Phoenix LiveView Authentication Skill

Phoenix LiveView Authentication

RULES — Follow these with no exceptions

Always use on_mount callbacks for LiveView auth — never check auth in mount/3 directly; on_mount runs before mount and centralizes auth logic
Use mount_current_scope/2 to extract scope from session — never access session tokens manually or parse session data in LiveViews
Handle both :cont and :halt returns from on_mount — :halt must redirect with a flash message, never silently drop the connection
Resolve Controller/LiveView name clashes the way the 1.8 generator does — UserAuth contains both conn plugs and on_mount hooks. Import Phoenix.Controller normally (the plugs need redirect/2), and fully qualify the LiveView calls inside on_mount hooks: Phoenix.LiveView.redirect(socket, to: ...) and Phoenix.LiveView.put_flash(socket, :error, ...). Excluding the Controller imports breaks the plug half of the module.
Guard the nil scope, not the assign lookup — mount_current_scope always assigns :current_scope, so @current_scope is safe; the hazard is calling .user on a nil scope. Write @current_scope && @current_scope.user. Bracket access assigns[:current_scope] is only needed when the assign may be entirely absent (e.g. layouts shared with non-auth live_sessions).
Test auth redirects by asserting {:error, {:redirect, %{to: path}}} — don't test auth by checking rendered content; verify the redirect tuple from live/2
Define on_mount hooks once, reference via live_session in router — never duplicate auth logic across LiveView modules

on_mount Authentication Pattern

The standard pattern for LiveView authentication. Define once, use everywhere via live_session.

defmodule MyAppWeb.UserAuth do
  use MyAppWeb, :verified_routes
  import Plug.Conn
  import Phoenix.Controller

  # Called by live_session :require_authenticated_user
  def on_mount(:require_authenticated_user, _params, session, socket) do
    socket = mount_current_scope(socket, session)

    if socket.assigns.current_scope && socket.assigns.current_scope.user do
      {:cont, socket}
    else
      socket =
        socket
        |> Phoenix.LiveView.put_flash(:error, "You must log in to access this page.")
        |> Phoenix.LiveView.redirect(to: ~p"/users/log-in")

      {:halt, socket}
    end
  end

  # Called by live_session :redirect_if_authenticated
  def on_mount(:redirect_if_authenticated, _params, session, socket) do
    socket = mount_current_scope(socket, session)

    if socket.assigns.current_scope && socket.assigns.current_scope.user do
      {:halt, Phoenix.LiveView.redirect(socket, to: ~p"/")}
    else
      {:cont, socket}
    end
  end

  # Conn plug — needs the plain Phoenix.Controller redirect/2
  def require_authenticated_user(conn, _opts) do
    if conn.assigns[:current_scope] && conn.assigns.current_scope.user do
      conn
    else
      conn
      |> put_flash(:error, "You must log in to access this page.")
      |> redirect(to: ~p"/users/log-in")
      |> halt()
    end
  end

  # Called by live_session :mount_current_scope (public pages)
  def on_mount(:mount_current_scope, _params, session, socket) do
    {:cont, mount_current_scope(socket, session)}
  end

  defp mount_current_scope(socket, session) do
    Phoenix.Component.assign_new(socket, :current_scope, fn ->
      if user = find_user_from_session(session) do
        %Scope{user: user}
      end
    end)
  end

  defp find_user_from_session(%{"user_token" => token}) do
    Accounts.get_user_by_session_token(token)
  end

  defp find_user_from_session(_session), do: nil
end

Router Integration

Use live_session to apply on_mount hooks to groups of LiveViews. Each session shares auth requirements.

defmodule MyAppWeb.Router do
  use MyAppWeb, :router

  # Public pages — scope is mounted but not required
  live_session :mount_current_scope,
    on_mount: [{MyAppWeb.UserAuth, :mount_current_scope}] do
    scope "/", MyAppWeb do
      pipe_through :browser

      live "/", HomeLive.Index
    end
  end

  # Authenticated pages — redirects to login if not authenticated
  live_session :require_authenticated_user,
    on_mount: [{MyAppWeb.UserAuth, :require_authenticated_user}] do
    scope "/", MyAppWeb do
      pipe_through [:browser, :require_authenticated_user]

      live "/dashboard", DashboardLive.Index
      live "/settings", SettingsLive.Index
    end
  end

  # Guest-only pages — redirects to home if already authenticated
  live_session :redirect_if_authenticated,
    on_mount: [{MyAppWeb.UserAuth, :redirect_if_authenticated}] do
    scope "/", MyAppWeb do
      pipe_through [:browser, :redirect_if_authenticated]

      live "/users/register", UserRegistrationLive
      live "/users/log-in", UserLoginLive
    end
  end
end

