Check client and token in GET /oauth/authorize

lib/pleroma/web/oauth/oauth_controller.ex

@@ -35,7 +35,7 @@ defmodule Pleroma.Web.OAuth.OAuthController do
     authorize(conn, Map.merge(params, auth_attrs))
   end
 
-  def authorize(%Plug.Conn{assigns: %{token: %Token{}}} = conn, params) do
+  def authorize(%Plug.Conn{assigns: %{token: %Token{}}} = conn, %{"force_login" => _} = params) do
     if ControllerHelper.truthy_param?(params["force_login"]) do
       do_authorize(conn, params)
     else
@@ -43,6 +43,22 @@ defmodule Pleroma.Web.OAuth.OAuthController do
     end
   end
 
+  # Note: the token is set in oauth_plug, but the token and client do not always go together.
+  # For example, MastodonFE's token is set if user requests with another client,
+  # after user already authorized to MastodonFE.
+  # So we have to check client and token.
+  def authorize(
+        %Plug.Conn{assigns: %{token: %Token{} = token}} = conn,
+        %{"client_id" => client_id} = params
+      ) do
+    with %Token{} = t <- Repo.get_by(Token, token: token.token) |> Repo.preload(:app),
+         ^client_id <- t.app.client_id do
+      handle_existing_authorization(conn, params)
+    else
+      _ -> do_authorize(conn, params)
+    end
+  end
+
   def authorize(%Plug.Conn{} = conn, params), do: do_authorize(conn, params)
 
   defp do_authorize(%Plug.Conn{} = conn, params) do

test/web/oauth/oauth_controller_test.exs

@@ -468,6 +468,29 @@ defmodule Pleroma.Web.OAuth.OAuthControllerTest do
       assert html_response(conn, 200) =~ ~s(type="submit")
     end
 
+    test "renders authentication page if user is already authenticated but user request with another client",
+         %{
+           app: app,
+           conn: conn
+         } do
+      token = insert(:oauth_token, app_id: app.id)
+
+      conn =
+        conn
+        |> put_session(:oauth_token, token.token)
+        |> get(
+          "/oauth/authorize",
+          %{
+            "response_type" => "code",
+            "client_id" => "another_client_id",
+            "redirect_uri" => OAuthController.default_redirect_uri(app),
+            "scope" => "read"
+          }
+        )
+
+      assert html_response(conn, 200) =~ ~s(type="submit")
+    end
+
     test "with existing authentication and non-OOB `redirect_uri`, redirects to app with `token` and `state` params",
          %{
            app: app,