Index: typo3/sysext/install/mod/class.tx_install.php
===================================================================
--- typo3/sysext/install/mod/class.tx_install.php	(Revision 8465)
+++ typo3/sysext/install/mod/class.tx_install.php	(Arbeitskopie)
@@ -301,6 +301,10 @@
 		}
 
 		$this->session = t3lib_div::makeInstance('tx_install_session');
+		$sessionCreationError = $this->session->getErrorCreationMessage();
+		if ($sessionCreationError !== NULL) {
+			$this->outputErrorAndExit('<p>' . $sessionCreationError . '</p>');
+		}
 
 			// *******************
 			// Check authorization
@@ -4870,6 +4874,23 @@
 		return $out;
 	}
 
+  	/**
+	 * Outputs an error and dies.
+	 * Should be used by all errors that occur before even starting the install tool process.
+	 *
+	 * @param string The content of the error
+	 * @return void
+	 */
+	function outputErrorAndExit($content, $title = 'Install Tool error') {
+			// Output the warning message and exit
+		header('Content-Type: text/html; charset=utf-8');
+		header('Cache-Control: no-cache, must-revalidate');
+		header('Pragma: no-cache');
+		echo '<h1>'.$title.'</h1>';
+		echo $content;
+		exit();
+	}
+
 	/**
 	 * [Describe function...]
 	 *
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	(Arbeitskopie)
@@ -75,6 +75,13 @@
 	var $regenerateSessionIdTime = 5;
 
 	/**
+	 * On creation of the session, store a potential error message here
+	 *
+	 * @var string
+	 */
+	var $sessionCreationError = NULL;
+
+	/**
 	 * Constructor. Starts PHP session handling in our own var store
 	 *
 	 * Side-effect: might set a cookie, so must be called before any other output.
@@ -113,10 +120,27 @@
 		if (version_compare(phpversion(), '5.2', '<')) {
 			ini_set('session.cookie_httponly', TRUE);
 		}
+		if (ini_get('session.auto_start')) {
+			$this->sessionCreationError = '<p><strong>Error: session.auto-start is enabled</strong></p>';
+			$this->sessionCreationError .= '<p>The PHP option session.auto-start is enabled. Disable this option in php.ini or .htaccess:</p>';
+			$this->sessionCreationError .= '<pre>php_value session.auto_start Off</pre>';
+		} else if (defined('SID')) {
+			$this->sessionCreationError = '<p><strong>Error: Session already started by session_start().</strong></p>';
+			$this->sessionCreationError .= '<p>Make sure no installed extension is starting a session in its ext_localconf.php or ext_tables.php.</p>';
+		}
 		session_start();
 	}
 
 	/**
+	 * Returns a potential error message for session handling. 
+	 *
+	 * @return string Error message on creating our session or NULL if there is no error
+	 */
+	public function getErrorCreationMessage() {
+		return $this->sessionCreationError;
+	}
+
+	/**
 	 * Returns the path where to store our session files
 	 */
 	function getSessionSavePath() {