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contracts...Mal.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.10;

contract Foo {
    Bar bar;

    constructor(address _bar) {
        bar = Bar(_bar);
    }

    function callBar() public {
        bar.log();
    }
}

contract Bar {
    event Log(string message);

    function log() public {
        emit Log("Bar was called");
    }
}

// This code is hidden in a separate file
contract Mal {
    event Log(string message);

    // function () external {
    //     emit Log("Mal was called");
    // }

    // Actually we can execute the same exploit even if this function does
    // not exist by using the fallback
    function log() public {
        emit Log("Mal was called");
    }
}