miner buying half the world’s machines, but a handful of mining pools coordinating the hashrate that thousands of independent miners point at them. In a traditional Stratum setup, the pool — not the miner — builds the block template and the coinbase transaction. When the two largest pools together direct a sizable chunk of total hashrate, the question shifts from « can someone attack the chain? » to « who already decides which transactions get mined? »
That is the centralization risk a 51% attack really illuminates: the danger lives in template control, not just raw machine ownership. Newer protocols address it directly. Stratum V2 lets miners construct their own templates through Job Declaration, and OCEAN’s DATUM overlay lets a miner running a full node build blocks from its own mempool and broadcast them itself — handing transaction-selection power back to the people running the hardware. Each is one more layer decentralized.
Why a home miner cares
If you run an ASIC at home, you are part of the answer. Every Bitaxe or ASIC pointed at a decentralization-focused pool, or better yet at solo mining against your own node, dilutes the share any single entity can command. Choosing where your hashrate goes is a vote on who controls block construction.
You can lean further into sovereignty by pairing a node with a template-choosing protocol, or by exploring open-source firmware that puts pool and template selection fully in your hands. The combination of open-source mining hardware and miner-built templates is how the network stays resistant to the very attack this term describes. For a side-by-side look at how different stacks handle pool and protocol choice, see the firmware comparison.
Related terms: Network Hashrate, Longest Chain Rule, Selfish Mining, Orphan Block, Mining Pool, Proof of Work
In Simple Terms
A theoretical attack requiring over 50% of network hashrate to double-spend or censor transactions.
51% Attack is a scenario in which a single entity controls more than half of a Proof-of-Work network's hashrate, giving it enough power to outpace every other miner and rewrite recent history on the blockchain. With a sustained majority of mining power, that entity can reorganize the chain, censor transactions, and double-spend coins it has already moved.
Also known as: majority attack, majority hashrate attack.
How a majority attack works
Bitcoin's security rests on the longest-chain rule: honest nodes accept whichever valid chain has accumulated the most proof-of-work. An attacker with the majority of network hashrate can mine a private chain faster than the rest of the world combined. By withholding that chain and then releasing it, the attacker forces a reorg in which honest blocks become orphaned and the attacker's version becomes canonical.
The practical payoff is the double-spend. The attacker pays someone in Bitcoin, waits for the merchant to see confirmations, then publishes a longer private chain that omits the original payment. A 51% attacker can also censor transactions by refusing to include them in blocks, and can selfishly mine to claim a disproportionate share of the block reward. What it cannot do is steal coins from arbitrary wallets, spend coins it never controlled, or alter old, deeply buried blocks — those would require forging digital signatures or redoing all the work since.
Why Bitcoin has held up
For Bitcoin, a sustained 51% attack is extraordinarily expensive. With the global network secured by a hashrate measured in hundreds of exahash per second, an attacker would need to acquire and power an equivalent fleet of ASICs — a capital and energy outlay that dwarfs anything a rational attacker could expect to recoup, since the very act of attacking would crater the value of the coins they hold. The difficulty adjustment also pushes back: as honest hashrate returns, the attacker must keep spending to maintain a lead. This is why majority attacks are a constant threat to small Proof-of-Work coins with cheap, rentable hashrate, but remain theoretical for Bitcoin itself.
The real risk: pool centralization
The more grounded concern is not one miner buying half the world's machines, but a handful of mining pools coordinating the hashrate that thousands of independent miners point at them. In a traditional Stratum setup, the pool — not the miner — builds the block template and the coinbase transaction. When the two largest pools together direct a sizable chunk of total hashrate, the question shifts from "can someone attack the chain?" to "who already decides which transactions get mined?"
That is the centralization risk a 51% attack really illuminates: the danger lives in template control, not just raw machine ownership. Newer protocols address it directly. Stratum V2 lets miners construct their own templates through Job Declaration, and OCEAN's DATUM overlay lets a miner running a full node build blocks from its own mempool and broadcast them itself — handing transaction-selection power back to the people running the hardware. Each is one more layer decentralized.
Why a home miner cares
If you run an ASIC at home, you are part of the answer. Every Bitaxe or ASIC pointed at a decentralization-focused pool, or better yet at solo mining against your own node, dilutes the share any single entity can command. Choosing where your hashrate goes is a vote on who controls block construction.
You can lean further into sovereignty by pairing a node with a template-choosing protocol, or by exploring open-source firmware that puts pool and template selection fully in your hands. The combination of open-source mining hardware and miner-built templates is how the network stays resistant to the very attack this term describes. For a side-by-side look at how different stacks handle pool and protocol choice, see the firmware comparison.
Related terms: Network Hashrate, Longest Chain Rule, Selfish Mining, Orphan Block, Mining Pool, Proof of Work