Which Type of Cryptocurrency Chains Are Most Vulnerable to a 51% Attack?

Smaller, newer, or less-established Proof-of-Work (PoW) cryptocurrency chains are most vulnerable to a 51% attack. This is because their total network hash rate is low, meaning the cost to acquire 51% of the hash power is relatively inexpensive.

An attacker can often rent the necessary hash power from a mining rental service to execute the attack, which is not economically feasible for large chains like Bitcoin.