Serial Killer 4 - CTF Writeup

Challenge Name: Serial Killer 4
Category: Osint
CTF: MOJO-JOJO
Description: Tracking the killer led investigators to another possible location.
A full search yielded almost nothing — except two mysterious files.
Somewhere inside them lies the next clue.
Your move.

Solution

#

Step 1: Analyzing the Text File - Zero-Width Steganography

#

Opening the .txt handout, the content appeared corrupted at first glance, which was immediately suspicious.

This behavior is characteristic of zero-width characters being used for steganography. To extract the hidden data, we used the Unicode Steganography Decoder.

Result: We successfully extracted a hidden username from the zero-width characters.

Step 2: Decoding the Audio File - Morse Code

#

Next, we examined the .wav handout. Upon listening, it was clearly Morse code.

We used a Morse Code Audio Decoder to translate the audio signal:

Decoded Message:

I HAVE ONLY 03 SUBSCRIBERS ON MY CHANNEL BROO

This revealed two critical pieces of information:

1. The user runs a YouTube channel
2. The channel had 3 subscribers (suggesting the two-digit number is 03)

Step 3: Finding the YouTube Channel

#

Using the username extracted from Step 1, we searched for the corresponding YouTube channel.

At the time of the CTF, the channel indeed had 3 subscribers, confirming our findings from the Morse code.

Step 4: Geolocation via Image OSINT

#

We proceeded to examine the video content on the channel for the next clue.

The video contained an image of a mosque. To geolocate it, we performed image-based OSINT using Yandex AI Images (also known as Yandex Image Search), which is particularly powerful for reverse image searching.

Result: The mosque was identified as the Oqba Bin Nafi Mosque (also known as the Great Mosque of Kairouan) located in Kairouan, Tunisia.

Flag Construction

#

With all the pieces gathered:

• Location: Kairouan (from the mosque geolocation)
• Number: 03 (from the Morse code message about subscribers)

Final Flag

#

MOJO-JOJO{Kairouan_03}