A blockchain is pretty ironclad. However, only one crucial exception exists to a blockchain network’s security failure, and it's called the 51% attack.
Malicious actors and attackers will need to get control of at least 51% of the blockchain's ownership to ‘hack’ it. Simply put, the party that owns 51% of the blockchain is the majority owner, so what they say goes.
Having said that, the reality is that it is extremely difficult to execute a 51% attack. To begin with, owning 51% of any major blockchain assets is supremely expensive. The threat of a Sybil attack can also be ruled out, as this would need people to create numerous fraudulent identities and accounts. But since 51% of the ownership cannot be divided, this attack strategy will not work.