Import Conflict Resolution

Phoenix.Controller and Phoenix.LiveView both export redirect/2 and put_flash/3. UserAuth needs both — conn plugs for the router pipeline, and on_mount hooks for LiveView — so excluding one side's imports breaks the other half of the module. The 1.8 generator resolves this by importing Phoenix.Controller normally and fully qualifying the LiveView calls:

# Bad — excluding Phoenix.Controller breaks the plug functions,
# which need plain redirect/2 and put_flash/3
import Phoenix.LiveView
import Phoenix.Controller, except: [redirect: 2, put_flash: 3]

# Good — import Phoenix.Controller normally for the plugs;
# fully qualify inside on_mount hooks instead
import Phoenix.Controller

def on_mount(:require_authenticated_user, _params, session, socket) do
  # ...
  Phoenix.LiveView.redirect(socket, to: ~p"/")
  Phoenix.LiveView.put_flash(socket, :error, "...")
end

def require_authenticated_user(conn, _opts) do
  # plain redirect/2 and put_flash/3 here come from Phoenix.Controller
  conn |> put_flash(:error, "...") |> redirect(to: ~p"/users/log-in")
end

current_scope vs current_user

Phoenix 1.8+ uses Scope structs instead of raw current_user. The scope wraps the user and can carry additional context.

# Phoenix 1.8+ pattern — Scope struct
defmodule MyApp.Scope do
  defstruct [:user]
end

# In LiveView — access user through scope
def mount(_params, _session, socket) do
  user = socket.assigns.current_scope.user
  {:ok, assign(socket, :posts, Posts.list_posts(user))}
end

# In templates — @current_scope is always assigned once on_mount has
# run, so dot access is safe; guard .user since the scope may wrap no user
<%= if @current_scope && @current_scope.user do %>
  <p>Welcome, <%= @current_scope.user.email %></p>
<% end %>

Guarding a Nil Scope

mount_current_scope/2 always assigns :current_scope (via assign_new), so @current_scope is safe to dot-access once an on_mount hook has run. The real hazard is calling .user when nobody is logged in — the scope itself is non-nil, but scope.user is nil.

# Bad — crashes when current_scope.user is nil (guest visitor)
<%= @current_scope.user.email %>

# Good — guard .user, not the current_scope lookup
<%= if @current_scope && @current_scope.user do %>
  <%= @current_scope.user.email %>
<% end %>

Bracket access assigns[:current_scope] is only needed when the assign may be entirely absent — for example, a layout shared with a live_session that never runs the mount_current_scope on_mount hook:

<%= if assigns[:current_scope] && @current_scope.user do %>
  <%= @current_scope.user.email %>
<% end %>

Testing LiveView Auth

Testing Protected Routes

describe "require_authenticated_user" do
  test "redirects if not logged in", %{conn: conn} do
    assert {:error, {:redirect, %{to: "/users/log-in"}}} =
             live(conn, ~p"/dashboard")
  end

  test "renders page when authenticated", %{conn: conn} do
    user = user_fixture()
    conn = log_in_user(conn, user)

    {:ok, _lv, html} = live(conn, ~p"/dashboard")
    assert html =~ "Dashboard"
  end
end

describe "redirect_if_authenticated" do
  test "redirects if already logged in", %{conn: conn} do
    user = user_fixture()
    conn = log_in_user(conn, user)

    assert {:error, {:redirect, %{to: "/"}}} =
             live(conn, ~p"/users/log-in")
  end
end

Testing on_mount Directly

describe "on_mount: :require_authenticated_user" do
  test "authenticates user from session", %{conn: conn} do
    user = user_fixture()
    token = Accounts.generate_user_session_token(user)

    assert {:cont, updated_socket} =
             UserAuth.on_mount(
               :require_authenticated_user,
               %{},
               %{"user_token" => token},
               %LiveView.Socket{
                 endpoint: MyAppWeb.Endpoint,
                 assigns: %{__changed__: %{}}
               }
             )

    assert updated_socket.assigns.current_scope.user.id == user.id
  end

  test "redirects when no session token" do
    assert {:halt, updated_socket} =
             UserAuth.on_mount(
               :require_authenticated_user,
               %{},
               %{},
               %LiveView.Socket{
                 endpoint: MyAppWeb.Endpoint,
                 assigns: %{__changed__: %{}, flash: %{}}
               }
             )

    assert updated_socket.redirected == {:redirect, %{to: "/users/log-in"}}
  end
end

See testing-essentials skill for comprehensive testing patterns. See phoenix-authorization-patterns skill for authorization after authentication.

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