diff --git a/phpBB/cron.php b/phpBB/cron.php
index ea4c4ddef2..6c7b4311aa 100644
--- a/phpBB/cron.php
+++ b/phpBB/cron.php
@@ -33,6 +33,33 @@ echo base64_decode('R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==
 // test without flush ;)
 // flush();
 
+// make sure cron doesn't run multiple times in parallel
+if ($config['cron_lock'])
+{
+	// if the other process is running more than an hour already we have to assume it
+	// aborted without cleaning the lock
+	$time = explode(' ', $config['cron_lock']);
+	$time = $time[0];
+
+	if ($time + 3600 >= time())
+	{
+		exit;
+	}
+}
+
+define('CRON_ID', time() . ' ' . unique_id());
+
+$sql = 'UPDATE ' . CONFIG_TABLE . "
+	SET config_value = '" . $db->sql_escape(CRON_ID) . "'
+	WHERE config_name = 'cron_lock' AND config_value = '" . $db->sql_escape($config['cron_lock']) . "'";
+$db->sql_query($sql);
+
+// another cron process altered the table between script start and UPDATE query so exit
+if ($db->sql_affectedrows() != 1)
+{
+	exit;
+}
+
 /**
 * Run cron-like action
 * Real cron-based layer will be introduced in 3.2
@@ -227,6 +254,11 @@ else
 	garbage_collection();
 }
 
+$sql = 'UPDATE ' . CONFIG_TABLE . "
+	SET config_value = '0'
+	WHERE config_name = 'cron_lock' AND config_value = '" . $db->sql_escape(CRON_ID) . "'";
+$db->sql_query($sql);
+
 exit;
 
 ?>
\ No newline at end of file
diff --git a/phpBB/docs/CHANGELOG.html b/phpBB/docs/CHANGELOG.html
index ad6e443d4b..56133a3a59 100644
--- a/phpBB/docs/CHANGELOG.html
+++ b/phpBB/docs/CHANGELOG.html
@@ -277,6 +277,7 @@ p a {
 		<li>[Fix] Quick-Mod tools now retaining the start parameter (Bug #13537)</li>
 		<li>[Fix] Several fixes for custom profile fields on multi-lingual boards (Bugs #13763, #13527, #13525, #11515)</li>
 		<li>[Fix] Return to the mode previously selected after disaproving a post (Bug #13796)</li>
+		<li>[Fix] Cron now uses a locking variable to make sure it does not spawn too many webserver processes (Bug #12741)</li>
 	</ul>
 
 	</div>
diff --git a/phpBB/install/database_update.php b/phpBB/install/database_update.php
index 395eefc1f1..d255688f55 100644
--- a/phpBB/install/database_update.php
+++ b/phpBB/install/database_update.php
@@ -1233,6 +1233,7 @@ if (version_compare($current_version, '3.0.RC3', '<='))
 	$db->sql_query($sql);
 
 	set_config('allow_birthdays', '1');
+	set_config('cron_lock', '0', true);
 }
 
 _write_result($no_updates, $errored, $error_ary);