Broken Access Control in Aspnet with Firestore

Broken Access Control in an ASP.NET application that uses Google Cloud Firestore typically arises when authorization checks are missing, inconsistent, or bypassed at the API or data layer. Firestore security rules provide the primary mechanism for enforcing access at the database level, but if the ASP.NET backend does not validate identities and authorization before constructing and forwarding requests to Firestore, the rules alone cannot fully protect sensitive records.

Consider an endpoint that retrieves user documents by ID without verifying that the requesting user is allowed to view that document. If the backend uses a service account with broad Firestore permissions and only relies on Firestore rules that assume authenticated user context, the backend may inadvertently expose data belonging to other users. This happens because Firestore rules evaluate the request based on the credentials used; if the backend authenticates with a highly privileged service account, rules designed to restrict access by UID may be ineffective.

Another common pattern is insecure direct object references (IDOR) where user-supplied identifiers such as documentId are passed directly to Firestore without confirming that the user owns or is permitted to access that document. For example, an endpoint like /api/users/{documentId} might call Firestore’s GetDocument using a path built from the user input, without confirming that the authenticated user’s UID matches the document’s owner field. Because Firestore rules may not enforce ownership checks if the request is authenticated as a service account, the request succeeds even when the user should not have access.

OWASP API Top 10 categorizes this as Broken Object Level Authorization (BOLA), a specific form of Broken Access Control. In Firestore, authorization is influenced by how security rules are written and how the backend authenticates. If rules assume request.auth != null and the backend uses a privileged account, those rules may not block unauthorized access. Additionally, insufficient logging and lack of per-request authorization in the application layer mean that suspicious access patterns are not detected early. Proper mitigation requires combining Firestore rules that enforce user-based conditions with explicit authorization logic in ASP.NET that confirms ownership or role-based access before any database operation.

To remediate Broken Access Control when using Firestore in ASP.NET, enforce strict per-request authorization in application code and align Firestore security rules with the principle of least privilege. Always resolve the user identity from the authenticated context and validate access before interacting with Firestore. Avoid relying solely on service account privileges or assuming Firestore rules will compensate for missing application-level checks.

Below are concrete code examples that demonstrate secure patterns.

1. Validate ownership before reading a document

Ensure that the requesting user’s UID matches the document’s owner field. Use the Firebase Admin SDK to retrieve the document and perform checks in C#.

using Google.Cloud.Firestore;
using System.Threading.Tasks;

public class DocumentService
{
    private readonly FirestoreDb _db;

    public DocumentService(FirestoreDb db)
    {
        _db = db;
    }

    public async Task<DocumentSnapshot> GetUserDocumentAsync(string userId, string documentId)
    {
        DocumentReference docRef = _db.Collection("userData").Document(documentId);
        DocumentSnapshot snapshot = await docRef.GetSnapshotAsync();
        if (!snapshot.Exists)
        {
            return null;
        }

        // Explicit ownership check in application code
        if (!snapshot.TryGetValue("owner_uid", out string ownerUid) || ownerUid != userId)
        {
            return null; // Access denied
        }

        return snapshot;
    }
}

2. Use parameterized queries with user UID

Instead of passing raw user input into document paths, query documents filtered by UID to ensure the user only accesses their own data.

using Google.Cloud.Firestore;
using System.Collections.Generic;
using System.Threading.Tasks;

public class QueryService
{
    private readonly CollectionReference _items;

    public QueryService(CollectionReference items)
    {
        _items = items;
    }

    public async Task<List<DocumentSnapshot>> GetItemsForUserAsync(string userId)
    {
        Query query = _items.WhereEqualTo("owner_uid", userId);
        QuerySnapshot querySnapshot = await query.GetSnapshotAsync();
        return querySnapshot.Documents;
    }
}

3. Firestore security rule examples

Define rules that enforce ownership and role-based conditions, assuming request.auth is present. These rules work alongside application checks for defense in depth.

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    match /userData/{documentId} {
      allow read, write: if request.auth != null && request.auth.uid == request.resource.data.owner_uid;
      allow list: if request.auth != null && request.auth.uid == request.query.owner_uid;
    }
  }
}

4. Middleware-based authorization in ASP.NET

Implement authorization early in the request pipeline to reject unauthorized calls before they reach Firestore. This example shows a simple policy check; extend it to integrate with your identity provider.

using Microsoft.AspNetCore.Http;
using System.Threading.Tasks;

public class DocumentOwnershipMiddleware
{
    private readonly RequestDelegate _next;

    public DocumentOwnershipMiddleware(RequestDelegate next)
    {
        _next = next;
    }

    public async Task InvokeAsync(HttpContext context, IDocumentService documentService)
    {
        if (context.Request.RouteValues.TryGetValue("documentId", out var rawId))
        {
            string userId = context.User.FindFirst("uid")?.Value;
            if (string.IsNullOrEmpty(userId))
            {
                context.Response.StatusCode = 401;
                return;
            }

            var document = await documentService.GetUserDocumentAsync(userId, rawId.ToString());
            if (document == null || !document.Exists)
            {
                context.Response.StatusCode = 403;
                return;
            }
        }

        await _next(context);
    }
}

By combining these practices—explicit ownership validation in C#, parameterized queries, secure Firestore rules, and middleware checks—you reduce the risk of Broken Access Control in ASP.NET applications using Firestore.