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)
@@ -318,6 +318,10 @@
 		}
 
 		$this->session = t3lib_div::makeInstance('tx_install_session');
+		$sessionCreationError = $this->session->getErrorCreationMessage();
+		if ($sessionCreationError !== NULL) {
+			$this->outputErrorAndExit('<p>' . $sessionCreationError . '</p>');
+		}
 
 			// *******************
 			// Check authorization
@@ -7735,6 +7739,47 @@
 	}
 
 	/**
+	 * 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') {
+			// Define the stylesheet
+		$stylesheet = '<link rel="stylesheet" type="text/css" href="' .
+			'../stylesheets/install/install.css" />';
+		$javascript = '<script type="text/javascript" src="' .
+			'../contrib/prototype/prototype.js"></script>' . LF;
+		$javascript .= '<script type="text/javascript" src="' .
+			'../sysext/install/Resources/Public/Javascript/install.js"></script>';
+
+			// Get the template file
+		$template = @file_get_contents(PATH_site . '/typo3/templates/install.html');
+			// Define the markers content
+		$markers = array(
+			'styleSheet' => $stylesheet,
+			'javascript' => $javascript,
+			'title' => $title,
+			'content' => $content,
+		);
+			// Fill the markers
+		$content = t3lib_parsehtml::substituteMarkerArray(
+			$template,
+			$markers,
+			'###|###',
+			1,
+			1
+		);
+			// 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 $content;
+		exit();
+	}
+
+	/**
 	 * Sends the page to the client.
 	 *
 	 * @param string $content The HTML page
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)
@@ -82,6 +82,13 @@
 	private $backendFile = 'backend.php';
 
 	/**
+	 * On creation of the session, store a potential error message here
+	 *
+	 * @var string
+	 */
+	private $sessionCreationError = NULL;
+
+	/**
 	 * Constructor. Starts PHP session handling in our own private store
 	 *
 	 * Side-effect: might set a cookie, so must be called before any other output.
@@ -120,10 +127,27 @@
 		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
 	 */
 	private function getSessionSavePath() {