A single user gets >50% stake in a currency, and can now control the validation of blocks, allowing them to add any blocks they wish.
- Including any transaction they can decide on
- Total control by the majority stake. Cost of a 51% Attack for Different Cryptocurrencies | Crypto51 shows chain vulnerability.
NiceHash is a platform for renting computing power for hashing, and can be used by attackers.
A fork of bitcoin that was a victim of a double-spend attack in 2018.
- Transferred $18.6 million of BTG on a crypto exchange, then reverted the public ledger using >51% mine power (hence keeping the BTG).
A double spend attack in 2018. ZenCash Statement on Double Spend Attack - Horizen Blog