diff --git a/wp-admin/about.php b/wp-admin/about.php
index dd7d22be0a..334997daee 100644
--- a/wp-admin/about.php
+++ b/wp-admin/about.php
@@ -45,6 +45,26 @@ include( ABSPATH . 'wp-admin/admin-header.php' );
 
 		<div class="changelog point-releases">
 			<h3><?php _e( 'Maintenance and Security Releases' ); ?></h3>
+			<p>
+				<?php
+				printf(
+					/* translators: %s: WordPress version number */
+					__( '<strong>Version %s</strong> addressed some security issues.' ),
+					'4.7.26'
+				);
+				?>
+				<?php
+				printf(
+					/* translators: %s: HelpHub URL */
+					__( 'For more information, see <a href="%s">the release notes</a>.' ),
+					sprintf(
+						/* translators: %s: WordPress version */
+						esc_url( __( 'https://wordpress.org/support/wordpress-version/version-%s/' ) ),
+						sanitize_title( '4.7.26' )
+					)
+				);
+				?>
+			</p>
 			<p>
 				<?php
 				printf(
diff --git a/wp-admin/includes/ajax-actions.php b/wp-admin/includes/ajax-actions.php
index 362f506006..c3fc617a6b 100644
--- a/wp-admin/includes/ajax-actions.php
+++ b/wp-admin/includes/ajax-actions.php
@@ -2197,6 +2197,10 @@ function wp_ajax_set_attachment_thumbnail() {
 		wp_send_json_error();
 	}
 
+	if ( false === check_ajax_referer( 'set-attachment-thumbnail', '_ajax_nonce', false ) ) {
+		wp_send_json_error();
+	}
+
 	$post_ids = array();
 	// For each URL, try to find its corresponding post ID.
 	foreach ( $_POST['urls'] as $url ) {
diff --git a/wp-includes/embed.php b/wp-includes/embed.php
index b7cca5de97..71374690a7 100644
--- a/wp-includes/embed.php
+++ b/wp-includes/embed.php
@@ -450,7 +450,7 @@ function get_post_embed_html( $width, $height, $post = null ) {
 		 * and edit wp-embed.js directly.
 		 */
 		$output .=<<<JS
-		!function(c,d){"use strict";var e=!1,n=!1;if(d.querySelector)if(c.addEventListener)e=!0;if(c.wp=c.wp||{},!c.wp.receiveEmbedMessage)if(c.wp.receiveEmbedMessage=function(e){var t=e.data;if(t.secret||t.message||t.value)if(!/[^a-zA-Z0-9]/.test(t.secret)){for(var r,a,i,s=d.querySelectorAll('iframe[data-secret="'+t.secret+'"]'),n=d.querySelectorAll('blockquote[data-secret="'+t.secret+'"]'),o=0;o<n.length;o++)n[o].style.display="none";for(o=0;o<s.length;o++)if(r=s[o],e.source===r.contentWindow){if(r.removeAttribute("style"),"height"===t.message){if(1e3<(i=parseInt(t.value,10)))i=1e3;else if(~~i<200)i=200;r.height=i}if("link"===t.message)if(a=d.createElement("a"),i=d.createElement("a"),a.href=r.getAttribute("src"),i.href=t.value,i.host===a.host)if(d.activeElement===r)c.top.location.href=t.value}}},e)c.addEventListener("message",c.wp.receiveEmbedMessage,!1),d.addEventListener("DOMContentLoaded",t,!1),c.addEventListener("load",t,!1);function t(){if(!n){n=!0;for(var e,t,r=-1!