Insecure Deserialization in Rails with Firestore

Insecure deserialization occurs when an application processes untrusted data without integrity checks, allowing an attacker to manipulate serialized objects to execute code or alter behavior. In a Ruby on Rails application using Google Cloud Firestore as a backend, risk arises at the intersection of Rails’ object handling and Firestore document payloads.

Firestore stores documents as JSON-like structures; however, Rails may convert incoming data into Ruby objects via ActiveModel or custom serializers before writing to or reading from Firestore. If deserialization of attacker-controlled data is performed—such as reconstructing Ruby objects from parameters stored in Firestore or from external input that is later merged with document data—methods like Marshal.load or unsafe YAML/JSON extensions are potential vectors. An attacker who can influence a Firestore document (e.g., through compromised user data or misconfigured rules) might craft payloads that, when deserialized by Rails code, lead to remote code execution or logic bypass. Common patterns include storing serialized Ruby objects in Firestore fields or using Firestore-triggered background jobs that deserialize data without strict validation.

The unauthenticated attack surface of Firestore can be enumerated by scanning endpoints that expose document reads or writes without proper authentication. Insecure deserialization findings may map to OWASP API Top 10 A01:2023 (Broken Object Level Authorization) and A08:2023 (Software and Data Integrity Failures), and can intersect with BFLA scenarios where privilege escalation is possible via tampered serialized state.

Example risk scenario: a Rails controller fetches a Firestore document containing a serialized user preferences blob and passes it to Marshal.load to restore an object. If an attacker can modify that document, they can supply a malicious payload. Even without direct document write access, insufficient input validation and missing integrity checks on retrieved data create an exploitable path.

To mitigate insecure deserialization in Rails when interacting with Firestore, avoid deserializing untrusted data and enforce strict schema validation. Prefer JSON parsing over Ruby-specific serialization, and treat all Firestore data as untrusted input.

1. Use safe JSON parsing instead of Marshal

Replace Marshal.load with JSON.parse and schema validation. Firestore documents are inherently JSON-compatible; deserialize with permitted structures only.

require "json"
require "google/cloud/firestore"

firestore = Google::Cloud::Firestore.new
user_doc = firestore.doc("users/#{user_id}").get
raw_data = user_doc.data # returns a Hash

# Safe: validate and permit known keys
allowed_keys = ["theme", "language", "notifications_enabled"]
preferences = raw_data.slice(*allowed_keys)
# Use preferences as a plain Hash; do not invoke Marshal.load on raw_data

2. Enforce schema validation on retrieved documents

Use a validation layer (e.g., dry-validation, ActiveModel::Validations) to ensure Firestore documents conform to expected shapes before use.

class UserPreferences
  include ActiveModel::Model
  attr_accessor :theme, :language, :notifications_enabled

  validates :theme, inclusion: { in: %w[light dark system] }, allow_nil: true
  validates :language, format: { with: /\A[a-z]{2}\z/ }, allow_nil: true
  validates :notifications_enabled, inclusion: { in: [true, false] }
end

attrs = UserPreferences.new(raw_data)
if attrs.valid?
  # proceed safely
else
  Rails.logger.warn("Invalid preferences: #{attrs.errors.full_messages}")
end

3. Avoid eval or YAML.load on Firestore content

Never use YAML.load or ERB.new on data sourced from Firestore. If you must handle complex structures, use JSON with symbol conversion disabled and maintain a strict whitelist of permitted fields.

require "yaml"

# Unsafe: YAML.load can instantiate arbitrary classes
# data = YAML.load(untrusted_yaml) # DO NOT DO THIS

# Safer alternative: use JSON or safe YAML loading with permitted classes only
safe_data = JSON.parse(untrusted_json, symbolize_names: false)

4. Principle of least privilege for Firestore rules

Ensure Firestore security rules limit write access to trusted sources and validate data shapes. Rules cannot prevent insecure deserialization in Rails code, but they reduce the risk of malicious document modification.

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    match /users/{userId} {
      allow read: if request.auth != null && request.auth.uid == userId;
      allow write: if request.auth != null && request.auth.uid == userId
        && request.resource.data.keys().hasAll(['theme', 'language', 'notifications_enabled'])
        && request.resource.data.theme.matches('^(light|dark|system)$')
        && request.resource.data.language.matches('^[a-z]{2}$');
    }
  }
}

By combining strict deserialization hygiene with properly defined Firestore rules and schema-aware validation, Rails applications can safely work with Firestore data without introducing object injection risks.