Insecure Direct Object Reference in Buffalo with Firestore

An Insecure Direct Object Reference (BOLA/IDOR) in a Buffalo application that uses Google Cloud Firestore occurs when an endpoint exposes a Firestore document ID or path derived solely from user-supplied input without verifying that the requesting subject has permission to access that specific document. Because Firestore security rules are not enforced at the application layer, the API must enforce ownership and authorization explicitly. If Buffalo routes a request such as /clients/{clientID}/invoices/{invoiceID} and uses clientID and invoiceID directly to build a Firestore path like clients/{clientID}/invoices/{invoiceID} without confirming the authenticated user belongs to that client, an attacker can manipulate IDs to access or enumerate other clients' invoices.

In this stack, the vulnerability is amplified by Firestore’s flat, scalable namespace and predictable document paths. An attacker can iterate through numeric or predictable IDs (e.g., invoice IDs 1001, 1002) and read data they should not see if the backend does not enforce tenant isolation. Unlike SQL databases, Firestore does not offer row-level security at the database level for multi-tenant access patterns unless rules are meticulously designed; therefore, the application must implement robust checks. Common root causes include missing tenant context validation, trusting URL path parameters alone, and failing to scope queries by the authenticated user’s ID or organization ID.

During a middleBrick scan, such misconfigurations are surfaced across multiple checks, including BOLA/IDOR and Property Authorization. The scanner correlates runtime observations with the OpenAPI spec to detect endpoints that accept resource identifiers without corresponding access controls. For instance, if an endpoint accepts a Firestore document ID in the URL but the request handler does not validate that the authenticated user’s ID matches the document’s owner field, middleBrick highlights this as a high-severity finding. Attack patterns like IDOR enable horizontal privilege escalation, where a user accesses another user’s resources, or vertical privilege escalation if the exposed references include administrative documents.

Real-world examples include endpoints that retrieve Firestore documents by ID without verifying the requesting user’s relationship to the document’s parent tenant. Consider an invoice service where the URL contains both a client slug and an invoice number. If the server builds a Firestore reference as clients/{clientID}/invoices/{invoiceID} and fetches the document without ensuring the authenticated user is a member of clientID, the API leaks data across client boundaries. middleBrick’s detection of such issues includes validating that path parameters are constrained by session or token-derived tenant identifiers and that Firestore document reads are scoped accordingly.

Remediation centers on enforcing tenant or user ownership at the handler level before constructing Firestore paths and ensuring queries are scoped to the authenticated subject. Always resolve the user or session context, then include the user or tenant identifier in the Firestore document path or query. Avoid using raw user input as the sole document key without validation.

Example: Secure invoice retrieval with tenant scoping

Assume a Buffalo app with a current user stored in the context and a Firestore collection structure where invoices are nested under clients. The client identifier must be derived from the user’s tenant membership, not from the URL alone.

// handlers/invoices.go
package handlers

import (
	"context"
	"net/http"

	"github.com/gobuffalo/buffalo"
	"cloud.google.com/go/firestore"
	"google.golang.org/api/iterator"
)

type InvoiceHandler struct {
	DB       *firestore.Client
	TenantID string // derived from session or JWT, not from URL
}

// Show retrieves an invoice for the current tenant and user.
func (h *InvoiceHandler) Show(params buffalo.Params) error {
	ctx := context.Background()
	// params["invoiceID"] is user-supplied; do not use it alone to build a path.
	invoiceID := params.Get("invoiceID")

	// Scope to the tenant derived from auth/session.
	invoiceRef := h.DB.Collection("tenants").Doc(h.TenantID).Collection("invoices").Doc(invoiceID)
	var invoice Invoice
	if err := invoiceRef.Get(ctx, &invoice); err != nil {
		if err == iterator.Done {
			return buffalo.WrapError(http.StatusNotFound, err)
		}
		return buffalo.WrapError(http.StatusInternalServerError, err)
	}
	return invoice.Render(buffalo.JSON, invoice)
}

In this approach, h.TenantID must be resolved from the authenticated session or token claims, ensuring that even if an attacker changes the invoiceID in the URL, they cannot access invoices belonging to other tenants. The Firestore document path explicitly includes the tenant ID, so cross-tenant reads are prevented at the reference level.

Example: Query-scoped access with owner field validation

If denormalization is used and the invoice document contains an owner or tenant field, always verify ownership after fetching. This double-check pattern is useful when Firestore security rules cannot enforce tenant boundaries at read time.

// handlers/clients.go
package handlers

import (
	"context"
	"net/http"

	"github.com/gobuffalo/buffalo"
	"cloud.google.com/go/firestore"
)

type ClientHandler struct {
	DB *firestore.Client
}

type Invoice struct {
	ID      string `firestore:"id"`
	OwnerID string
	Amount  float64
}

// GetInvoiceForCurrentUser ensures the invoice belongs to the current user.
func (h *ClientHandler) GetInvoiceForCurrentUser(params buffalo.Params) error {
	ctx := context.Background()
	invoiceID := params.Get("invoiceID")
	userID := getCurrentUserID(ctx) // implementation-specific

	iter := h.DB.Collection("invoices").
		Where("owner_id", "==", userID).
		Where("id", "==", invoiceID).
		Limit(1).Documents(ctx)

	defer iter.Stop()
	doc, err := iter.Next()
	if err == iterator.Done {
		return buffalo.WrapError(http.StatusNotFound, err)
	}
	if err != nil {
		return buffalo.WrapError(http.StatusInternalServerError, err)
	}
	var invoice Invoice
	if err := doc.DataTo(&invoice); err != nil {
		return buffalo.WrapError(http.StatusInternalServerError, err)
	}
	return invoice.Render(buffalo.JSON, invoice)
}

This query explicitly scopes reads to documents where the owner matches the authenticated user, mitigating IDOR even if the invoiceID is guessable. For Firestore, combine path scoping (tenant collection hierarchy) with field-level ownership checks where appropriate.

Additional hardening

• Use UUIDs or opaque identifiers instead of sequential numeric IDs to reduce enumeration risk.
• Validate that tenant or organization identifiers in session/JWT claims are used to scope all Firestore references.
• Audit Firestore security rules to ensure they align with application-level checks; remember that rules are not a substitute for handler validation in multi-tenant applications.