Ethereum co-founder Vitalik Buterin has cautioned that the blockchain’s cryptographic guarantees stop where external trust begins. On October 26, Buterin explained that even a 51% attack cannot validate an invalid block. This means that even if a majority of validators collude or experience a software bug, they cannot seize users’ funds or forge transactions. Buterin Reignites Debate Over a Blockchain Validators This is because each blockchain node independently verifies new blocks and automatically rejects any that break the protocol’s rules
source: https://beincrypto.com/ethereum-vitalik-exposes-blind-spot-blockchain-security/