[ACTF2020 新生赛]BackupFile1

进入靶机没有任何提示，source文件也找不到，那就先dirsearch扫一下吧

扫到备份文件index.php.bak

下载后打开可以看到该网页的源码

<?php
include_once "flag.php";

if(isset($_GET['key'])) {
    $key = $_GET['key'];
    if(!is_numeric($key)) {
        exit("Just num!");
    }
    $key = intval($key);
    $str = "123ffwsfwefwf24r2f32ir23jrw923rskfjwtsw54w3";
    if($key == $str) {
        echo $flag;
    }
}
else {
    echo "Try to find out source file!";
}

简单看一下就知道是弱比较

1	payload:?key=123

拿到flag