Insecure Direct Object Reference in Django with Firestore

Insecure Direct Object Reference (IDOR) occurs when an API exposes a reference to an internal object—such as a Firestore document ID—and allows an authenticated user to act on or retrieve another user’s object without authorization checks. In a Django application using Cloud Firestore as the backend, this typically arises when a view uses a user-supplied identifier (e.g., a document name or numeric key) to directly fetch or modify a Firestore document, relying solely on user identity and not on object-level ownership or permissions.

Consider a Django view that retrieves a user’s profile document by an profile_id provided in the URL. If the view does not verify that the requested profile_id belongs to the requesting user, an attacker can enumerate IDs and access or modify other users’ profiles. Firestore security rules alone do not protect an unauthenticated or over-privileged service account used by Django; if the Django backend has broad read/write permissions, missing authorization logic in the application layer becomes a direct path to data exposure, modification, or lateral movement across user boundaries.

Firestore’s document-centric model makes IDOR straightforward to trigger: predictable document names or auto-generated IDs can be iterated or guessed. For example, using a numeric sequence in a URL like /api/profile/123 allows automated enumeration. Even with user authentication, if the view does not scope the query to the authenticated user’s data—such as by enforcing that a document’s user_id field matches the request’s user—Firestore will return the requested document as long as the backend credentials permit it. This shifts the responsibility to the application to implement strict ownership checks, a step often missed when developers assume Firestore rules suffice.

Another common pattern involves associating resources by path, such as users/{user_id}/orders/{order_id}. If the Django handler extracts user_id from the token but then retrieves order_id without confirming the tuple (user_id, order_id) maps to the requester, the endpoint becomes vulnerable. Firestore queries can fetch the document, but without explicit ownership validation—like ensuring the document’s user_id matches the authenticated user’s ID—an attacker can substitute any order_id and access or alter other users’ orders. This risk is compounded when Firestore rules allow read/write at the collection or document level without granular constraints, placing the onus on Django to enforce per-object authorization.

IDOR in this stack also intersects with other checks middleBrick performs, such as BOLA/IDOR and Property Authorization. The scanner tests whether endpoints properly scope data access and whether returned fields respect authorization boundaries. For instance, an endpoint that returns a Firestore document containing sensitive fields like ssn or internal pointers may leak data if field-level authorization is omitted. Because Firestore returns the full document when a reference is supplied, Django must explicitly select and filter fields based on the requester’s permissions before serialization, preventing unintended exposure.

Real-world attack patterns mirror OWASP API Top 10’s A1: Broken Object Level Authorization. Unlike black-box tests that only observe behavior, middleware scans like middleBrick cross-reference OpenAPI specs with runtime findings to highlight mismatches between documented parameters and actual authorization logic. For Django-Firestore integrations, this means validating that every document reference is accompanied by a per-object check tying the resource to the requesting user, and that responses do not overexpose data.

To mitigate IDOR in Django with Firestore, enforce object-level ownership on every request and avoid direct exposure of Firestore document references to the client. Always scope queries to the authenticated user and validate that the requested resource belongs to them before reading or writing. Below are concrete, Firestore-specific examples that demonstrate secure patterns.

Secure document retrieval with ownership check

Instead of fetching a document solely by client-provided ID, bind the lookup to the authenticated user’s ID and verify ownership explicitly.

from django.contrib.auth.decorators import login_required
from google.cloud import firestore
from django.http import JsonResponse, HttpResponseForbidden

@login_required
def get_user_profile(request, profile_id):
    db = firestore.Client()
    user_id = request.user.id  # authenticated user identity from your auth system
    profile_ref = db.collection('profiles').document(profile_id)
    doc = profile_ref.get()
    if not doc.exists:
        return JsonResponse({'error': 'Not found'}, status=404)
    # Ownership check: ensure the document’s user_id matches the authenticated user
    if doc.get('user_id') != user_id:
        return HttpResponseForbidden('Access denied')
    return JsonResponse(doc.to_dict())

Scoped query using user_id field

Prefer querying a collection with a composite filter that includes the user identifier, so Firestore only returns documents the user owns. This avoids unnecessary document reads and makes ownership explicit.

from google.cloud import firestore
from django.http import JsonResponse

def list_user_orders(request):
    db = firestore.Client()
    user_id = request.user.id
    orders_ref = db.collection('orders').where('user_id', '==', user_id)
    results = orders_ref.stream()
    orders = [doc.to_dict() for doc in results]
    return JsonResponse(orders, safe=False)

Transactional updates with ownership verification

When modifying a document, use a transaction to re-check ownership and ensure the document hasn’t changed between read and write. This pattern is critical for operations like updating payment methods or profile details.

from google.cloud import firestore
from django.http import JsonResponse

def update_user_email(request, profile_id):
    db = firestore.Client()
    user_id = request.user.id
    profile_ref = db.collection('profiles').document(profile_id)

    @firestore.transactional
    def update_in_transaction(transaction):
        snapshot = profile_ref.get(transaction=transaction)
        if not snapshot.exists:
            raise ValueError('Not found')
        if snapshot.get('user_id') != user_id:
            raise PermissionError('Forbidden')
        transaction.update(profile_ref, {'email': request.POST.get('email')})

    transaction = db.transaction()
    try:
        update_in_transaction(transaction)
        return JsonResponse({'status': 'ok'})
    except (ValueError, PermissionError) as e:
        return JsonResponse({'error': str(e)}, status=403)

Field-level filtering before serialization

When returning Firestore documents in API responses, strip fields that the requesting user is not allowed to see, rather than relying on Firestore rules to hide them.

def serialize_profile(doc, user_id):
    data = doc.to_dict()
    # Remove sensitive fields unless the viewer is the owner or an admin
    if data.get('user_id') != user_id and not user_is_admin(request):
        data.pop('ssn', None)
        data.pop('internal_notes', None)
    return data

These patterns ensure that object-level authorization is consistently enforced in Django, reducing the risk of IDOR. By combining per-request ownership checks, scoped queries, and careful field filtering, you align with security best practices that complement Firestore’s rule-based model.