51% Attack
When a single miner controls over 50% of a blockchain’s hash rate (Proof of Work) or owns over 50% of the tokens (Proof of Stake). They have the power to change the historical record of transactions on the blockchain. They are also positioned to manipulate the current functioning of the network and engage in double spending and other malicious activities.
The way attacks commonly work is when the 50% threshold is exceeded, the controlling actor starts to manipulate how blocks are validated. Since they can create and validate blocks faster than other network participants. They can establish the protocols canonical chain at will to say what they want.
Invalidating the transactions of others or changing them to a more favorable outcome. For example saying the tokens exchanged in a transaction were meant for them instead of the intended recipient.
Note attacks like these can be carried out without 51% control because of the randomness of blockchain verification. But, the likelihood of sustained success to reverse years of previously confirmed transactions is low. Eventually other network participants should win out on enough block confirmations to restore the proper chain of historical records.
Since any holder of the network’s token can be wiped out during a successful 51% attack by having their tokens stolen or records of their transactions reversed. A fundamental principle of blockchain security is preventing any single actor from gaining a level of control permitting them to launch a 51% attack.
Further Reading on 51% Attack’s
What is a 51% Attack? – CoinDesk