==navigator.appVersion.indexOf("MSIE 10"),a=!!navigator.userAgent.match(/Trident.*rv:11\./),i=d.querySelectorAll("iframe.wp-embedded-content"),s=0;s<i.length;s++){if(!(e=i[s]).getAttribute("data-secret"))t=Math.random().toString(36).substr(2,10),e.src+="#?secret="+t,e.setAttribute("data-secret",t);if(r||a)(t=e.cloneNode(!0)).removeAttribute("security"),e.parentNode.replaceChild(t,e)}}}}(window,document);
+		!function(d,l){"use strict";var e=!1,n=!1;if(l.querySelector)if(d.addEventListener)e=!0;if(d.wp=d.wp||{},!d.wp.receiveEmbedMessage)if(d.wp.receiveEmbedMessage=function(e){var t=e.data;if(t.secret||t.message||t.value)if(!/[^a-zA-Z0-9]/.test(t.secret)){for(var r,a,i,s=l.querySelectorAll('iframe[data-secret="'+t.secret+'"]'),n=l.querySelectorAll('blockquote[data-secret="'+t.secret+'"]'),o=new RegExp("^https?:$","i"),c=0;c<n.length;c++)n[c].style.display="none";for(c=0;c<s.length;c++)if(r=s[c],e.source===r.contentWindow){if(r.removeAttribute("style"),"height"===t.message){if(1e3<(i=parseInt(t.value,10)))i=1e3;else if(~~i<200)i=200;r.height=i}if("link"===t.message)if(a=l.createElement("a"),i=l.createElement("a"),a.href=r.getAttribute("src"),i.href=t.value,o.test(i.protocol))if(i.host===a.host)if(l.activeElement===r)d.top.location.href=t.value}}},e)d.addEventListener("message",d.wp.receiveEmbedMessage,!1),l.addEventListener("DOMContentLoaded",t,!1),d.addEventListener("load",t,!1);function t(){if(!n){n=!0;for(var e,t,r=-1!==navigator.appVersion.indexOf("MSIE 10"),a=!!navigator.userAgent.match(/Trident.*rv:11\./),i=l.querySelectorAll("iframe.wp-embedded-content"),s=0;s<i.length;s++){if(!(e=i[s]).getAttribute("data-secret"))t=Math.random().toString(36).substr(2,10),e.src+="#?secret="+t,e.setAttribute("data-secret",t);if(r||a)(t=e.cloneNode(!0)).removeAttribute("security"),e.parentNode.replaceChild(t,e)}}}}(window,document);
 JS;
 	}
 	$output .= "\n//--><!]]>";
diff --git a/wp-includes/js/wp-embed.js b/wp-includes/js/wp-embed.js
index 3be0dc7245..d0a372662c 100644
--- a/wp-includes/js/wp-embed.js
+++ b/wp-includes/js/wp-embed.js
@@ -37,6 +37,7 @@
 
