Use CSP nonces for script and style tags (#61)

This pull request adds a new `:csp_nonce_assign_key` option to the
`error_tracker_dashboard/2` macro. If provided, the error tracker will
fetch the nonce from the given assign key and use it in the `<style>`
and `<script>` tags.

This allows using the ErrorTracker dashboard in environments with a
restricted content security policy without requiring the usage of
`unsafe-inline`, which should be avoided.

This implementation is based on the [Phoenix
LiveDashboard](https://hexdocs.pm/phoenix_live_dashboard/Phoenix.LiveDashboard.Router.html#live_dashboard/2)
one.

I've updated the `dev.exs` script to use CSP headers. If we remove the
new option we will see that the ErrorTracker dashboard doesn't have any
styles.

Closes #58

dev.exs

@@ -115,7 +115,7 @@ defmodule ErrorTrackerDevWeb.Router do
     get "/exit", ErrorTrackerDevWeb.PageController, :exit
 
     scope "/dev" do
-      error_tracker_dashboard "/errors"
+      error_tracker_dashboard "/errors", csp_nonce_assign_key: :my_csp_nonce
     end
   end
 end
@@ -142,10 +142,24 @@ defmodule ErrorTrackerDevWeb.Endpoint do
   plug Plug.RequestId
   plug Plug.Telemetry, event_prefix: [:phoenix, :endpoint]
   plug :maybe_exception
+  plug :set_csp
   plug ErrorTrackerDevWeb.Router
 
   def maybe_exception(%Plug.Conn{path_info: ["plug-exception"]}, _), do: raise("Plug exception")
   def maybe_exception(conn, _), do: conn
+
+  defp set_csp(conn, _opts) do
+    nonce = 10 |> :crypto.strong_rand_bytes() |> Base.encode64()
+
+    policies = [
+      "script-src 'self' 'nonce-#{nonce}';",
+      "style-src 'self' 'nonce-#{nonce}';"
+    ]
+
+    conn
+    |> Plug.Conn.assign(:my_csp_nonce, "#{nonce}")
+    |> Plug.Conn.put_resp_header("content-security-policy", Enum.join(policies, " "))
+  end
 end
 
 defmodule ErrorTrackerDev.Telemetry do

lib/error_tracker/web/components/layouts/root.html.heex

@@ -10,10 +10,10 @@
 
     <title><%= assigns[:page_title] || "🐛 ErrorTracker" %></title>
 
-    <style>
+    <style nonce={@csp_nonces[:style]}>
       <%= raw get_content(:css) %>
     </style>
-    <script>
+    <script nonce={@csp_nonces[:script]}>
       <%= raw get_content(:js) %>
     </script>
   </head>

lib/error_tracker/web/hooks/set_assigns.ex

@@ -1,7 +1,11 @@
 defmodule ErrorTracker.Web.Hooks.SetAssigns do
   @moduledoc false
 
-  def on_mount({:set_dashboard_path, path}, _params, _session, socket) do
-    {:cont, %{socket | private: Map.put(socket.private, :dashboard_path, path)}}
+  import Phoenix.Component, only: [assign: 2]
+
+  def on_mount({:set_dashboard_path, path}, _params, session, socket) do
+    socket = %{socket | private: Map.put(socket.private, :dashboard_path, path)}
+
+    {:cont, assign(socket, csp_nonces: session["csp_nonces"])}
   end
 end

lib/error_tracker/web/router.ex

@@ -3,16 +3,18 @@ defmodule ErrorTracker.Web.Router do
   ErrorTracker UI integration into your application's router.
   """
 
+  alias ErrorTracker.Web.Hooks.SetAssigns
+
   @doc """
   Creates the routes needed to use the `ErrorTracker` web interface.
 
   It requires a path in which you are going to serve the web interface.
 
   ## Security considerations
 
-  Errors may contain sensitive information so it is recommended to use the `on_mount`
-  option to provide a custom hook that implements authentication and authorization
-  for access control.
+  The dashboard inlines both the JS and CSS assets. This means that, if your
+  application has a Content Security Policy, you need to specify the
+  `csp_nonce_assign_key` option, which is explained below.
 
   ## Options
 
@@ -21,6 +23,10 @@ defmodule ErrorTracker.Web.Router do
 
   * `as`: a session name to use for the dashboard LiveView session. By default
   it uses `:error_tracker_dashboard`.
+
+  * `csp_nonce_assign_key`: an assign key to find the CSP nonce value used for assets.
+  Supports either `atom()` or a map of type
+  `%{optional(:img) => atom(), optional(:script) => atom(), optional(:style) => atom()}`
   """
   defmacro error_tracker_dashboard(path, opts \\ []) do
     quote bind_quoted: [path: path, opts: opts] do
@@ -45,17 +51,32 @@ defmodule ErrorTracker.Web.Router do
   @doc false
   def parse_options(opts, path) do
     custom_on_mount = Keyword.get(opts, :on_mount, [])
-
-    on_mount =
-      [{ErrorTracker.Web.Hooks.SetAssigns, {:set_dashboard_path, path}}] ++ custom_on_mount
-
     session_name = Keyword.get(opts, :as, :error_tracker_dashboard)
 
+    csp_nonce_assign_key =
+      case opts[:csp_nonce_assign_key] do
+        nil -> nil
+        key when is_atom(key) -> %{img: key, style: key, script: key}
+        keys when is_map(keys) -> Map.take(keys, [:img, :style, :script])
+      end
+
     session_opts = [
-      on_mount: on_mount,
+      session: {__MODULE__, :__session__, [csp_nonce_assign_key]},
+      on_mount: [{SetAssigns, {:set_dashboard_path, path}}] ++ custom_on_mount,
       root_layout: {ErrorTracker.Web.Layouts, :root}
     ]
 
     {session_name, session_opts}
   end
+
+  @doc false
+  def __session__(conn, csp_nonce_assign_key) do
+    %{
+      "csp_nonces" => %{
+        img: conn.assigns[csp_nonce_assign_key[:img]],
+        style: conn.assigns[csp_nonce_assign_key[:style]],
+        script: conn.assigns[csp_nonce_assign_key[:script]]
+      }
+    }
+  end
 end