Existing vulnerabilities in Proof-of-Work
There are vulnerabilities that exist in Bitcoin's blockchain and the system that it uses to verify data integrity - Proof-of-Work(PoW). These range from minor ones, dealing with single transactions, or major ones, dealing with whole blocks.
Race attack is one of the vulnerabilities that exists in Proof-of-Work and is the least threatening one.
Finney attack refers to a vulnerability that exists in Proof-of-Work and can work with many transaction forks instead of simply forking one transaction.
51% attack is the most dangerous vulnerability in Proof-of-Work and is a flaw that can render a whole block of transactions useless.