A 51% attack occurs when a single entity controls more than half the network's mining power (PoW) or staking weight (PoS), allowing them to rewrite recent blockchain history, enabling double-spending, transaction censorship, or blocking confirmations. It does not let them steal coins from other addresses or create coins beyond protocol rules.
What Is a 51% Attack?
2 min read
The short version
Imagine a voting system where one party controls 51% of the votes. They can outvote everyone else and rewrite the latest minutes of the meeting, but they can't forge other people's signatures or change the constitution. A 51% attacker can reorganize recent blocks, but can't steal your private keys or mint new coins.
How It Works
In PoW: the attacker mines a private chain faster than the honest network. When their private chain is longer, they broadcast it and the network switches to it (longest chain wins). Transactions confirmed on the shorter (honest) chain are reversed. The attacker can double-spend by sending coins, waiting for confirmation, then reverting that block on their longer chain. Cost depends on the network's hash rate, attacking Bitcoin at 600 EH/s would require more ASIC hardware than exists. In PoS: an attacker controlling >2/3 of stake could finalize conflicting blocks. But slashing would destroy their entire stake, making it a one-time, self-destructive attack.
The Ethereum Classic 51% attack (2019)
In January 2019, an attacker rented hash power (ETC's network was small enough to attack profitably). They mined a private chain for several hours, double-spent approximately $1.1 million in ETC across exchanges, then released their longer chain. Exchanges that accepted deposits during the reorganized blocks lost funds. The attack cost an estimated $5,000-$10,000 in rented hash power but netted $1.1M. Result: exchanges increased ETC confirmation requirements from 12 to 400+ blocks.
What People Get Wrong
A 51% attacker can steal anyone's coins
They cannot forge signatures. They can only revert their own transactions (double-spend) or censor others' transactions. Your coins in your wallet remain safe from 51% attacks.
51% attacks are impossible on large networks
On Bitcoin, the cost is prohibitive (~$10B+ in hardware). But smaller PoW chains with low hash rates are routinely attacked. It's an economic question, not a theoretical impossibility.
Proof of stake eliminates 51% attacks
PoS replaces the attack with a different form: controlling >2/3 of stake. The difference is that the attacker's stake gets slashed, making it destructively expensive rather than merely operationally expensive.
Keep Reading
Sources & Further Reading
- Crypto51 Attack Cost Calculator
Estimates the cost of 51% attacks on various PoW chains using NiceHash rental prices
- Bitcoin Hash Rate (Blockchain.com)
Live chart showing total Bitcoin network hash rate over time
Questions People Also Ask
- Has Bitcoin ever been 51% attacked?
- No. Bitcoin's enormous hash rate (600+ EH/s) makes the cost of attack prohibitive, estimated at $5-10 billion in specialized hardware alone, plus ongoing electricity costs.
- How many confirmations protect against a 51% attack?
- More confirmations mean the attacker must maintain majority power longer. Six confirmations on Bitcoin is standard. Smaller chains may need hundreds of confirmations for equivalent security.
- Can I protect myself from a 51% attack?
- As a user: wait for more confirmations before considering large payments final. As a merchant: increase confirmation requirements on smaller-cap PoW chains. Your held coins are never at risk.