Bonjour Ondine!

BreizhCTF 2023 - Web.

BreizhCTF 2023 - Bonjour Ondine!

Challenge details

Event Challenge Category Points Solves
BreizhCTF 2023 Bonjour Ondine! Web ??? ???

pokemon

Je cherche ma collection de cartes pokemon, l’avez-vous vu ?

Auteur: Zeecka

https://ondine.ctf.bzh/

TL;DR

Le fichier robots.txt nous amène vers une page contenant une table HTML vide avec des cases ayant une class noire ou blanche. L’ajout d’attributs CSS sur ces classes permet de dévoiler un QRcode nous indiquant une 3eme page. Cette page contient un prompt en JS Fuck servant de mot de passe. Une fois décodé, l’utilisateur peut rentrer le mot de passe et récupérer le flag.

Méthodologie

step1

On commence par faire une reconnaissance des fichiers du serveur web à l’aide d’outils comme dirb, dirsearch, ffuf…

$ dirsearch -u "https://ondine.ctf.bzh/" -x 403

  _|. _ _  _  _  _ _|_    v0.4.3
 (_||| _) (/_(_|| (_| )

Extensions: php, aspx, jsp, html, js | HTTP method: GET | Threads: 25 | Wordlist size: 11601

Output: output.txt

Target: https://ondine.ctf.bzh/

[18:23:56] Starting: 
[18:25:48] 200 -   45B  - /robots.txt
[18:25:57] 301 -    0B  - /static  ->  /static/

Task Completed

On récupère le fichier robots.txt suivant:

User-agent: *
Disallow : /ondine_secrets.html

L’accès à la page ondine_secrets.html nous emmène vers une page a priori vide:

step2

L’inspection du document (ctrl+u) nous informe cependdant de la présence d’une table avec des classes black et white.

table

L’ajout d’attributs CSS (dans l’inspecteur ou en sauvegardant une copie du document) nous permet d’ajouter une taille aux cellules ainsi qu’un fond de couleur correspondant à leurs classes:

td{
    width: 10px;
    height: 10px;
}
.black{
    background: black;
}

step3

Le décodage du QRcode nous renvoie /poke_hidden_form.html, dernière étape du challenge:

step4

Un mot de passe nous est demandé. Ce type de d’interaction est liée à une fonctionnalité javascript. L’inspection du code source HTML révèle une code étrange:

<body>
    <script>
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    </script>
</body>

Ce code est en realité du code JavaScript obfusqué. Il est possible de décoder le programme manuellement ou à l’aide d’outils comme Decoder-JSFuck.

jsfuck

var a = prompt("Entrez le mot de passe."); if (a == atob('cGlLYS1waUthIQ==')){ window.location = "/"+a+".html";}

On voit ici que le mot de passe attendu doit être équivalent à atob('cGlLYS1waUthIQ=='), soit la chaine piKa-piKa! une fois décodée de la base64.

Une fois le mot de passe rentré, nous soommes redirigés vers /piKa-piKa!.html qui contient le flag:

flag

Flag

BZHCTF{piKa-piKa!}

Auteur: Zeecka