Harmony Horizon Bridge Hack

The Harmony Horizon Bridge hack was distinct from many other bridge exploits as it didn't involve a smart contract vulnerability but rather a compromise of private keys. The bridge used a multisig wallet requiring 2-of-5 signatures to validate transactions, but these keys were insufficiently secured.

Investigation revealed several critical security flaws:

1. The bridge employed a multi-signature scheme with a low threshold (2-of-5 signatures needed)
2. The private keys were encrypted and stored in a way that allowed the attacker to decrypt them
3. No abnormal transaction monitoring system was in place to detect unusual withdrawal patterns

The attack method has been linked to sophisticated social engineering, possibly combined with compromised endpoint security, rather than a blockchain code vulnerability:

// Simplified representation of the vulnerable multisig implementation
contract MultiSigWallet {
    // Only 2 out of 5 signatures needed - too low for high-value bridge
    uint256 public required = 2;
    uint256 public owners = 5;
    
    // Transaction execution function
    function executeTransaction(
        address destination,
        uint value,
        bytes data,
        bytes[] signatures
    ) public {
        // Verify at least 2 valid signatures
        require(
            _validateSignatures(destination, value, data, signatures),
            "Invalid signatures"
        );
        
        // Execute the transaction
        (bool success, ) = destination.call{value: value}(data);
        require(success, "Transaction execution failed");
    }
    
    // ... code omitted for brevity
}