After Upgrading My Site to Magento version 2.4.3 I am facing this issue on my admin login at 2FA Login screen.
Please Help me About this why its showing the invalid form key.
3 Answers
Magento 2.4.3 has a known bug causing this problem. One user reported your exact problem. From that Github issue, this solution is reported to work until Magento patches this in a future release. I'm unable to paste it here as it doesn't appear to play nice with StackExchange formatting tools, but those links should get you to a solution.
- Thank You @kookaburraMark Philips– Mark Philips2021-10-01 12:13:27 +00:00Commented Oct 1, 2021 at 12:13
Run below command in terminal Magento root path :
bin/magento module:disable Magento_TwoFactorAuth bin/magento cache:flush 
- 1I Want TwoFactor on my siteMark Philips– Mark Philips2021-09-30 12:46:13 +00:00Commented Sep 30, 2021 at 12:46
- This is not an issue,Just refresh your page and try it again Or try by increasing values in php.ini max_input_vars = 200000 max_input_time = 600 max_execution_time = 3600Fatma– Fatma2021-09-30 13:03:18 +00:00Commented Sep 30, 2021 at 13:03
You have to change default Magento's File
Go to , Magento/Backend/App/Action/Plugin/Authentication.php
find protected function _redirectIfNeededAfterLogin() function
And Replace With Below Code :
protected function _redirectIfNeededAfterLogin(\Magento\Framework\App\RequestInterface $request) { $requestUri = null; // Checks, whether secret key is required for admin access or request uri is explicitly set if ($this->_url->useSecretKey()) { $requestParts = strpos(trim($request->getRequestUri(), '/'), $request->getFrontName()) === 0 ? explode('/', trim($request->getRequestUri(), '/'), 4) : explode('/', trim($request->getRequestUri(), '/'), 3); if (($key = array_search($request->getFrontName(), $requestParts)) !== false) { unset($requestParts[$key]); } $requestParams = $request->getParams(); unset($requestParams['key'], $requestParams['form_key']); $requestUri = $this->_url->getUrl(implode('/', $requestParts), $requestParams); } elseif ($request) { $requestUri = $request->getRequestUri(); } if (!$requestUri) { return false; } $this->_response->setRedirect($requestUri); $this->_actionFlag->set('', \Magento\Framework\App\ActionInterface::FLAG_NO_DISPATCH, true); return true; } Changes in Default Magento files is not a god practice, until you have to wait Magento official to fix this bug...Or Use this Solution Its Working in my case
Ref : Github