What Is a “51% Attack” and How Does PoW Prevent It?

A 51% attack is a hypothetical scenario where a single entity or group gains control of more than 50% of a blockchain network's total hashing power. This control would allow them to prevent new transactions from gaining confirmations and, more critically, to reverse their own transactions, enabling double-spending.

PoW prevents this by making the acquisition and maintenance of such a large amount of computing power prohibitively expensive and economically unviable.

What Is the Risk of ‘Unwinding’ a Hedge Too Early or Too Late?
What Is a 51% Attack and How Does Hashing Power Overcome the Security Provided by SHA-256’s Output Size?
What Is the Main Security Benefit of Combining PoW and PoS in Proof-of-Activity?
What Is the “51% Attack” and How Does It Relate to the Hashing Power of a Cryptocurrency Network?
Compare the Capital Cost of a PoS Attack to the Energy Cost of a PoW Attack
How Do Energy Costs Influence a Miner’s Profit Margin in PoA?
What Is a “51% Attack” and How Does It Relate to PoW’s Probabilistic Finality?
What Are the Potential Consequences of Setting a TWAP Time Period That Is Too Short or Too Long?

Glossar