diff --git src/wp-includes/functions.php src/wp-includes/functions.php
index 9c24d0b..26390f0 100644
--- src/wp-includes/functions.php
+++ src/wp-includes/functions.php
@@ -4439,7 +4439,7 @@ function wp_allowed_protocols() {
 	static $protocols;
 
 	if ( empty( $protocols ) ) {
-		$protocols = array( 'http', 'https', 'ftp', 'ftps', 'mailto', 'news', 'irc', 'gopher', 'nntp', 'feed', 'telnet', 'mms', 'rtsp', 'svn', 'tel', 'fax', 'xmpp' );
+		$protocols = array( 'http', 'https', 'ftp', 'ftps', 'mailto', 'news', 'irc', 'gopher', 'nntp', 'feed', 'telnet', 'mms', 'rtsp', 'svn', 'tel', 'fax', 'xmpp', 'webcal' );
 
 		/**
 		 * Filter the list of protocols allowed in HTML attributes.