Insecure Deserialization in Rails with Basic Auth

Insecure deserialization in Ruby on Rails occurs when an application reconstructs objects from untrusted data without sufficient validation. Rails provides Marshal.load and related methods that can instantiate arbitrary classes, making deserialization a high-risk operation if applied to attacker-controlled input. Basic Authentication (Basic Auth) is often used to provide a simple access boundary for an endpoint. When Basic Auth protects a deserialization path, developers may assume the protection is sufficient. In practice, this combination can expose or amplify the vulnerability in several ways.

• Trust boundary confusion: Basic Auth is commonly implemented via the authenticate_or_request_with_http_basic helper in controllers. If this check occurs before deserialization, an attacker who obtains or guesses a valid username and password can deliver malicious serialized payloads to the deserialization routine. The authentication layer grants access, but the deserialization layer performs the unsafe reconstruction of objects.
• Parameter handling: Rails parameters can be nested deeply and may include serialized objects via JSON, XML, or custom formats. Developers sometimes use YAML.safe_load with permissive allowed classes or, less safely, Marshal.load on user input. When Basic Auth is used to gate these actions, the deserialization code path is reachable, and the deserializer may instantiate classes that execute code during reconstruction, such as via custom _load or marshal_dump/marshal_restore hooks.
• SSRF and network exposure: Basic Auth over HTTP transmits credentials in easily decoded Base64. If the deserialization endpoint is also reachable internally (e.g., on localhost or internal services), an authenticated user may coerce the application to fetch and deserialize remote content, facilitating Server-Side Request Forgery (SSRF) and expanding the impact of insecure deserialization.

An example scenario: a Rails controller uses authenticate_or_request_with_http_basic to protect an import endpoint, then passes the request body to YAML.safe_load with a broad allowed class list or to Marshal.load. An authenticated attacker can send a crafted serialized payload that triggers remote code execution during deserialization, leading to arbitrary code execution in the application process.

To illustrate the risk context, this aligns with OWASP API Top 10 categories such as API1:2023 – Broken Object Level Authorization when deserialization is tied to object access, and broader insecure deserialization risks highlighted in the OWASP Top 10. Real-world CVEs affecting Ruby libraries and Rails plugins have demonstrated how deserialization of malicious payloads can lead to full server compromise.

Securing deserialization when Basic Auth is used requires strict controls on both authentication and deserialization practices. The goal is to ensure that deserialization never operates on untrusted data and that authentication does not create a false sense of security.

• Avoid deserializing user input: Refrain from using Marshal.load, YAML.load, or unsafe deserializers on request data. Prefer safe formats such as JSON with schema validation and use YAML.safe_load with an explicit, minimal allow list.
• Use strong parameters and whitelisting: Process input with permitted scalar values and avoid passing raw serialized blobs to deserialization routines.
• Apply authentication as one layer: Treat Basic Auth as transport-layer protection, not a substitute for input validation and object-level authorization.

Concrete Rails examples:

# Unsafe pattern to avoid: deserializing user input with Marshal
# Do NOT use this in production
raw_data = request.body.read
object = Marshal.load(raw_data) # High risk

# Safer alternative: use JSON with schema validation and strict permitted classes
require "json"
begin
  payload = JSON.parse(request.body.read, symbolize_names: true)
  # Validate payload structure and types explicitly
  title = payload.fetch(:title) { raise "missing title" }
  # Process scalar values only
rescue JSON::ParserError, KeyError => e
  render json: { error: "invalid input" }, status: :bad_request
end

# If YAML is required, use safe_load with a strict allow list
begin
  data = YAML.safe_load(
    request.body.read,
    permitted_classes: [Date, Time], # explicit allow list
    aliases: false
  )
rescue Psych::DisallowedClass => e
  render json: { error: "disallowed class" }, status: :bad_request
end

# Controller with Basic Auth using authenticate_or_request_with_http_basic
class Api::V1::ImportsController < ApplicationController
  before_action :set_http_basic_auth

  def create
    # Authentication happens before the action via before_action
    # Ensure deserialization is safe inside the action
    payload = JSON.parse(request.body.read, symbolize_names: true)
    # Process payload safely
    render json: { status: "ok" }
  end

  private

  def set_http_basic_auth
    authenticate_or_request_with_http_basic do |username, password|
      # Use secure password comparison (e.g., ActiveSupport::SecurityUtils.secure_compare)
      username == ENV["API_USER"] && password == ENV["API_PASS"]
    end
  end
end

Additional recommendations: enforce HTTPS to protect credentials in transit, rotate credentials regularly, and monitor for anomalous deserialization attempts. When using the middleBrick CLI (e.g., middlebrick scan <url>) or the GitHub Action to add API security checks to your CI/CD pipeline, ensure that scans include authenticated scenarios if Basic Auth is used, and review the findings for insecure deserialization indicators.