diff --git a/wp-admin/includes/class-wp-upgrader.php b/wp-admin/includes/class-wp-upgrader.php
index a9f230fd4d..6c399c7ec0 100644
--- a/wp-admin/includes/class-wp-upgrader.php
+++ b/wp-admin/includes/class-wp-upgrader.php
@@ -99,7 +99,7 @@ class WP_Upgrader {
 					break;
 				default:
 					if ( ! $wp_filesystem->find_folder($dir) )
-						return new WP_Error('fs_no_folder', sprintf($this->strings['fs_no_folder'], $dir));
+						return new WP_Error( 'fs_no_folder', sprintf( $this->strings['fs_no_folder'], esc_html( basename( $dir ) ) ) );
 					break;
 			}
 		}
@@ -1144,7 +1144,7 @@ class WP_Upgrader_Skin {
 		} elseif ( is_wp_error($errors) && $errors->get_error_code() ) {
 			foreach ( $errors->get_error_messages() as $message ) {
 				if ( $errors->get_error_data() )
-					$this->feedback($message . ' ' . $errors->get_error_data() );
+					$this->feedback($message . ' ' . esc_html( $errors->get_error_data() ) );
 				else
 					$this->feedback($message);
 			}
@@ -1158,8 +1158,11 @@ class WP_Upgrader_Skin {
 		if ( strpos($string, '%') !== false ) {
 			$args = func_get_args();
 			$args = array_splice($args, 1);
-			if ( !empty($args) )
+			if ( $args ) {
+				$args = array_map( 'strip_tags', $args );
+				$args = array_map( 'esc_html', $args );
 				$string = vsprintf($string, $args);
+			}
 		}
 		if ( empty($string) )
 			return;
@@ -1199,11 +1202,11 @@ class Plugin_Upgrader_Skin extends WP_Upgrader_Skin {
 	function after() {
 		$this->plugin = $this->upgrader->plugin_info();
 		if ( !empty($this->plugin) && !is_wp_error($this->result) && $this->plugin_active ){
-			echo '<iframe style="border:0;overflow:hidden" width="100%" height="170px" src="' . wp_nonce_url('update.php?action=activate-plugin&networkwide=' . $this->plugin_network_active . '&plugin=' . $this->plugin, 'activate-plugin_' . $this->plugin) .'"></iframe>';
+			echo '<iframe style="border:0;overflow:hidden" width="100%" height="170px" src="' . wp_nonce_url('update.php?action=activate-plugin&networkwide=' . $this->plugin_network_active . '&plugin=' . urlencode( $this->plugin ), 'activate-plugin_' . $this->plugin) .'"></iframe>';
 		}
 
 		$update_actions =  array(
-			'activate_plugin' => '<a href="' . wp_nonce_url('plugins.php?action=activate&amp;plugin=' . $this->plugin, 'activate-plugin_' . $this->plugin) . '" title="' . esc_attr__('Activate this plugin') . '" target="_parent">' . __('Activate Plugin') . '</a>',
+			'activate_plugin' => '<a href="' . wp_nonce_url('plugins.php?action=activate&amp;plugin=' . urlencode( $this->plugin ), 'activate-plugin_' . $this->plugin) . '" title="' . esc_attr__('Activate this plugin') . '" target="_parent">' . __('Activate Plugin') . '</a>',
 			'plugins_page' => '<a href="' . self_admin_url('plugins.php') . '" title="' . esc_attr__('Go to plugins page') . '" target="_parent">' . __('Return to Plugins page') . '</a>'
 		);
 		if ( $this->plugin_active || ! $this->result || is_wp_error( $this->result ) || ! current_user_can( 'activate_plugins' ) )
@@ -1255,8 +1258,11 @@ class Bulk_Upgrader_Skin extends WP_Upgrader_Skin {
 		if ( strpos($string, '%') !== false ) {
 			$args = func_get_args();
 			$args = array_splice($args, 1);
-			if ( !empty($args) )
+			if ( $args ) {
+				$args = array_map( 'strip_tags', $args );
+				$args = array_map( 'esc_html', $args );
 				$string = vsprintf($string, $args);
+			}
 		}
 		if ( empty($string) )
 			return;
@@ -1280,7 +1286,7 @@ class Bulk_Upgrader_Skin extends WP_Upgrader_Skin {
 		if ( is_wp_error($error) ) {
 			foreach ( $error->get_error_messages() as $emessage ) {
 				if ( $error->get_error_data() )
-					$messages[] = $emessage . ' ' . $error->get_error_data();
+					$messages[] = $emessage . ' ' . esc_html( $error->get_error_data() );
 				else
 					$messages[] = $emessage;
 			}
@@ -1442,12 +1448,12 @@ class Plugin_Installer_Skin extends WP_Upgrader_Skin {
 		$from = isset($_GET['from']) ? wp_unslash( $_GET['from'] ) : 'plugins';
 
