Monsieur C
Here is my solution for the OSINT challenge Monsieur C as part of the 2026 edition of the 404CTF held online by the French secret service (DGSE) and Telecom SudParis.
This problem is divided in 4 steps as followed :
- Monsieur C Découverte : intro
- Monsieur C Somebody is watching me : medium
- Monsieur C Voyage voyage : medium
- Monsieur C Fan de sciences : hard
Monsieur C : Découverte
Un étrange personnage se présentant comme “Monsieur C” est venu me voir, moi, créateur du chall, en DM pour faire de la pub pour son nouveau blog en partageant son URL. Il était si sympathique que je n’ai pu refuser et ai mis son URL dans le premier endroit que j’avais sous les yeux. Quel est l’URL du blog ?
We need to retrieve Mister C’s blog. We know the author of the challenge (Sherpearce) advertised the URL somewhere.
I had a look at X, Instagram, Github and finally Discord. In the author’s profile on the event’s Discord server :
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404CTF{lab.c.404ctf.fr}
Monsieur C : Somebody is watching me
L’individu surveillé cache probablement des informations d’importance capitale chez lui. D’après nos renseignements, il dispose d’un dispositif de surveillance à son domicile. Encore faut-il trouver un moyen d’y accéder…
From lab.c.404ctf.fr/cv hosting our target’s resume, we get his name : Carlos Int
In a few blog posts, the target mentionned a Youtube channel that can be found easily using "Carlos Int" site:youtube.com : the channel is located at youtube.com/@IntCarlos.
The channel hosts multiple videos and Shorts among which there is Ma chambre, a slideshow with fast-moving pictures of our target’s room. One of them, holds a username and a password written on a whiteboard (rootcam:W19ub7d08!) :
I provided these credentials to the login page located at lab.c.404ctf.fr/login and was granted the access to /camera :
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404CTF{67}
Monsieur C : Voyage voyage
Monsieur C est récemment parti en vacances. Il y a quelques siècles, un explorateur français s’était lui aussi rendu pour la première fois sur ce lieu, accompagné d’un naturaliste. Comment s’appelle ce naturaliste ?
Our target went on a trip lately. We need to find where and what French naturalist explored this location centuries ago.
On the Youtube channel, the video Souvenirs appears promising. While it does not mention the destination explicitly, we have hints about the location : flowers growing there, has both beaches and mountains, a drawing from someone he met there.
A reverse image search for the flowers leads to this page : Leucheria leontopodioides - Pl@ntNet identify.
Leucheria is native to South America and the Falkland Islands1. A quick search will then mention Antoine-Joseph PERNETY as the French naturalist who went on a expedition to the Falkland Islands with the explorer Louis-Antoine de Bougainville.
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404CTF{antoine-joseph_pernety}
Monsieur C : Fan de sciences
Qui est le scientifique préféré de “Monsieur C” ?
From lab.c.404ctf.fr/cv hosting our target’s resume, we get this phone number : +33650237918
On WhatsApp, the contact associated with this phone number is :
A reverse image search of the profile picture then leads to Camille Guérin.
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404CTF{camille_guerin}