A 51% attack is a blockchain nightmare.

It’s when an individual or a group takes over > 51% of a blockchain’s hashing power (computational power used while mining)

This gives them major control over the blockchain.

The simple reason behind this:

The longest chain rule ⛓️

Blockchain validates transactions with the longest chain

if two parties are looking to validate a transaction, the longest chain wins.

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With this, they can:

But they can’t: