akkoma/lib/pleroma/web/plugs/http_security_plug.ex

# Pleroma: A lightweight social networking server
# Copyright © 2017-2021 Pleroma Authors <https://pleroma.social/>
# SPDX-License-Identifier: AGPL-3.0-only

defmodule Pleroma.Web.Plugs.HTTPSecurityPlug do
  alias Pleroma.Config
  import Plug.Conn

  require Logger

  def init(opts), do: opts

  def call(conn, _options) do
    if Config.get([:http_security, :enabled]) do
      conn
      |> merge_resp_headers(headers(conn))
      |> maybe_send_sts_header(Config.get([:http_security, :sts]))
    else
      conn
    end
  end

  def primary_frontend do
    with %{"name" => frontend} <- Config.get([:frontends, :primary]),
         available <- Config.get([:frontends, :available]),
         %{} = primary_frontend <- Map.get(available, frontend) do
      {:ok, primary_frontend}
    end
  end

  def custom_http_frontend_headers do
    with {:ok, %{"custom-http-headers" => custom_headers}} <- primary_frontend() do
      custom_headers
    else
      _ -> []
    end
  end

  @spec headers(Plug.Conn.t()) :: [{String.t(), String.t()}]
  def headers(conn) do
    referrer_policy = Config.get([:http_security, :referrer_policy])
    report_uri = Config.get([:http_security, :report_uri])
    custom_http_frontend_headers = custom_http_frontend_headers()

    headers = [
      {"x-xss-protection", "0"},
      {"x-permitted-cross-domain-policies", "none"},
      {"x-frame-options", "DENY"},
      {"x-content-type-options", "nosniff"},
      {"referrer-policy", referrer_policy},
      {"content-security-policy", csp_string(conn)},
      {"permissions-policy", "interest-cohort=()"}
    ]

    headers =
      if custom_http_frontend_headers do
        custom_http_frontend_headers ++ headers
      else
        headers
      end

    if report_uri do
      report_group = %{
        "group" => "csp-endpoint",
        "max-age" => 10_886_400,
        "endpoints" => [
          %{"url" => report_uri}
        ]
      }

      [{"report-to", Jason.encode!(report_group)} | headers]
    else
      headers
    end
  end

  static_csp_rules = [
    "default-src 'none'",
    "base-uri 'none'",
    "frame-ancestors 'none'",
    "manifest-src 'self'"
  ]

  @csp_start [Enum.join(static_csp_rules, ";") <> ";"]

  defp csp_string(conn) do
    scheme = Config.get([Pleroma.Web.Endpoint, :url])[:scheme]
    static_url = Pleroma.Web.Endpoint.static_url()
    websocket_url = Pleroma.Web.Endpoint.websocket_url()
    report_uri = Config.get([:http_security, :report_uri])
    %{assigns: %{csp_nonce: nonce}} = conn
    nonce_tag = "nonce-" <> nonce
    img_src = "img-src 'self' data: blob:"
    media_src = "media-src 'self'"

    # Strict multimedia CSP enforcement only when MediaProxy is enabled
    {img_src, media_src} =
      if Config.get([:media_proxy, :enabled]) &&
           !Config.get([:media_proxy, :proxy_opts, :redirect_on_failure]) do
        sources = build_csp_multimedia_source_list()
        {[img_src, sources], [media_src, sources]}
      else
        {[img_src, " https:"], [media_src, " https:"]}
      end

    connect_src =
      if Config.get([:media_proxy, :enabled]) do
        sources = build_csp_multimedia_source_list()
        ["connect-src 'self' blob: ", static_url, ?\s, websocket_url, ?\s, sources]
      else
        ["connect-src 'self' blob: ", static_url, ?\s, websocket_url]
      end

    style_src = "style-src 'self' '#{nonce_tag}'"
    font_src = "font-src 'self' '#{nonce_tag}' data:"

    script_src =
      if Config.get(:env) == :dev do
        "script-src 'self' 'unsafe-eval' '#{nonce_tag}'"
      else
        "script-src 'self' '#{nonce_tag}'"
      end

    report = if report_uri, do: ["report-uri ", report_uri, ";report-to csp-endpoint"]
    insecure = if scheme == "https", do: "upgrade-insecure-requests"

    @csp_start
    |> add_csp_param(img_src)
    |> add_csp_param(media_src)
    |> add_csp_param(connect_src)
    |> add_csp_param(script_src)
    |> add_csp_param(font_src)
    |> add_csp_param(style_src)
    |> add_csp_param(insecure)
    |> add_csp_param(report)
    |> :erlang.iolist_to_binary()
  end

  defp build_csp_from_whitelist([], acc), do: acc

  defp build_csp_from_whitelist([last], acc) do
    [build_csp_param_from_whitelist(last) | acc]
  end

  defp build_csp_from_whitelist([head | tail], acc) do
    build_csp_from_whitelist(tail, [[?\s, build_csp_param_from_whitelist(head)] | acc])
  end

  # TODO: use `build_csp_param/1` after removing support bare domains for media proxy whitelist
  defp build_csp_param_from_whitelist("http" <> _ = url) do
    build_csp_param(url)
  end

  defp build_csp_param_from_whitelist(url), do: url

  defp build_csp_multimedia_source_list do
    media_proxy_whitelist =
      [:media_proxy, :whitelist]
      |> Config.get()
      |> build_csp_from_whitelist([])

    captcha_method = Config.get([Pleroma.Captcha, :method])
    captcha_endpoint = Config.get([captcha_method, :endpoint])

    base_endpoints =
      [
        [:media_proxy, :base_url],
        [Pleroma.Upload, :base_url],
        [Pleroma.Uploaders.S3, :public_endpoint]
      ]
      |> Enum.map(&Config.get/1)

    [captcha_endpoint | base_endpoints]
    |> Enum.map(&build_csp_param/1)
    |> Enum.reduce([], &add_source(&2, &1))
    |> add_source(media_proxy_whitelist)
  end

  defp add_source(iodata, nil), do: iodata
  defp add_source(iodata, []), do: iodata
  defp add_source(iodata, source), do: [[?\s, source] | iodata]

  defp add_csp_param(csp_iodata, nil), do: csp_iodata

  defp add_csp_param(csp_iodata, param), do: [[param, ?;] | csp_iodata]

  defp build_csp_param(nil), do: nil

  defp build_csp_param(url) when is_binary(url) do
    %{host: host, scheme: scheme} = URI.parse(url)

    if scheme do
      [scheme, "://", host]
    end
  end

  def warn_if_disabled do
    unless Config.get([:http_security, :enabled]) do
      Logger.warn("
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HTTP Security is disabled. Please re-enable it to prevent users from attacking
your instance and your users via malicious posts:

      config :pleroma, :http_security, enabled: true
      ")
    end
  end

  defp maybe_send_sts_header(conn, true) do
    max_age_sts = Config.get([:http_security, :sts_max_age])

    merge_resp_headers(conn, [
      {"strict-transport-security", "max-age=#{max_age_sts}; includeSubDomains; preload"}
    ])
  end

  defp maybe_send_sts_header(conn, _), do: conn
end