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contracts...Challenge.sol

pragma solidity 0.8.10;

// The goal of this challenge is to be able to sign offchain a message
// with an address stored in winners.
contract Challenge{

    address[] public winners;
    bool lock;

    function exploit_me(address winner) public{
        lock = false;

        msg.sender.call("");

        require(lock);
        winners.push(winner);
    }

    function lock_me() public{
        lock = true;
    }
}