Insecure Direct Object Reference in Fiber with Firestore

Insecure Direct Object Reference (BOLA/IDOR) occurs when an API exposes internal object references such as Firestore document IDs without enforcing access control per request. In a Fiber application, route handlers often extract a document ID from the URL (for example, /users/:userID/documents/:docID) and directly query Firestore using that ID. If the handler does not validate that the authenticated user is authorized to access the specific Firestore document, an attacker can modify the ID to access or manipulate other users' data.

Consider a typical Fiber handler that retrieves a document by ID:

// WARNING: Insecure example — missing authorization check
app.Get("/documents/:docID", func(c *fiber.Ctx) error {
    docID := c.Params("docID")
    ctx := context.Background()
    client, err := firestore.NewClient(ctx, "my-project-id")
    if err != nil {
        return c.Status(fiber.StatusInternalServerError).JSON(fiber.Map{"error": "failed to create client"})
    }
    defer client.Close()

    docRef := client.Collection("documents").Doc(docID)
    docSnap, err := docRef.Get(ctx)
    if err != nil {
        return c.Status(fiber.StatusInternalServerError).JSON(fiber.Map{"error": "failed to fetch document"})
    }
    if !docSnap.Exists() {
        return c.Status(fiber.StatusNotFound).JSON(fiber.Map{"error": "document not found"})
    }
    return c.JSON(docSnap.Data())
})

In this example, the Firestore document ID comes directly from the URL with no verification that the requesting user owns or is permitted to access that document. An authenticated user can change the docID parameter to reference any document in the collection, potentially reading or triggering operations on other users' data. Because Firestore permissions are enforced at the database level, this missing authorization check means the API relies solely on the developer to implement proper checks; if omitted, the endpoint is vulnerable to BOLA/IDOR.

The risk is compounded when the endpoint exposes sensitive Firestore fields (such as role flags or PII) or when IDs are predictable (e.g., sequential integers or UUIDs that are not randomly assigned). Attackers may combine this with other techniques like probing for open endpoints or scanning for misconfigured rules. MiddleBrick detects such patterns during its 12 parallel security checks, including BOLA/IDOR and Property Authorization, and surfaces findings with severity and remediation guidance.

Because this is an unauthenticated attack surface scan, middleBrick can evaluate the endpoint without credentials, identifying endpoints where object references are used without proper authorization. Findings map to relevant compliance frameworks such as OWASP API Top 10 (A01:2023 Broken Object Level Authorization) and help prioritize fixes before deploying to production.

To remediate IDOR in Fiber when working with Firestore, enforce ownership or role-based access control within each handler. The key is to resolve the user identity (from session or JWT) and use it to scope Firestore queries so that users can only access documents they are permitted to view or modify.

Example: a user-specific endpoint that retrieves a document only if it belongs to the requesting user:

// Secure example — user-based Firestore scoping
app.Get("/documents/:docID", func(c *fiber.Ctx) error {
    docID := c.Params("docID")
    // Assume you have an authenticated user with a UID extracted from token/session
    userID, ok := c.Locals("userID").(string)
    if !ok || userID == "" {
        return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "missing user identity"})
    }

    ctx := context.Background()
    client, err := firestore.NewClient(ctx, "my-project-id")
    if err != nil {
        return c.Status(fiber.StatusInternalServerError).JSON(fiber.Map{"error": "failed to create client"})
    }
    defer client.Close()

    // Scope the document reference to the user’s collection
    docRef := client.Collection("users").Doc(userID).Collection("documents").Doc(docID)
    docSnap, err := docRef.Get(ctx)
    if err != nil {
        return c.Status(fiber.StatusInternalServerError).JSON(fiber.Map{"error": "failed to fetch document"})
    }
    if !docSnap.Exists() {
        return c.Status(fiber.StatusNotFound).JSON(fiber.Map{"error": "document not found"})
    }
    return c.JSON(docSnap.Data())
})

In this secure pattern, the Firestore path includes the authenticated user ID (e.g., users/{userID}/documents/{docID}), ensuring that document IDs are scoped to a user namespace. Even if an attacker guesses or iterates over document IDs, they cannot access documents outside their own user collection. For shared or admin resources, implement an additional authorization layer (e.g., role checks or allow-lists) before performing reads or writes.

When using Firestore security rules, align backend checks with rules that validate request.auth.uid matches the document’s owner field. MiddleBrick’s BOLA/IDOR and Property Authorization checks can highlight endpoints where runtime behavior diverges from intended rules, helping prioritize fixes.

For broader protection, apply consistent scoping across all Firestore interactions in Fiber routes, validate input IDs for format and length, and return generic error messages to avoid leaking information. The Pro plan’s continuous monitoring can alert you if new endpoints introduce missing authorization, and the GitHub Action can fail builds when risk thresholds are exceeded.