Important: #91242 - Introduce Backend Route Referrer Check

See Issue #91242

Description

Public backend routes (those having option 'access' => 'public' in Configuration/Backend/Routes.php) do not require any session token, but can be used to internally redirect to a route that requires a session token. For this context it is required that a backend user is currently logged in having a valid session.

This scenario can lead to situations that an existing cross-site scripting vulnerability (XSS) allows to bypass mentioned session token - which can be considered as 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.

In order to mitigate described potential backend routes can enforce the existence of a HTTP Referer header by adding new option referrer to routes in Configuration/Backend/Routes.php.

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

Values for option 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

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.