Add the HTTPS protocol to the Google Tag Manager path if it is path relative.

google-analytics-for-wordpress-security-fix.php

<?php
/**
 * Google Analytics for WordPress Security Fix
 *
 * @package gafwsf
 * @author  Bernhard Kau
 * @license GPLv3
 *
 * @wordpress-plugin
 * Plugin Name: Google Analytics for WordPress Security Fix
 https://gist.github.com/2ndkauboy/bcab679895b7be376c5b9ebc35897165
 * Description: Add the HTTPS protocol to the Google Tag Manager path if it is path relative.
 * Version: 1.0.0
 * Author: Bernhard Kau
 * Author URI: https://kau-boys.de
 * Text Domain: gafwsf
 * License: GPLv3
 * License URI: https://www.gnu.org/licenses/gpl-3.0.txt
 */

/**
 * Add the HTTPS protocol to the Google Tag Manager path if it is path relative.
 *
 * @param string $src The current Google Tag Manager Tag path.
 *
 * @return string
 */
function gafwsf_gtag_src( $src ) {
	if ( 0 === strpos( $src, '//' ) ) {
		$src = 'https:' . $src;
	}

	return $src;
}
add_filter( 'monsterinsights_frontend_output_gtag_src',  'gafwsf_gtag_src' );