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The 51% Attack at a Glance
| Question | Answer |
|---|---|
| What is a 51% attack | A single entity gains over 50% of a blockchain's hash power or stake and manipulates transaction history |
| What can attackers do | Reverse their own transactions, double spend coins, and censor other users' payments |
| What can attackers not do | Steal funds from wallets they do not control or create new coins beyond protocol rules |
| Has Bitcoin ever been 51% attacked | No. Bitcoin's hash rate makes an attack economically unfeasible |
| Which blockchains have been attacked | Bitcoin Gold (2018), Ethereum Classic (2020), Monero (2025) |
| How much does it cost to attack Bitcoin | Hundreds of millions of dollars per hour as of 2026 |
What Is a 51% Attack in Blockchain
Every blockchain relies on a majority of honest participants to validate transactions and add new blocks. A 51 attack in blockchain networks targets this exact assumption. When a single actor controls more than half of the network's computing power or staked tokens, the trust model breaks.
The attacker gains the ability to rewrite recent transaction history. This makes majority hash rate takeovers one of the most serious threats to any Proof of Work blockchain.
How a Single Entity Takes Over a Network
On Proof of Work blockchains, miners compete to solve complex math problems. The first miner to solve the problem adds the next block and earns a reward. A 51 percent attack begins when one miner or mining pool accumulates more hash power than the rest of the network combined.
This can happen through massive hardware purchases. It can also happen through hash rate rental services like NiceHash. On smaller networks, renting enough power to cross the 51% threshold takes just hours and a modest budget.
What Happens to Transactions During the Attack
Once the attacker controls majority hash power, they mine blocks on a private version of the blockchain. The public network continues processing transactions normally. Users and exchanges accept payments as confirmed. Nobody knows a parallel chain exists.
When the attacker releases their longer private chain, the network accepts it as the valid version. All transactions on the shorter public chain get discarded. Confirmed payments vanish. Funds return to the attacker's wallet.
Key takeaway: A 51% attack gives a single entity control over which transactions are valid on a blockchain. The attacker mines a secret chain and overwrites the public one. This breaks the trust model that makes blockchain technology work. Smaller networks with lower hash rates face the greatest risk.
How a 51% Attack Works Step by Step
The attack follows a predictable sequence that exploits how blockchains resolve disagreements between competing chains. The attacker first accumulates hash power, then mines privately, and finally broadcasts a longer chain to overwrite the real one.
Building a Secret Chain
The attacker starts mining blocks without broadcasting them to the network. Because they hold more than 50% of the total hash power, their private chain grows faster than the public one. During this time, the attacker sends cryptocurrency to an exchange on the public chain and converts it to cash or another asset.
Once the exchange confirms the deposit, the attacker broadcasts the private chain. This version does not contain the deposit transaction. Instead, it shows the funds sent back to the attacker's own wallet.
The Longest Chain Rule and Why It Matters
Most Proof of Work blockchains follow the longest chain rule. When two competing versions appear, nodes automatically accept the one with the most accumulated work. This rule normally prevents forks and keeps the network in agreement.
During a blockchain 51 attack, this rule becomes a weapon. The attacker's chain has more work because they controlled more hash power. Honest nodes have no choice but to adopt the attacker's version and discard their own blocks.
Key takeaway: The attacker mines a private chain while spending coins on the public chain. They release the longer chain to overwrite the public version. The longest chain rule forces all honest nodes to accept the attacker's history. This allows the attacker to reverse confirmed transactions and spend the same coins twice.
What Attackers Can and Cannot Do
A majority hash rate takeover does not give total control over a blockchain. 51% attacks allow manipulation of recent transaction history but cannot break the fundamental cryptographic rules of the protocol.
Double Spending Explained
Double spending is the primary goal of most attacks. The attacker spends coins on the public chain and then erases that transaction by publishing a longer private chain. The result is that the attacker keeps both the coins and whatever they received in exchange.
The 2018 Bitcoin Gold attack demonstrated this clearly. The attacker deposited BTG on exchanges, sold it, and reorganized the chain to erase the deposits. Exchanges lost roughly $18 million.
Transaction Censorship and Denial of Service
An attacker with majority control can refuse to include specific transactions in their blocks. This effectively censors certain users or addresses from using the network. The attacker can also prevent other miners from earning rewards by orphaning their blocks.
