Broken Access Control in Phoenix

Broken Access Control in Phoenix applications often stems from Phoenix's flexible routing and authorization patterns. The most common manifestation occurs when developers rely on implicit authorization rather than explicit permission checks. For example, a Phoenix controller might fetch a user's profile using Repo.get(User, id) without verifying that the authenticated user actually owns that profile.

Phoenix's plug system creates another vulnerability vector. When authorization plugs are placed incorrectly in the pipeline, unauthenticated requests can bypass critical checks. A typical mistake is mounting a resource route before the :authenticate plug, allowing anyone to access /api/users/:id without proper authentication.

Phoenix's Ecto queries compound these issues. Developers often write queries like User |> Repo.get(id) without scoping to the current user's accessible records. This pattern is particularly dangerous in admin interfaces where users might manipulate query parameters to access other users' data.

Cross-tenant data exposure is another Phoenix-specific issue. In multi-tenant applications, developers might forget to scope queries by tenant ID, allowing users to access data from other organizations. Phoenix's context pattern, while helpful, can lead to over-trusting context functions that don't validate user permissions.

Phoenix LiveView introduces additional complexity. State management in LiveView components can inadvertently expose data to unauthorized users if the component doesn't properly validate user permissions before rendering sensitive information. The reactive nature of LiveView means that authorization failures might not be immediately obvious until the component attempts to access restricted data.

Detecting Broken Access Control in Phoenix applications requires examining both the code structure and runtime behavior. Start by analyzing your router configuration. Look for routes that are publicly accessible but should be protected, particularly around resource routes that handle sensitive operations like user management or financial transactions.

Code review should focus on controller actions that accept IDs or other identifiers. Search for patterns like Repo.get(User, params[:id]) without subsequent authorization checks. Pay special attention to actions that perform updates or deletions, as these are high-risk operations.

middleBrick's Phoenix-specific scanning can identify these vulnerabilities automatically. The scanner tests unauthenticated endpoints for BOLA (Broken Object Level Authorization) vulnerabilities by attempting to access resources with manipulated IDs. It also checks for missing authentication requirements on sensitive routes.

middleBrick analyzes your Phoenix application's OpenAPI spec (if available) to identify endpoints that should require authentication but don't. The scanner then attempts active exploitation of these findings, providing concrete evidence of the vulnerability rather than just theoretical warnings.

Runtime detection involves monitoring for unusual access patterns. Phoenix's logging can help identify when users access resources they shouldn't have permission to view. Look for patterns where users access records with IDs significantly different from their own, or where they access records at a rate suggesting automated enumeration attempts.

middleBrick's continuous monitoring in the Pro plan can detect these runtime anomalies. The scanner maintains a baseline of normal access patterns and alerts you when users attempt to access resources outside their normal scope, providing early warning of potential Broken Access Control exploitation.

Phoenix provides several native mechanisms for fixing Broken Access Control. The Guardian library (or similar JWT implementations) integrates well with Phoenix's plug system for authentication. Create a dedicated authorization plug that verifies user permissions before allowing access to sensitive resources.

defmodule MyAppWeb.Plugs.Authorize do
  import Plug.Conn
  import Phoenix.Controller

  def init(opts), do: opts

  def call(conn, _opts) do
    current_user = conn.assigns[:current_user]
    resource_id = conn.params["id"]
    
    if authorized?(current_user, resource_id) do
      conn
    else
      conn
      |> put_status(403)
      |> json(%{error: "Forbidden"})
      |> halt()
    end
  end

  defp authorized?(user, resource_id) do
    # Implement your authorization logic
    # Check if user owns the resource or has required permissions
    case Repo.get(User, resource_id) do
      %User{id: id} when id == user.id -> true
      _ -> false
    end
  end
end

Phoenix contexts provide an excellent place to centralize authorization logic. Create context functions that automatically scope queries to the current user:

defmodule MyApp.Accounts do
  alias MyApp.Repo
  alias MyApp.Accounts.User

  def get_user_by_id(user_id, current_user_id) do
    if user_id == current_user_id do
      Repo.get(User, user_id)
    else
      nil
    end
  end

  def list_user_resources(user_id, current_user_id) do
    if user_id == current_user_id do
      # Return only resources owned by this user
      from(r in Resource, where: r.user_id == ^user_id) |> Repo.all()
    else
      []
    end
  end
end

For LiveView applications, implement authorization at the component level. Before mounting a LiveView, verify that the user has permission to access the data:

defmodule MyAppWeb.UserLive.Show do
  use MyAppWeb, :live_view

  def mount(%{"id" => id}, %{"user_id" => current_user_id}, socket) do
    if authorized?(id, current_user_id) do
      user = Accounts.get_user_by_id(id, current_user_id)
      
      socket =
        socket
        |> assign(:user, user)
        |> assign(:page_title, "User: #{user.name}")

      {:ok, socket}
    else
      {:error, :unauthorized}
    end
  end

  defp authorized?(requested_id, current_user_id) do
    requested_id == current_user_id
  end
end

middleBrick's CLI tool can help validate your remediation efforts. After implementing fixes, run middlebrick scan to verify that previously vulnerable endpoints now properly enforce authorization. The tool provides detailed reports showing which endpoints are now secure and which still need attention.

For continuous assurance, integrate middleBrick's GitHub Action into your CI/CD pipeline. Configure it to fail builds if any API endpoints score below a security threshold, ensuring that Broken Access Control vulnerabilities are caught before deployment. The action can scan your staging environment automatically on pull requests, providing immediate feedback to developers.