[BUGFIX] Information disclosure during backend login

Change-Id: I04cffe3eae59b281be409f70e6adaa7539a3a409
Resolves: #24456
Reviewed-on: http://review.typo3.org/3739
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader

t3lib/class.t3lib_userauth.php

@@ -252,6 +252,9 @@ function start() {
 			// Make certain that NO user is set initially
 		$this->user = '';
 
+			// We need a PHP session session for most login levels
+		session_start();
+
 			// Check to see if anyone has submitted login-information and if so register the user with the session. $this->user[uid] may be used to write log...
 		$this->checkAuthentication();
 
@@ -1299,7 +1302,6 @@ function compareUident($user, $loginData, $security_level = '') {
 
 					// Check challenge stored in cookie:
 				if ($this->challengeStoredInCookie) {
-					session_start();
 					if ($_SESSION['login_challenge'] !== $loginData['chalvalue']) {
 						if ($this->writeDevLog) {
 							t3lib_div::devLog('PHP Session stored challenge "' . $_SESSION['login_challenge'] . '" and submitted challenge "' . $loginData['chalvalue'] . '" did not match, so authentication failed!', 't3lib_userAuth', 2);