Bug #72345 [Opn->Ana]: session_start() is not subject to max_execution_time

Edit report at https://bugs.php.net/bug.php?id=72345&edit=1 ID: 72345 Updated by: yohgaki@php.net Reported by: maggus dot staab at googlemail dot com Summary: session_start() is not subject to max_execution_time -Status: Open +Status: Analyzed Type: Bug Package: Session related PHP Version: Irrelevant -Assigned To: +Assigned To: yohgaki Block user comment: N Private report: N New Comment: Thank you for reporting. It's possible attackers setup clients that exploit session locking mechanism to use all server processes/threads. Counter measure is difficult because "locks" are handled by save handlers. "lock" is implemented by save handler in various ways. Any lock mechanism in PHP could be exploited. i.e. It's not a session only issue. Can we handle timeout signal in the engine? Previous Comments: ------------------------------------------------------------------------ [2016-07-14 13:00:46] maggus dot staab at googlemail dot com put the following code into a file called "experiment.php" and you can reproduce the issue. with my custom session files handler (see comment above) registered $handler = new FileSessionHandler(200); session_set_save_handler($handler, true); the queued up requests immediately fail. with the native php-session handler 20 apache workers are are bound with just 1 request. depending on how "slow" you simulate the first 8 workers, the things get very fast a lot worse as the apache nearly needs forever to fullfill all the requests. if (!empty($_GET['worker'])) { $worker = $_GET['worker']; echo "<xmp>"; echo "ID: ". $worker."\n"; $start = microtime(true); echo "ACQUIRE LOCK: ". $start ."\n"; try { session_start(); } catch (Exception $e) { var_dump($e->getMessage()); exit(); } $waited = ((microtime(true) - $start) * 1000); // simulate work $sleep = 500; if ($worker < 8) { $sleep = 5000; } usleep($sleep * 1000); // make sure the server needs to update the sess file after each request $_SESSION['worker'][$worker]++; $finished = ((microtime(true) - $start) * 1000); echo "FINISHED: ". $finished ."ms\n"; // highlight runs which were blocked by others // (so session_start() couldn't return immediately) if ($waited > 10) { echo '</xmp><span style="color:red">'; } echo 'BLOCKED FOR '. $waited .'ms'; return; } foreach(range(1,20) as $workerId) { echo '<iframe src="experiment.php?worker='. $workerId .'"></iframe>'; } ------------------------------------------------------------------------ [2016-07-14 12:56:50] maggus dot staab at googlemail dot com I created a custom FileSessionHandler class which works arround this (in my eyes php bug) using a non-blocking flock() call. https://gist.github.com/staabm/ce225d4b01d7b4e2560848646e13d6fc As anyone can easily DDOS any web-sapi based php application because of the blocking default-file-session-handler I hope for a fix at php-src though. ------------------------------------------------------------------------ [2016-07-14 08:12:36] maggus dot staab at googlemail dot com in the last 2 months we had at least 4 DDOS attacks which succeeded because php "never" returns from session_start() of parallel running requests. A few attackers were able to DOS a apache2 (mod_php) with 400 workers, because all the workers were waiting for a session lock. would it be possible to add a "lock_timeout" option (or similar) so one could configure that session_start() should return (or throw) after a certain amount of milliseconds if it could not acquire a session lock? ------------------------------------------------------------------------ [2016-06-06 17:48:57] maggus dot staab at googlemail dot com I agree that it is a documented/known issue. Like https://bugs.php.net/bug.php?id=72346 it is a very fundamental problem.. The fact that there are a bunch of functions which are not subject to any timeout makes it really easy to abuse/attack php based applications. One just needs to block all available php workers and the website will be unavailble. There are no means in php to protect against such kind of attacks (except you do everything async, but this is not something 99% of php apps will do) ------------------------------------------------------------------------ [2016-06-06 16:36:58] cmb@php.net It is documented[1] that: | The maximum execution time is not affected by system calls, | stream operations etc. Please see the set_time_limit() function | for more details. So, in my opinion, this is not a bug. [1] <http://php.net/manual/en/info.configuration.php#ini.max-execution-time> ------------------------------------------------------------------------ The remainder of the comments for this report are too long. To view the rest of the comments, please view the bug report online at https://bugs.php.net/bug.php?id=72345 -- Edit this bug report at https://bugs.php.net/bug.php?id=72345&edit=1