51% Attack
Last Updated: October 30, 2026
Quick Answer: A 51% attack is an attack on a blockchain in which one party, or a group acting together, controls more than half of the network’s mining power (in proof of work) or a majority of staked tokens (in proof of stake), letting it rewrite recent blocks, reverse its own payments (double spend) and censor transactions. It cannot create coins out of nothing or take funds from other people’s wallets, but it can seriously damage trust in a network.
Key Takeaways
- Bitcoin’s white paper assumes security holds “as long as a majority of CPU power is controlled by nodes that are not cooperating to attack the network”
- Bitcoin’s hashrate was about 1,048 EH/s (roughly 1.05 zettahashes per second) on October 30, 2026, which makes a majority attack extremely costly
- Mining pools are not single miners, but concentration matters: in the week to October 30, 2026, the two largest pools mined about 46.8% of Bitcoin blocks
- Smaller proof of work coins are the usual victims: the Ethereum Classic project says its network suffered two 51% attacks in January 2019 and three in August 2020
- On proof of stake Ethereum, different thresholds (about 33%, 50% and 66% of stake) enable different attacks, and attackers risk losses through slashing, the inactivity leak or a community fork
How a 51% Attack Works
On a proof of work chain, nodes follow the valid chain with the most accumulated work. An attacker with majority hashrate can:
- Send coins to an exchange or merchant on the public chain.
- Secretly mine a competing chain in which those coins go back to the attacker’s own address.
- Wait until the exchange credits the deposit and the attacker withdraws other assets.
- Publish the longer private chain. Nodes switch to it (a “reorg”), and the original payment disappears.
The Bitcoin white paper shows why confirmations matter. If an attacker has 10% of the hashrate, the chance of catching up from 5 blocks behind falls below 0.1% (0.0009137). With 30%, a recipient needs to wait 24 blocks for the same level of safety. Above 50%, the attacker eventually catches up however long the victim waits. The paper also argues that a rational attacker “ought to find it more profitable to play by the rules.”
| Attacker can | Attacker cannot |
|---|---|
| Reverse its own recent transactions (double spend) | Spend coins from other people’s addresses |
| Exclude or delay other users’ transactions | Change the coin supply rules or block rewards accepted by nodes |
| Orphan blocks mined by others | Rewrite very old history cheaply, since each block adds more work to redo |
Proof of stake is different. According to ethereum.org, an attacker with 33% of staked ETH can stop the chain from finalizing, one with 34% could cause conflicting finality under network delays, one with more than 50% can censor transactions and perform short reorgs, and one with 66% could finalize its preferred chain. Defenses include slashing, the inactivity leak that drains non participating validators, and a social layer response that could adopt an honest minority fork, making the attacker’s stake far less valuable.
Examples
Bitcoin (2026). Mempool.space estimated Bitcoin’s hashrate at about 1,048 EH/s on October 30, 2026. In the week to that date, Foundry USA mined about 26.4% of blocks, AntPool 20.4% and F2Pool 17.5%. Pools coordinate many independent miners who can switch pools, so a large pool share is not the same as one owner controlling that hashrate.
Ethereum Classic (2019 and 2020). Ethereum Classic, the original Ethereum chain that continued after the 2016 DAO fork and still uses proof of work, is a frequently cited case. According to a November 21, 2023 post on the project’s official website, there were two 51% attacks on the network on January 5 and 7, 2019, and three in August 2020.
Monero (2025). In August 2025, Qubic, a separate blockchain project, said it had taken majority control of Monero’s hashrate, according to media reports at the time. A later academic paper on the campaign, published on arXiv, reported elevated orphaning and deeper reorganizations but no sustained majority control.
Risks and Common Misconceptions
- “A 51% attack lets hackers steal anyone’s coins.” Transactions still need valid signatures, so attackers can only reverse their own payments and censor others.
- “Bitcoin cannot be attacked.” It can in theory, but the hardware and energy required at over 1,000 EH/s, plus the likely fall in the value of the coins an attacker holds, make it extremely expensive.
- “Proof of stake makes 51% attacks impossible.” It changes the economics: an attacker must buy a majority of stake and risks losing it through slashing or a community fork.
- Who is most at risk: small proof of work coins that share a mining algorithm with a much larger coin, because hashrate can be rented or redirected cheaply.
Related Terms
Frequently Asked Questions
What is a 51% attack in simple terms?
A 51% attack happens when one person or group controls more than half of a blockchain’s mining power or staked tokens. With that majority, they can build a longer competing chain, reverse their own recent payments and block other transactions. They cannot steal coins from other wallets or change the protocol rules, but the attack can cause losses for exchanges and merchants.
Has Bitcoin ever had a 51% attack?
There is no publicly documented successful 51% attack on Bitcoin’s main network. Its hashrate was about 1,048 EH/s on October 30, 2026, so an attacker would need huge amounts of specialized mining hardware and energy. Large mining pools occasionally approach high shares of block production, which draws attention, but pools are made up of many independent miners who can leave if a pool misbehaves.
How much does a 51% attack cost?
The cost depends on the network. For a proof of work coin, the attacker needs more hashrate than all honest miners for the duration of the attack, either by buying hardware or renting it. For small coins this can be cheap; for Bitcoin it is enormous. On proof of stake networks, the attacker needs a majority of staked tokens and risks losing them through slashing.
Can a 51% attack happen on proof of stake?
Yes, in principle, but the thresholds and penalties differ. On Ethereum, about a third of stake can stop finality, a majority can censor and perform short reorgs, and two thirds could finalize a chain of its choice. Validators who break the rules can be slashed, and the community can coordinate a fork that leaves the attacker’s stake far less valuable.
How do exchanges protect against 51% attacks?
Exchanges mainly protect themselves by requiring more block confirmations before crediting deposits, especially for smaller proof of work coins with low or rentable hashrate. They also monitor for deep reorganizations, pause deposits when hashrate shifts suddenly and may delist coins with repeated security problems. Waiting longer reduces the risk that a payment disappears after a reorg.
Data sources: Bitcoin white paper (bitcoin.org), mempool.space mining API (October 30, 2026), ethereum.org (proof of stake attack and defense), Ethereum Classic project website (November 21, 2023), arXiv paper on Qubic’s Monero mining campaign (2512.01437); Qubic’s claims as reported by media in August 2025.