bugku | flagphp | 反序列化

1.开脑洞

2.读源码得到条件是要求反序列化后的‘ISecer’的值等于$KEY

<?php
    error_reporting(0);
    include_once("flag.php");
    $cookie = $_COOKIE['ISecer'];
    if(isset($_GET['hint']))
    {
        show_source(__FILE__);
    }
    elseif (unserialize($cookie) === "$KEY")
    {
        echo "$flag";
    }
    else
    {
        ?>

            <html>
            <head>
            <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
            <title>Login</title>
            <link rel="stylesheet" href="admin.css" type="text/css">
            </head>
            <body>
            <br>
            <div class="container" align="center">
              <form method="POST" action="#">
                <p><input name="user" type="text" placeholder="Username"></p>
                <p><input name="password" type="password" placeholder="Password"></p>
                <p><input value="Login" type="button"/></p>
              </form>
            </div>
            </body>
            </html>

        <?php
    }
    $KEY='ISecer:www.isecer.com';
?>

3.这里的$KEY其实是空的（它在后方定义），这里一直坑，一开始一直用NULL来反序列化，谁知道是“”Orz ,岂可休啊