Insecure Design in Aspnet with Firestore

Insecure Design in an ASP.NET application that uses Google Cloud Firestore often stems from mismatched trust boundaries and weak authorization logic at the service layer. Firestore security rules operate server-side and are enforced independently of the ASP.NET runtime, but the application can inadvertently weaken overall protection by designing endpoints that rely solely on client-supplied identifiers without server-side verification.

Consider an endpoint that retrieves a user document using a path built directly from an incoming query parameter, such as users/{uid}/profile. If the ASP.NET handler does not validate that the authenticated principal matches {uid}, the endpoint may leak data across users. Because Firestore rules may permit read access based on request authentication tokens, a missing or incomplete BOLA (Broken Object Level Authorization) check in the ASP.NET layer can allow an attacker to enumerate or access other users' documents by altering the resource identifier. This is a classic Insecure Design where authorization logic is underimplemented in the application, even when backend rules exist.

Another common pattern is coarse-grained Firestore rules that allow broad read or write access for authenticated users, combined with an ASP.NET API that exposes high-level operations without scoping requests to the caller's identity. For example, an endpoint that accepts a document ID and passes it directly to Firestore without confirming ownership can lead to IDOR (Insecure Direct Object Reference). Attackers can chain this with missing rate limiting to perform mass enumeration. The design fails to enforce tenant isolation at the application layer, over-relying on Firestore rules that may be misconfigured or evolve over time.

Insecure Design also appears when Firestore is used to store sensitive configuration or secrets that the ASP.NET app references without integrity checks. If the application loads rules or policies from Firestore at startup and does not re-validate them for each request, changes in Firestore may not be reflected promptly, leading to inconsistent enforcement. Similarly, if the app uses Firestore to store user roles or permissions but does not re-evaluate them on each call, privilege escalation can occur when roles change. The design gap is not in Firestore itself but in how the ASP.NET app integrates with it, particularly around caching, context propagation, and dynamic scoping.

These issues map to the OWASP API Top 10 BOLA/IDOR and BFLA/Privilege Escalation categories, and they are detectable by middleBrick’s parallel security checks, which include Authorization and Property Authorization scans. By analyzing both OpenAPI/Swagger specs (with full $ref resolution) and runtime behavior, middleBrick can highlight mismatches between declared authentication expectations and actual endpoint behavior, including unauthenticated LLM endpoint exposure when AI-related routes are inadvertently exposed.

Remediation centers on enforcing strict ownership checks and scoping every Firestore operation to the authenticated user's identity. In ASP.NET, resolve the user's UID from the claims principal and use it to constrain Firestore queries rather than trusting path parameters or request bodies.

// Example: secure document retrieval in ASP.NET Core
[ApiController]
[Route("api/[controller]")]
public class ProfileController : ControllerBase
{
    private readonly FirestoreDb _firestore;

    public ProfileController(FirestoreDb firestore)
    {
        _firestore = firestore;
    }

    [HttpGet("profile")]
    public async Task<IActionResult> GetProfile()
    {
        var userId = User.FindFirst(ClaimTypes.NameIdentifier)?.Value;
        if (string.IsNullOrEmpty(userId))
        {
            return Unauthorized();
        }

        DocumentReference docRef = _firestore.Collection("users").Document(userId).Collection("profile").Document("current");
        DocumentSnapshot snapshot = await docRef.GetSnapshotAsync();
        if (!snapshot.Exists)
        {
            return NotFound();
        }

        return Ok(snapshot.ConvertTo<Profile>());
    }
}

For operations that accept a document ID, always recompute the full path from the authenticated UID and compare it to the provided ID to prevent IDOR:

// Example: safe document access with explicit ownership check
[HttpGet("documents/{documentId}")]
public async Task<IActionResult> GetDocument(string documentId)
{
    var userId = User.FindFirst(ClaimTypes.NameIdentifier)?.Value;
    if (string.IsNullOrEmpty(userId))
    {
        return Unauthorized();
    }

    // Recompute expected path and reject mismatches
    string expectedPath = $"users/{userId}/documents/{documentId}";
    if (!documentId.StartsWith(userId, StringComparison.Ordinal))
    {
        return Forbid();
    }

    DocumentReference docRef = _firestore.Document(expectedPath);
    DocumentSnapshot snapshot = await docRef.GetSnapshotAsync();
    if (!snapshot.Exists)
    {
        return NotFound();
    }

    return Ok(snapshot.ConvertTo<Document>());
}

Apply Firestore security rules that mirror these checks, using request.auth.uid to enforce tenant isolation. Avoid broad rules like allow read, write: if request.auth != null;; instead scope to user-specific collections:

// Firestore rule example: user-scoped access
rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    match /users/{userId}/profile {
      allow read, write: if request.auth != null && request.auth.uid == userId;
    }
    match /users/{userId}/documents/{documentId} {
      allow read, write: if request.auth != null && request.auth.uid == userId;
    }
  }
}

To address Privilege Escalation, avoid storing role assignments in Firestore that the ASP.NET app caches without revalidation. Instead, issue short-lived claims via identity providers and validate them on each request. middleBrick’s checks for BFLA and Property Authorization can highlight endpoints where roles or permissions are inferred from mutable Firestore data without proper scoping.

Finally, for applications that expose LLM endpoints, ensure that any routes tied to AI functionality are not inadvertently unauthenticated. middleBrick’s LLM Security checks include unauthenticated LLM endpoint detection, which complements the above fixes by identifying design gaps where AI services might be exposed without proper access controls.