Difficulty: easy

Category: crypto

Author: xnull

Description:

### Encrypted Session System

This web application stores your session data in an encrypted cookie. When you visit the site, you get a cookie that marks you as a regular user.

Only users with admin privileges can see the flag. The cookie is encrypted using AES encryption, but the implementation might have a weakness you can exploit.

**Goal:** Modify your encrypted session cookie to gain admin access and retrieve the flag.

Solution

import requests
from pwn import *
from rich import print
import re

URL = "https://a5b24fe0-4a84-462c-ad75-23f71647a996.ctf.endolum.io:1337"


def get_cookie(st):
    """
    get cookie from page
    """
    return requests.post(f"{URL}/register", data={"username": st}).json()["profile"]


def splits(s):
    """
    split into strings of 16 bytes
    """
    a = b64d(s)
    parts = [a[x : x + 16] for x in range(0, len(a), 16)]
    return parts


def pad(s):
    """
    fake ecb padding
    """
    todo = 16 - len(s)
    return s + "\x0b" * todo


c = get_cookie(
        # user=
        "aaaaaaaaaaa" +  # (=16)
        # starts at a new block, pads the entire 16 bytes, so admin\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b
        pad("admin") +
        # dummy end, leaves us with [bbbbbbbbbbroole=] and [user]
        "bbbbbbbbbb")
success(f"Got cookie: {c}")

parts = splits(c)
# parts[0]: user=aaaaaaaaaaa
# parts[2]: bbbbbbbbbbroole=
# parts[1]: admin
payload = b64e(parts[0] + parts[2] + parts[1])
success(f"Exploit payload: {b64e(parts[0] + parts[2] + parts[1])}")

solution = requests.get(URL+"/profile", cookies={"session": payload}).text
flag = re.search('<div class="flag">(.*)</div>', solution).group(1)
success(f"Got flag: {flag}")

# Output:
# [+] Got cookie: K6rUmoCQ75fiXWC4/F6KEFzYtEUHsKACRE/ZOHEf73RjXRVK1M9rZF7ThQRaUFf1ZATR/+z0i5hMIb1RFfoYvw==
# [+] Exploit payload: K6rUmoCQ75fiXWC4/F6KEGNdFUrUz2tkXtOFBFpQV/Vc2LRFB7CgAkRP2ThxH+90
# [+] Got flag: ENDLM{977e0ca98c4a5d9da30f60ad2227e68fcc07f53de436fb30}

Flag: ENDLM{977e0ca98c4a5d9da30f60ad2227e68fcc07f53de436fb30}

Solution#

Solution