FrontendUserAuthentication::loginBySessionId() 

\nn\t3::FrontendUserAuthentication()->loginBySessionId($sessionId = ''); 

Login of an FE user using a session ID.

The session ID corresponds to the TYPO3 cookie fe_typo_user. As a rule, there is one entry for one entry in the fe_sessions table for each FE user session. Up to Typo3 v10, the the ses_id column corresponded exactly to the cookie value.

As of Typo3 v10, the value is also hashed.

See also \nn\t3::Encrypt()->hashSessionId( $sessionId );

\nn\t3::FrontendUserAuthentication()->loginBySessionId( $sessionId );

| @return array

public function loginBySessionId( $sessionId = '' )
{
	if (!trim($sessionId)) return [];
	$sessionId = \nn\t3::Encrypt()->hashSessionId( $sessionId );
	$session = \nn\t3::Db()->findOneByValues( 'fe_sessions', ['ses_id'=>$sessionId] );
	if (!$session) return [];
	if ($feUserUid = $session['ses_userid']) {
		return $this->loginUid( $feUserUid );
	}
	return [];
}