June 6, 2023

The #20th DOJO CHALLENGE, Butters Adventure aims to exploit a DOM based Cross Site Scripting (XSS) vulnerability and change the connection of the user Butters to become offline so that he can escape from virtual reality!

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We are glad to announce the #20 DOJO Challenge winners list.


  • The best write-ups reports were submitted by: Tihmz, bosstester and Braxoia! Congrats 🥳

The swag is on its way! 🎁

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Read on to find the best write-up as well as the challenge author’s recommendations.

The challenge

DOM XSS – Butters Adventure

See the challenge page >

We asked you to produce a qualified write-up report explaining the logic allowing such exploitation. This write-up serves two purposes:

  • Ensure no copy-paste would occur.
  • Determine the contestant ability to properly describe a vulnerability and its vectors inside a professionally redacted report. This capacity gives us invaluable hints on your own, unique, talent as a bug hunter.


We would like to thank everyone who participated. The #20 DOJO challenge provided us with a large amount of high quality reports and everyone did a great job!

We had to make a selection of the best ones. These challenges allow to see that there are almost as many different solutions… as long as there is creativity! 😉

Thanks again for all your submissions and thanks for playing with us!

Tihmz Write-Up

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Cross-site Scripting (XSS) – DOM trought ($cmd) parameter using octal encoding (CWE-79)

Cross-Site Scripting (XSS) is a type of vulnerability that allows an attacker to inject malicious code into a web page. This code is executed by the victim’s web browser, allowing the attacker to steal sensitive information such as login credentials, manipulate the contents of the web page, or perform other unauthorized actions. In a DOM-based XSS attack, the attacker injects malicious code into the DOM (Document Object Model), which is then executed by the browser when the page is loaded or when a user interacts with the page in a certain way.

Exploitation – steps to reproduce

source code

In this situation, we have 2 inputs parameters : $username and $cmd

Let’s take a look at the javascript source code :


//Cass to create a new player:
class Player {
  constructor(status, name, spawn, grade, quotes) { 
    this.whoami = name;
    this.groups = grade;
    this.pwd = spawn; = quotes;
    this.connection = status;

//Create new players: 'butters' and 'cartman':
const butters = new Player( 'online', 'Butters', '~/School', '4th Grade', Array('Hey Fellas!', 'Oh boy!', 'A little chaos...') );  
const cartman = new Player( 'online', 'Cartman', '~/School', '4th Grade', Array('Kewl', 'WassUP!!') );

//Add players to the `users` object:
let users = {};
let addUser = Object.assign(users, { butters, cartman });

//Cartman to rescue!!
var username = '$username';
var cmd = '$cmd';

/* [Help Guide]
* (1) => Check `user` if it's valid, if not valid (||) user = 'butters'. 
* (2) => Check if the `command` exists, if not valid (||) command = 'not found'
var user = users[username] || users['butters']//(1)
var command = user[cmd] || 'Command \''+cmd+'\' not found.';//(2)

document.getElementById('inpt').innerHTML = cmd+'<br>'+command;
document.getElementById('user').innerText = user.whoami+'@vr';


The code create 2 object Player and initialize them with different attributes.
Then, each attributes can be accessed using the $username parameter to select a player, and $cmd parameter to select which attribute to print. If the username specified or the command specified is not found, the user fallback to ‘butters’ and the command printed is ‘not found’.

What is interesting here is this line :

document.getElementById('inpt').innerHTML = cmd+'<br>'+command;

This line use the .innerHTML tag, who’s property is used for DOM manipulation. I can be used to examine/modify the content of the HTML page, meaning we can use it to inject some javascript code for Cross-site scripting.

Blacklisted characters

All the following characters are blacklisted, and cannot be injected : "'`<>
We cannot also use the escape characters \x \u used for hexadecimal and unicode character injection inside a string.
It look like we cannot inject any character useful to execute javascript code.

The flaw

Actually, even if \x \u are blacklisted, we can still inject some special characters like <> or ', using another escape sequence character : the octal escape sequence character. We can use it just like the Unicode or Hexadecimal one to inject any ASCII character using it’s octal code.

For example the string Hello, world would be \110\145\154\154\157\54\40\167\157\162\154\144 in octal numbers.

Testing the injection

Using a simple python code, we can translate any payload easily to octal code :

import re
payload = input("Enter payload : ")
char_array = [oct(ord(c)) for c in payload]
pattern = r"[' ,\[\]]+"
result = re.sub(pattern,'',str(char_array))
print("Formated payload :\n",result.replace('0o','\\'))

This code take for input your payload, convert it to octal code for each character and then print all the codes in the good format for our injection. We can try to use with a common XSS payload :

<img src='x' onerror='alert("XSS")'>

We get the following encoded payload :


Now let’s try try it on the website :

The octal encoding method is working, we can successfully execute javascript code !


Completing the challenge

Now that we have a working XSS, we can finish this challenge, who’s main goal is to change the status of the player “Butters” from “online” to “offline”.

This can be done using the following XSS payload :

<img src='x' onerror='users.butters.connection = "offline";'>

and in octal code :


We can complete the challenge by login in as Cartman and modifying Butters connection status :

How to prevent :

  • Validation of user input before using it in .innerHTML
  • Better filtering of users input, like adding octal encoding to the blacklist
  • Use of content security policy to specify which sources are allowed to execute scripts

More here


As always , thanks for the dojo, it’s still really fun to play with




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