 		if ( 'import' == $from )
-			$install_actions['activate_plugin'] = '<a href="' . wp_nonce_url('plugins.php?action=activate&amp;from=import&amp;plugin=' . $plugin_file, 'activate-plugin_' . $plugin_file) . '" title="' . esc_attr__('Activate this plugin') . '" target="_parent">' . __('Activate Plugin &amp; Run Importer') . '</a>';
+			$install_actions['activate_plugin'] = '<a href="' . wp_nonce_url('plugins.php?action=activate&amp;from=import&amp;plugin=' . urlencode( $plugin_file ), 'activate-plugin_' . $plugin_file) . '" title="' . esc_attr__('Activate this plugin') . '" target="_parent">' . __('Activate Plugin &amp; Run Importer') . '</a>';
 		else
-			$install_actions['activate_plugin'] = '<a href="' . wp_nonce_url('plugins.php?action=activate&amp;plugin=' . $plugin_file, 'activate-plugin_' . $plugin_file) . '" title="' . esc_attr__('Activate this plugin') . '" target="_parent">' . __('Activate Plugin') . '</a>';
+			$install_actions['activate_plugin'] = '<a href="' . wp_nonce_url('plugins.php?action=activate&amp;plugin=' . urlencode( $plugin_file ), 'activate-plugin_' . $plugin_file) . '" title="' . esc_attr__('Activate this plugin') . '" target="_parent">' . __('Activate Plugin') . '</a>';
 
 		if ( is_multisite() && current_user_can( 'manage_network_plugins' ) ) {
-			$install_actions['network_activate'] = '<a href="' . wp_nonce_url('plugins.php?action=activate&amp;networkwide=1&amp;plugin=' . $plugin_file, 'activate-plugin_' . $plugin_file) . '" title="' . esc_attr__('Activate this plugin for all sites in this network') . '" target="_parent">' . __('Network Activate') . '</a>';
+			$install_actions['network_activate'] = '<a href="' . wp_nonce_url('plugins.php?action=activate&amp;networkwide=1&amp;plugin=' . urlencode( $plugin_file ), 'activate-plugin_' . $plugin_file) . '" title="' . esc_attr__('Activate this plugin for all sites in this network') . '" target="_parent">' . __('Network Activate') . '</a>';
 			unset( $install_actions['activate_plugin'] );
 		}
 
@@ -1682,4 +1688,4 @@ class File_Upload_Upgrader {
 
 		return true;
 	}
-}
\ No newline at end of file
+}
diff --git a/wp-admin/update.php b/wp-admin/update.php
index 687142e218..caed6e794d 100644
--- a/wp-admin/update.php
+++ b/wp-admin/update.php
@@ -57,7 +57,7 @@ if ( isset($_GET['action']) ) {
 		require_once(ABSPATH . 'wp-admin/admin-header.php');
 
 		$nonce = 'upgrade-plugin_' . $plugin;
-		$url = 'update.php?action=upgrade-plugin&plugin=' . $plugin;
+		$url = 'update.php?action=upgrade-plugin&plugin=' . urlencode( $plugin );
 
