honkoma/lib/pleroma/web/plugs/http_signature_plug.ex

# Pleroma: A lightweight social networking server
# Copyright © 2017-2022 Pleroma Authors <https://pleroma.social/>
# SPDX-License-Identifier: AGPL-3.0-only

defmodule Pleroma.Web.Plugs.HTTPSignaturePlug do
  import Plug.Conn
  import Phoenix.Controller, only: [get_format: 1, text: 2]
  require Logger

  def init(options) do
    options
  end

  def call(%{assigns: %{valid_signature: true}} = conn, _opts) do
    conn
  end

  def call(conn, _opts) do
    if get_format(conn) == "activity+json" do
      conn
      |> maybe_assign_valid_signature()
      |> maybe_require_signature()
    else
      conn
    end
  end

  defp validate_signature(conn, request_target) do
    # Newer drafts for HTTP signatures now use @request-target instead of the
    # old (request-target). We'll now support both for incoming signatures.
    conn =
      conn
      |> put_req_header("(request-target)", request_target)
      |> put_req_header("@request-target", request_target)

    HTTPSignatures.validate_conn(conn)
  end

  defp validate_signature(conn) do
    # This (request-target) is non-standard, but many implementations do it
    # this way due to a misinterpretation of
    # https://datatracker.ietf.org/doc/html/draft-cavage-http-signatures-06
    # "path" was interpreted as not having the query, though later examples
    # show that it must be the absolute path + query. This behavior is kept to
    # make sure most software (Pleroma itself, Mastodon, and probably others)
    # do not break.
    request_target = String.downcase("#{conn.method}") <> " #{conn.request_path}"

    # This is the proper way to build the @request-target, as expected by
    # many HTTP signature libraries, clarified in the following draft:
    # https://www.ietf.org/archive/id/draft-ietf-httpbis-message-signatures-11.html#section-2.2.6
    # It is the same as before, but containing the query part as well.
    proper_target = request_target <> "?#{conn.query_string}"

    cond do
      # Normal, non-standard behavior but expected by Pleroma and more.
      validate_signature(conn, request_target) ->
        true

      # Has query string and the previous one failed: let's try the standard.
      conn.query_string != "" ->
        validate_signature(conn, proper_target)

      # If there's no query string and signature fails, it's rotten.
      true ->
        false
    end
  end

  defp maybe_assign_valid_signature(conn) do
    if has_signature_header?(conn) do
      # we replace the digest header with the one we computed in DigestPlug
      conn =
        case conn do
          %{assigns: %{digest: digest}} = conn -> put_req_header(conn, "digest", digest)
          conn -> conn
        end

      assign(conn, :valid_signature, validate_signature(conn))
    else
      Logger.debug("No signature header!")
      conn
    end
  end

  defp has_signature_header?(conn) do
    conn |> get_req_header("signature") |> Enum.at(0, false)
  end

  defp maybe_require_signature(%{assigns: %{valid_signature: true}} = conn), do: conn

  defp maybe_require_signature(conn) do
    if Pleroma.Config.get([:activitypub, :authorized_fetch_mode], false) do
      conn
      |> put_status(:unauthorized)
      |> text("Request not signed")
      |> halt()
    else
      conn
    end
  end
end