Overview
A 51% attack occurs when a malicious actor or coordinated group acquires more than half of the total mining hash power on a proof-of-work network. With majority control, the attacker can mine blocks faster than the rest of the network, enabling them to manipulate the blockchain in several dangerous ways.
What an Attacker Can Do
With majority hash power, an attacker can:
- Double spend: Send bitcoin to a merchant, wait for goods or services, then secretly mine a longer chain that excludes that transaction, effectively spending the same coins again.
- Prevent confirmations: Refuse to include specific transactions in blocks, censoring certain users or addresses.
- Reverse recent transactions: Reorganize the most recent blocks to undo confirmed transactions.
What an Attacker Cannot Do
Even with 51% of hash power, an attacker cannot:
- Create coins out of thin air (violates consensus rules)
- Spend coins they do not own (requires valid signatures)
- Change the block reward amount
- Alter historical transactions buried deep in the chain
Normal chain: [A]──[B]──[C]──[D]──[E]
↑ honest tip
Attacker's chain: [C']──[D']──[E']──[F']
↑ attacker's longer chain replaces [C]-[E]
Why Bitcoin Is Resilient
Bitcoin's network hash rate is enormous, measured in hundreds of exahashes per second. The cost of acquiring and operating enough ASIC hardware and electricity to sustain 51% of this hash power would run into billions of dollars, making such an attack economically irrational. Smaller proof-of-work networks with lower hash rates have been successfully attacked, underscoring the security advantage of Bitcoin's scale.