Rating:
The page source hints that we can view the server source code at [http://52.59.124.14:5013/source](http://52.59.124.14:5013/source).
```py
from flask import Flask, request, redirect, render_template_string
import sys
import os
import bcrypt
import urllib.parse
app = Flask(__name__)
app.secret_key = os.urandom(16)
# This is super strong! The password was generated quite securely. Here are the first 70 bytes, since you won't be able to brute-force the rest anyway...
# >>> strongpw = bcrypt.hashpw(os.urandom(128),bcrypt.gensalt())
# >>> strongpw[:71]
# b'\xec\x9f\xe0a\x978\xfc\xb6:T\xe2\xa0\xc9<\x9e\x1a\xa5\xfao\xb2\x15\x86\xe5$\x86Z\x1a\xd4\xca#\x15\xd2x\xa0\x0e0\xca\xbc\x89T\xc5V6\xf1\xa4\xa8S\x8a%I\xd8gI\x15\xe9\xe7$M\x15\xdc@\xa9\xa1@\x9c\xeee\xe0\xe0\xf76'
app.ADMIN_PW_HASH = b'$2b$12$8bMrI6D9TMYXeMv8pq8RjemsZg.HekhkQUqLymBic/cRhiKRa3YPK'
FLAG = open("flag.txt").read();
@app.route('/source')
def source():
return open(__file__).read()
@app.route('/', methods=["GET"])
def index():
username = request.form.get("username", None)
password = request.form.get("password", None)
if username and password:
username = urllib.parse.unquote_to_bytes(username)
password = urllib.parse.unquote_to_bytes(password)
if username != b"admin":
return "Wrong user!"
if len(password) > 128:
return "Password too long!"
if not bcrypt.checkpw(password, app.ADMIN_PW_HASH):
return "Wrong password!"
return f"""Congrats! It appears you have successfully bf'ed the password. Here is your {FLAG}""" # Use f-string formatting within the template string
# ...
```
Our goal is to find a string that hashes to `app.ADMIN_PW_HASH`.
The comment gives 71 bytes of the password, despite saying it gives 70. Giving 71 bytes of the password seems strange, so we'll do some research on the `bcrypt.hashpw` format (e.g. how many bytes the salt takes up, etc) to see how much information 71 bytes really gives us.
After finding the [documentation](https://passlib.readthedocs.io/en/stable/lib/passlib.hash.bcrypt.html#security-issues), it turns out that ``only the first 72 bytes of a password are hashed,'' so we only need to brute force the last (72nd) byte of the effective password to find a string that matches the admin password hash.
```py
import bcrypt
import requests
admin = b'$2b$12$8bMrI6D9TMYXeMv8pq8RjemsZg.HekhkQUqLymBic/cRhiKRa3YPK'
salt = b'$2b$12$8bMrI6D9TMYXeMv8pq8Rje'
for b in range(256):
password = b'\xec\x9f\xe0a\x978\xfc\xb6:T\xe2\xa0\xc9<\x9e\x1a\xa5\xfao\xb2\x15\x86\xe5$\x86Z\x1a\xd4\xca#\x15\xd2x\xa0\x0e0\xca\xbc\x89T\xc5V6\xf1\xa4\xa8S\x8a%I\xd8gI\x15\xe9\xe7$M\x15\xdc@\xa9\xa1@\x9c\xeee\xe0\xe0\xf76' + b.to_bytes()
h = bcrypt.hashpw(password, salt)
if h == admin:
break
# password = b"\xec\x9f\xe0a\x978\xfc\xb6:T\xe2\xa0\xc9<\x9e\x1a\xa5\xfao\xb2\x15\x86\xe5$\x86Z\x1a\xd4\xca#\x15\xd2x\xa0\x0e0\xca\xbc\x89T\xc5V6\xf1\xa4\xa8S\x8a%I\xd8gI\x15\xe9\xe7$M\x15\xdc@\xa9\xa1@\x9c\xeee\xe0\xe0\xf76\xaa"
session = requests.Session()
print(session.get("http://52.59.124.14:5013", data={"username": "admin", "password": password}).text)
```
Flag: `ENO{BCRYPT_FAILS_TO_B_COOL_IF_THE_PW_IS_TOO_LONG}`
