diff --git a/wp-includes/kses.php b/wp-includes/kses.php
index 3b65ee9496..ab7aa75fe4 100644
--- a/wp-includes/kses.php
+++ b/wp-includes/kses.php
@@ -536,7 +536,7 @@ function wp_kses( $string, $allowed_html, $allowed_protocols = array() ) {
  * @return string Filtered attribute.
  */
 function wp_kses_one_attr( $string, $element ) {
-	$uris = array('xmlns', 'profile', 'href', 'src', 'cite', 'classid', 'codebase', 'data', 'usemap', 'longdesc', 'action');
+	$uris = wp_kses_uri_attributes();
 	$allowed_html = wp_kses_allowed_html( 'post' );
 	$allowed_protocols = wp_allowed_protocols();
 	$string = wp_kses_no_null( $string, array( 'slash_zero' => 'keep' ) );
@@ -733,6 +733,56 @@ function wp_kses_split( $string, $allowed_html, $allowed_protocols ) {
 	return preg_replace_callback( '%(<!--.*?(-->|$))|(<[^>]*(>|$)|>)%', '_wp_kses_split_callback', $string );
 }
 
+/**
+ * Helper function listing HTML attributes containing a URL.
+ *
+ * This function returns a list of all HTML attributes that must contain
+ * a URL according to the HTML specification.
+ *
+ * This list includes URI attributes both allowed and disallowed by KSES.
+ *
+ * @link https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes
+ *
+ * @since 5.0.1
+ *
+ * @return array HTML attributes that must include a URL.
+ */
+function wp_kses_uri_attributes() {
+	$uri_attributes = array(
+		'action',
+		'archive',
+		'background',
+		'cite',
+		'classid',
+		'codebase',
+		'data',
+		'formaction',
+		'href',
+		'icon',
+		'longdesc',
+		'manifest',
+		'poster',
+		'profile',
+		'src',
+		'usemap',
+		'xmlns',
+	);
+
+	/**
+	 * Filters the list of attributes that are required to contain a URL.
+	 *
+	 * Use this filter to add any `data-` attributes that are required to be
+	 * validated as a URL.
+	 *
+	 * @since 5.0.1
+	 *
+	 * @param array $uri_attributes HTML attributes requiring validation as a URL.
+	 */
+	$uri_attributes = apply_filters( 'wp_kses_uri_attributes', $uri_attributes );
+
+	return $uri_attributes;
+}
+
 /**
  * Callback for wp_kses_split.
  *
@@ -930,7 +980,7 @@ function wp_kses_hair($attr, $allowed_protocols) {
 	$attrarr = array();
 	$mode = 0;
 	$attrname = '';
-	$uris = array('xmlns', 'profile', 'href', 'src', 'cite', 'classid', 'codebase', 'data', 'usemap', 'longdesc', 'action');
+	$uris = wp_kses_uri_attributes();
 
 	// Loop through the whole attribute list
 
diff --git a/wp-includes/version.php b/wp-includes/version.php
index f13e4de4a1..f841cdb024 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.9.9-alpha-43997';
+$wp_version = '4.9.9-alpha-44020';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.