diff --git a/wp-admin/includes/class-custom-background.php b/wp-admin/includes/class-custom-background.php
index b62db4fb05..bc3c082bc3 100644
--- a/wp-admin/includes/class-custom-background.php
+++ b/wp-admin/includes/class-custom-background.php
@@ -581,6 +581,8 @@ class Custom_Background {
 	 * @deprecated 3.5.0
 	 */
 	public function wp_set_background_image() {
+		check_ajax_referer( 'custom-background' );
+
 		if ( ! current_user_can( 'edit_theme_options' ) || ! isset( $_POST['attachment_id'] ) ) {
 			exit;
 		}
diff --git a/wp-admin/js/custom-background.js b/wp-admin/js/custom-background.js
index a08fb96c47..7f57d8a9b4 100644
--- a/wp-admin/js/custom-background.js
+++ b/wp-admin/js/custom-background.js
@@ -126,11 +126,13 @@
 			frame.on( 'select', function() {
 				// Grab the selected attachment.
 				var attachment = frame.state().get('selection').first();
+				var nonceValue = $( '#_wpnonce' ).val() || '';
 
 				// Run an Ajax request to set the background image.
 				$.post( ajaxurl, {
 					action: 'set-background-image',
 					attachment_id: attachment.id,
+					_ajax_nonce: nonceValue,
 					size: 'full'
 				}).done( function() {
 					// When the request completes, reload the window.
diff --git a/wp-admin/js/custom-background.min.js b/wp-admin/js/custom-background.min.js
index 467e6822e8..75c80e3867 100644
--- a/wp-admin/js/custom-background.min.js
+++ b/wp-admin/js/custom-background.min.js
@@ -1,2 +1,2 @@
 /*! This file is auto-generated */
-!function(e){e(document).ready(function(){var o,t=e("#custom-background-image");e("#background-color").wpColorPicker({change:function(n,c){t.css("background-color",c.color.toString())},clear:function(){t.css("background-color","")}}),e('select[name="background-size"]').change(function(){t.css("background-size",e(this).val())}),e('input[name="background-position"]').change(function(){t.css("background-position",e(this).val())}),e('input[name="background-repeat"]').change(function(){t.css("background-repeat",e(this).is(":checked")?"repeat":"no-repeat")}),e('input[name="background-attachment"]').change(function(){t.css("background-attachment",e(this).is(":checked")?"scroll":"fixed")}),e("#choose-from-library-link").click(function(n){var c=e(this);n.preventDefault(),o||(o=wp.media.frames.customBackground=wp.media({title:c.data("choose"),library:{type:"image"},button:{text:c.data("update"),close:!1}})).on("select",function(){var n=o.state().get("selection").first();e.post(ajaxurl,{action:"set-background-image",attachment_id:n.id,size:"full"}).done(function(){window.location.reload()})}),o.open()})})}(jQuery);
\ No newline at end of file
+!function(e){e(document).ready(function(){var o,a=e("#custom-background-image");e("#background-color").wpColorPicker({change:function(n,c){a.css("background-color",c.color.toString())},clear:function(){a.css("background-color","")}}),e('select[name="background-size"]').change(function(){a.css("background-size",e(this).val())}),e('input[name="background-position"]').change(function(){a.css("background-position",e(this).val())}),e('input[name="background-repeat"]').change(function(){a.css("background-repeat",e(this).is(":checked")?"repeat":"no-repeat")}),e('input[name="background-attachment"]').change(function(){a.css("background-attachment",e(this).is(":checked")?"scroll":"fixed")}),e("#choose-from-library-link").click(function(n){var c=e(this);n.preventDefault(),o||(o=wp.media.frames.customBackground=wp.media({title:c.data("choose"),library:{type:"image"},button:{text:c.data("update"),close:!1}})).on("select",function(){var n=o.state().get("selection").first(),c=e("#_wpnonce").val()||"";e.post(ajaxurl,{action:"set-background-image",attachment_id:n.id,_ajax_nonce:c,size:"full"}).done(function(){window.location.reload()})}),o.open()})})}(jQuery);
\ No newline at end of file
diff --git a/wp-admin/js/media-gallery.js b/wp-admin/js/media-gallery.js
index 7fe77a64ee..e96222c67a 100644
--- a/wp-admin/js/media-gallery.js
+++ b/wp-admin/js/media-gallery.js
@@ -11,7 +11,7 @@ jQuery(function($) {
 	 * Adds a click event handler to the element with a 'wp-gallery' class.
 	 */
 	$( 'body' ).bind( 'click.wp-gallery', function(e) {
-		var target = $( e.target ), id, img_size;
+		var target = $( e.target ), id, img_size, nonceValue;
 
 		if ( target.hasClass( 'wp-set-header' ) ) {
 			// Opens the image to preview it full size.
@@ -21,6 +21,7 @@ jQuery(function($) {
 			// Sets the image as background of the theme.
 			id = target.data( 'attachment-id' );
 			img_size = $( 'input[name="attachments[' + id + '][image-size]"]:checked').val();
+			nonceValue = $( '#_wpnonce' ).val() && '';
 
 			/**
 			 * This Ajax action has been deprecated since 3.5.0, see custom-background.php
@@ -28,6 +29,7 @@ jQuery(function($) {
 			jQuery.post(ajaxurl, {
 				action: 'set-background-image',
 				attachment_id: id,
+				_ajax_nonce: nonceValue,
 				size: img_size
 			}, function() {
 				var win = window.dialogArguments || opener || parent || top;
diff --git a/wp-admin/js/media-gallery.min.js b/wp-admin/js/media-gallery.min.js
index 690a93c99e..1d996236cb 100644
--- a/wp-admin/js/media-gallery.min.js
+++ b/wp-admin/js/media-gallery.min.js
@@ -1,2 +1,2 @@
 /*! This file is auto-generated */
-jQuery(function(o){o("body").bind("click.wp-gallery",function(a){var e,t,n=o(a.target);n.hasClass("wp-set-header")?((window.dialogArguments||opener||parent||top).location.href=n.data("location"),a.preventDefault()):n.hasClass("wp-set-background")&&(e=n.data("attachment-id"),t=o('input[name="attachments['+e+'][image-size]"]:checked').val(),jQuery.post(ajaxurl,{action:"set-background-image",attachment_id:e,size:t},function(){var a=window.dialogArguments||opener||parent||top;a.tb_remove(),a.location.reload()}),a.preventDefault())})});
\ No newline at end of file
+jQuery(function(r){r("body").bind("click.wp-gallery",function(a){var e,t,n,o=r(a.target);o.hasClass("wp-set-header")?((window.dialogArguments||opener||parent||top).location.href=o.data("location"),a.preventDefault()):o.hasClass("wp-set-background")&&(e=o.data("attachment-id"),t=r('input[name="attachments['+e+'][image-size]"]:checked').val(),n=r("#_wpnonce").val()&&"",jQuery.post(ajaxurl,{action:"set-background-image",attachment_id:e,_ajax_nonce:n,size:t},function(){var a=window.dialogArguments||opener||parent||top;a.tb_remove(),a.location.reload()}),a.preventDefault())})});
\ No newline at end of file
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 9b826bf034..f51d6634c4 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.6-beta2-49387';
+$wp_version = '5.6-beta2-49388';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.