 		var iframes = document.querySelectorAll( 'iframe[data-secret="' + data.secret + '"]' ),
 			blockquotes = document.querySelectorAll( 'blockquote[data-secret="' + data.secret + '"]' ),
+			allowedProtocols = new RegExp( '^https?:$', 'i' ),
 			i, source, height, sourceURL, targetURL;
 
 		for ( i = 0; i < blockquotes.length; i++ ) {
@@ -72,6 +73,11 @@
 				sourceURL.href = source.getAttribute( 'src' );
 				targetURL.href = data.value;
 
+				/* Only follow link if the protocol is in the allow list. */
+				if ( ! allowedProtocols.test( targetURL.protocol ) ) {
+					continue;
+				}
+
 				/* Only continue if link hostname matches iframe's hostname. */
 				if ( targetURL.host === sourceURL.host ) {
 					if ( document.activeElement === source ) {
diff --git a/wp-includes/js/wp-embed.min.js b/wp-includes/js/wp-embed.min.js
index 4d37bce0b5..57e22365c5 100644
--- a/wp-includes/js/wp-embed.min.js
+++ b/wp-includes/js/wp-embed.min.js
@@ -1 +1 @@
-!function(c,d){"use strict";var e=!1,n=!1;if(d.querySelector)if(c.addEventListener)e=!0;if(c.wp=c.wp||{},!c.wp.receiveEmbedMessage)if(c.wp.receiveEmbedMessage=function(e){var t=e.data;if(t.secret||t.message||t.value)if(!/[^a-zA-Z0-9]/.test(t.secret)){for(var r,a,i,s=d.querySelectorAll('iframe[data-secret="'+t.secret+'"]'),n=d.querySelectorAll('blockquote[data-secret="'+t.secret+'"]'),o=0;o<n.length;o++)n[o].style.display="none";for(o=0;o<s.length;o++)if(r=s[o],e.source===r.contentWindow){if(r.removeAttribute("style"),"height"===t.message){if(1e3<(i=parseInt(t.value,10)))i=1e3;else if(~~i<200)i=200;r.height=i}if("link"===t.message)if(a=d.createElement("a"),i=d.createElement("a"),a.href=r.getAttribute("src"),i.href=t.value,i.host===a.host)if(d.activeElement===r)c.top.location.href=t.value}}},e)c.addEventListener("message",c.wp.receiveEmbedMessage,!1),d.addEventListener("DOMContentLoaded",t,!1),c.addEventListener("load",t,!1);function t(){if(!n){n=!0;for(var e,t,r=-1!==navigator.appVersion.indexOf("MSIE 10"),a=!!navigator.userAgent.match(/Trident.*rv:11\./),i=d.querySelectorAll("iframe.wp-embedded-content"),s=0;s<i.length;s++){if(!(e=i[s]).getAttribute("data-secret"))t=Math.random().toString(36).substr(2,10),e.src+="#?secret="+t,e.setAttribute("data-secret",t);if(r||a)(t=e.cloneNode(!0)).removeAttribute("security"),e.parentNode.replaceChild(t,e)}}}}(window,document);
\ No newline at end of file
+!function(d,l){"use strict";var e=!1,n=!1;if(l.querySelector)if(d.addEventListener)e=!0;if(d.wp=d.wp||{},!d.wp.receiveEmbedMessage)if(d.wp.receiveEmbedMessage=function(e){var t=e.data;if(t.secret||t.message||t.value)if(!/[^a-zA-Z0-9]/.test(t.secret)){for(var r,a,i,s=l.querySelectorAll('iframe[data-secret="'+t.secret+'"]'),n=l.querySelectorAll('blockquote[data-secret="'+t.secret+'"]'),o=new RegExp("^https?:$","i"),c=0;c<n.length;c++)n[c].style.display="none";for(c=0;c<s.length;c++)if(r=s[c],e.source===r.contentWindow){if(r.removeAttribute("style"),"height"===t.message){if(1e3<(i=parseInt(t.value,10)))i=1e3;else if(~~i<200)i=200;r.height=i}if("link"===t.message)if(a=l.createElement("a"),i=l.createElement("a"),a.href=r.getAttribute("src"),i.href=t.value,o.test(i.protocol))if(i.host===a.host)if(l.activeElement===r)d.top.location.href=t.value}}},e)d.addEventListener("message",d.wp.receiveEmbedMessage,!1),l.addEventListener("DOMContentLoaded",t,!1),d.addEventListener("load",t,!1);function t(){if(!n){n=!0;for(var e,t,r=-1!==navigator.appVersion.indexOf("MSIE 10"),a=!!navigator.userAgent.match(/Trident.*rv:11\./),i=l.querySelectorAll("iframe.wp-embedded-content"),s=0;s<i.length;s++){if(!(e=i[s]).getAttribute("data-secret"))t=Math.random().toString(36).substr(2,10),e.src+="#?secret="+t,e.setAttribute("data-secret",t);if(r||a)(t=e.cloneNode(!0)).removeAttribute("security"),e.parentNode.replaceChild(t,e)}}}}(window,document);
\ No newline at end of file
diff --git a/wp-includes/media.php b/wp-includes/media.php
index 3d2f850df9..cdb51ca0c4 100644
--- a/wp-includes/media.php
+++ b/wp-includes/media.php
@@ -3405,7 +3405,8 @@ function wp_enqueue_media( $args = array() ) {
 		/** This filter is documented in wp-admin/includes/media.php */
 		'captions'  => ! apply_filters( 'disable_captions', '' ),
 		'nonce'     => array(
-			'sendToEditor' => wp_create_nonce( 'media-send-to-editor' ),
+			'sendToEditor'           => wp_create_nonce( 'media-send-to-editor' ),
+			'setAttachmentThumbnail' => wp_create_nonce( 'set-attachment-thumbnail' ),
 		),
 		'post'    => array(
 			'id' => 0,
diff --git a/wp-includes/version.php b/wp-includes/version.php
index bcf1fd38f8..2fba9c274b 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.7.25';
+$wp_version = '4.7.26';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.