by rewriting parts of the blockchain, though not creating new coins or altering past blocks, posing a major threat to smaller Proof-of-Work (PoW) coins.

How it Works:

1. Control the Majority: An attacker acquires more than 50% of a blockchain's mining power (hashrate).
2. Create a Private Chain: They start mining a secret, longer version of the blockchain.
3. Double Spend: They send coins to an exchange, wait for the transaction to confirm on the public chain, then use their majority power to create a longer, secret chain where those coins are sent back to themselves, effectively spending them twice.
4. Broadcast: Once their secret chain is longer, they broadcast it, making it the new official history, invalidating the exchange's transactions.

What an Attacker Can Do:

1. Prevent Confirmations: Stop new transactions from being added.
2. Reverse Transactions: Cancel transactions already confirmed by others (double-spending).
3. Monopolize Mining: Take all block rewards for themselves.

What They Cannot Do:

1. Create Money: Cannot mint new coins or steal from others' wallets.
2. Alter Past Blocks: Cannot change the fundamental history of blocks mined before the attack.
3. Why It's a Threat (Especially for Smaller Coins):
4. It's extremely expensive for large networks like Bitcoin, but feasible for smaller cryptocurrencies with less hashing power, making them vulnerable.
5. It destroys trust and can crash the coin's value, which is often the attacker's goal.

Real-World Example:

The Ghash.io mining pool temporarily exceeded 50% of Bitcoin's hashrate in 2014, causing concern and price drops, leading them to voluntarily reduce power.