Index: typo3/sysext/install/mod/class.tx_install_session.php
===================================================================
--- typo3/sysext/install/mod/class.tx_install_session.php	(revision 8465)
+++ typo3/sysext/install/mod/class.tx_install_session.php	(working copy)
@@ -120,6 +120,12 @@
 		if (version_compare(phpversion(), '5.2', '<')) {
 			ini_set('session.cookie_httponly', TRUE);
 		}
+		if (ini_get('session.auto_start') === '1') {
+			die('The PHP option session.auto-start is enabled. Disable this option in php.ini or .htaccess!');
+		}
+		if (defined('SID')) {
+			die('Session has already been started by session.auto-start or session_start()!');
+		}
 		session_start();
 	}