Trusted Client (web)

PUBLISHED ON 22/03/2020 — EDITED ON 11/12/2023 — 247CTF, INFOSEC


This is my write-up of a Web challenge Trusted Client on the CTF site


Developers don’t always have time to setup a backend service when prototyping code. Storing credentials on the client side should be fine as long as it’s obfuscated right?


At the start of the challenge we get presented a website with login form. Inspecting the source code yields the following interesting part:

window.onload=function() {
  document.getElementById('login').onsubmit=function() {
  return false;


We can see in the instructions and in the source code, that we are dealing with obfuscated JS code.

Quick search will show that we are dealing with JSFuck and we can use this nifty online tool to deobfuscate the code.

From Wikipedia JSFuck article:

JSFuck is an esoteric subset of JavaScript, where code is written using only six characters: [, ], (, ), !, and +. The name is derived from Brainfuck, an esoteric programming language that also uses a minimalistic alphabet of only punctuation. Unlike Brainfuck, which requires its own compiler or interpreter, JSFuck is valid JavaScript code, meaning that JSFuck programs can be run in any web browser or engine that interprets JavaScript. JSFuck is able to recreate all JavaScript functionality using such a limited set of characters because JavaScript is a weakly typed programming language, and it allows the evaluation of any expression as any type.

if (this.username.value == 'the_flag_is' && this.password.value == '247CTF{xxxx}'){ alert('Valid username and password!'); } else { alert('Invalid username and password!'); }