TYPO3 Core

Additionally, this block

https://github.com/TYPO3/typo3/blob/836007411cd2b8d37faee12da7ae4194b6b90b09/typo3/sysext/felogin/Classes/Controller/LoginController.php#L69

should not be there, but outside

https://github.com/TYPO3/typo3/blob/836007411cd2b8d37faee12da7ae4194b6b90b09/typo3/sysext/felogin/Classes/Controller/LoginController.php#L68

otherwise the warning message here

https://github.com/TYPO3/typo3/blob/836007411cd2b8d37faee12da7ae4194b6b90b09/typo3/sysext/felogin/Resources/Private/Templates/Login/Logout.html#L3

cannot be shown.

I debugged this some hours and obviously, this feature seems broken since a long time. It does not even work in the old legacy version of the ext:felogin plugin (TYPO3 v8 - v10, did not test below).

I tried to fix the issue, but the currently used code/approach in ext:felogin is not good to really verify, if the browser supports cookies. Moving the check out of the condition as suggested does not work, since feUser->isCookieSet() will always return true if authentication was successful.

However, ext:felogin can not properly ensure, that a browser supports cookies except by testing it as suggested via JavaScript. The current TYPO3 backend login form does so, as it renders the "Cookies are disabled" warning first and then uses the AJAX route https://website.tld/typo3/ajax/login/preflight with a preflight cookie to verify, that cookies can be set and finally hides the cookie warning, if cookies are enabled/can be set.

In case of ext:felogin, this would be a completely new feature which requires breaking changes (JavaScrip, AJAX route).

Since the current "cookie waring" in ext:felogin does not work and can not be fixed, I think it is best to remove it completely and rely on the default message shown, when the login was not successful. This message includes a hint, that authentication was not successful, either caused by wrong credentials or because of disabled cookies.