 		$upgrader = new Plugin_Upgrader( new Plugin_Upgrader_Skin( compact('title', 'nonce', 'url', 'plugin') ) );
 		$upgrader->upgrade($plugin);
@@ -70,9 +70,9 @@ if ( isset($_GET['action']) ) {
 
 		check_admin_referer('activate-plugin_' . $plugin);
 		if ( ! isset($_GET['failure']) && ! isset($_GET['success']) ) {
-			wp_redirect( admin_url('update.php?action=activate-plugin&failure=true&plugin=' . $plugin . '&_wpnonce=' . $_GET['_wpnonce']) );
+			wp_redirect( admin_url('update.php?action=activate-plugin&failure=true&plugin=' . urlencode( $plugin ) . '&_wpnonce=' . $_GET['_wpnonce']) );
 			activate_plugin( $plugin, '', ! empty( $_GET['networkwide'] ), true );
-			wp_redirect( admin_url('update.php?action=activate-plugin&success=true&plugin=' . $plugin . '&_wpnonce=' . $_GET['_wpnonce']) );
+			wp_redirect( admin_url('update.php?action=activate-plugin&success=true&plugin=' . urlencode( $plugin ) . '&_wpnonce=' . $_GET['_wpnonce']) );
 			die();
 		}
 		iframe_header( __('Plugin Reactivation'), true );
@@ -107,7 +107,7 @@ if ( isset($_GET['action']) ) {
 
 		$title = sprintf( __('Installing Plugin: %s'), $api->name . ' ' . $api->version );
 		$nonce = 'install-plugin_' . $plugin;
-		$url = 'update.php?action=install-plugin&plugin=' . $plugin;
+		$url = 'update.php?action=install-plugin&plugin=' . urlencode( $plugin );
 		if ( isset($_GET['from']) )
 			$url .= '&from=' . urlencode(stripslashes($_GET['from']));
 
@@ -132,7 +132,7 @@ if ( isset($_GET['action']) ) {
 		$submenu_file = 'plugin-install.php';
 		require_once(ABSPATH . 'wp-admin/admin-header.php');
 
-		$title = sprintf( __('Installing Plugin from uploaded file: %s'), basename( $file_upload->filename ) );
+		$title = sprintf( __('Installing Plugin from uploaded file: %s'), esc_html( basename( $file_upload->filename ) ) );
 		$nonce = 'plugin-upload';
 		$url = add_query_arg(array('package' => $file_upload->id), 'update.php?action=upload-plugin');
 		$type = 'upload'; //Install plugin type, From Web or an Upload.
@@ -160,7 +160,7 @@ if ( isset($_GET['action']) ) {
 		require_once(ABSPATH . 'wp-admin/admin-header.php');
 
 		$nonce = 'upgrade-theme_' . $theme;
-		$url = 'update.php?action=upgrade-theme&theme=' . $theme;
+		$url = 'update.php?action=upgrade-theme&theme=' . urlencode( $theme );
 
 		$upgrader = new Theme_Upgrader( new Theme_Upgrader_Skin( compact('title', 'nonce', 'url', 'theme') ) );
 		$upgrader->upgrade($theme);
@@ -213,7 +213,7 @@ if ( isset($_GET['action']) ) {
 
 		$title = sprintf( __('Installing Theme: %s'), $api->name . ' ' . $api->version );
 		$nonce = 'install-theme_' . $theme;
-		$url = 'update.php?action=install-theme&theme=' . $theme;
+		$url = 'update.php?action=install-theme&theme=' . urlencode( $theme );
 		$type = 'web'; //Install theme type, From Web or an Upload.
 
 		$upgrader = new Theme_Upgrader( new Theme_Installer_Skin( compact('title', 'url', 'nonce', 'plugin', 'api') ) );
@@ -238,7 +238,7 @@ if ( isset($_GET['action']) ) {
 
 		require_once(ABSPATH . 'wp-admin/admin-header.php');
 
-		$title = sprintf( __('Installing Theme from uploaded file: %s'), basename( $file_upload->filename ) );
+		$title = sprintf( __('Installing Theme from uploaded file: %s'), esc_html( basename( $file_upload->filename ) ) );
 		$nonce = 'theme-upload';
 		$url = add_query_arg(array('package' => $file_upload->id), 'update.php?action=upload-theme');
 		$type = 'upload'; //Install plugin type, From Web or an Upload.