Backend Routing

Each request to the backend is eventually executed by a controller. A list of routes is defined which maps a given request to a controller and an action.

Routes are defined inside extensions, in file Configuration/Backend/Routes.php for general requests and in Configuration/Backend/AjaxRoutes.php for AJAX calls.

Here is an extract of typo3/sysext/backend/Configuration/Backend/Routes.php:

<?php

use TYPO3\CMS\Backend\Controller;

/**
 * Definitions for routes provided by EXT:backend
 * Contains all "regular" routes for entry points
 *
 * Please note that this setup is preliminary until all core use-cases are set up here.
 * Especially some more properties regarding modules will be added until TYPO3 CMS 7 LTS, and might change.
 *
 * Currently the "access" property is only used so no token creation + validation is made,
 * but will be extended further.
 */
return [
    // Login screen of the TYPO3 Backend
    'login' => [
        'path' => '/login',
        'access' => 'public',
        'target' => Controller\LoginController::class . '::formAction'
    ],

    // Main backend rendering setup (previously called backend.php) for the TYPO3 Backend
    'main' => [
        'path' => '/main',
        'referrer' => 'required,refresh-always',
        'target' => Controller\BackendController::class . '::mainAction'
    ],
   // ...
];

So a routes file essentially returns an array containing routes mapping. A route is defined by a key, a path, a referrer and a target. The “public” access property indicates that no authentication is required for that action.

Public backend routes (those having option 'access' => 'public') do not require any session token, but can be used to redirect to a route that requires a session token internally. For this context, the backend user logged in must have a valid session.

This scenario can lead to situations where an existing cross-site scripting vulnerability (XSS) bypasses the mentioned session token, which can be considered cross-site request forgery (CSRF). The difference in terminology is that this scenario occurs on same-site requests and not cross-site - however, potential security implications are still the same.

Backend routes can enforce an HTTP Referer header’s existence by adding a referrer to routes to mitigate the described scenario.

'main' => [
    'path' => '/main',
    'referrer' => 'required,refresh-empty',
    'target' => Controller\BackendController::class . '::mainAction'
],

Values for referrer are declared as comma-separated list:

  • required enforces existence of HTTP Referer header that has to match the currently used backend URL (e.g. https://example.org/typo3/), the request will be denied otherwise.
  • refresh-empty triggers a HTML based refresh in case HTTP Referer header is not given or empty - this attempt uses an HTML refresh, since regular HTTP Location redirect still would not set a referrer. It implies this technique should only be used on plain HTML responses and won’t have any impact e.g. on JSON or XML response types.

This technique should be used on all public routes (without session token) that internally redirect to a restricted route (having a session token). The goal is to protect and keep information about the current session token internal.

The request sequence in the TYPO3 core looks like this:

  • HTTP request to https://example.org/typo3/ having a valid user session
  • internally public backend route /login is processed
  • internally redirects to restricted backend route /main since an existing and valid backend user session was found + HTTP redirect to https://example.org/typo3/index.php?route=/main&token=... + exposing the token is mitigated with referrer route option mentioned above

Important

Please keep in mind these steps are part of a mitigation strategy, which requires to be aware of mentioned implications when implementing custom web applications.

Default Route Parameters

Routes definitions are extended by the possibility to define default parameters. Those parameters can be overridden during the regular URI generation process.

Several AJAX routes inhibited the backend session update to not keep the session alive by periodic polling. Those skipSessionUpdate parameters have been removed from the specific URI generation invocations and moved to the central route definitions.

Default route parameters are defined in an associative key-value-array using the index parameters. This definition can be used for both, plain routes and AJAX routes.

'systeminformation_render' => [
    'path' => '/system-information/render',
    'target' => \TYPO3\CMS\Backend\Backend\ToolbarItems\SystemInformationToolbarItem::class . '::renderMenuAction',
    'parameters' => [
        'skipSessionUpdate' => 1
    ]
]

More Information

Please refer to the following external resources and look at how the TYPO3 source code handles backend routing in your TYPO3 version.