fix security issue, don't allow to go back in the path

apps/files_external/ajax/removeRootCertificate.php

@@ -5,7 +5,7 @@ OCP\JSON::checkLoggedIn();
 OCP\JSON::callCheck();
 
 $view = \OCP\Files::getStorage("files_external");
-$cert = $_POST['cert'];
+$cert = ltrim($_POST['cert'], "/.");
 $file = \OCP\Config::getSystemValue('datadirectory').$view->getAbsolutePath("").'uploads/'.$cert;
 unlink($file);
 OC_Mount_Config::createCertificateBundle();