Ethereum co-founder Vitalik Buterin has issued a cautionary note about the limitations of blockchain security, emphasizing that even a 51% attack cannot validate invalid blocks. His warning comes at a time when many protocols rely heavily on off-chain validation for access to external data. Buterin argues that while blockchain networks are resilient against such attacks within consensus rules, this protection doesn’t extend to systems that rely on off-chain validator trust. This vulnerability is particularly significant in DeFi, where oracles and validator-based systems play a crucial role in bridging blockchain and real-world data. Recent incidents like the 2020 bZx and Compound protocol failures highlight the importance of maintaining verifiable on-chain data integrity to prevent similar risks from occurring again.