diff --git a/wp-admin/includes/class-wp-plugins-list-table.php b/wp-admin/includes/class-wp-plugins-list-table.php
index f87f3218bd..b12be2a487 100644
--- a/wp-admin/includes/class-wp-plugins-list-table.php
+++ b/wp-admin/includes/class-wp-plugins-list-table.php
@@ -550,16 +550,16 @@ class WP_Plugins_List_Table extends WP_List_Table {
 				if ( $is_active ) {
 					if ( current_user_can( 'manage_network_plugins' ) ) {
 						/* translators: %s: plugin name */
-						$actions['deactivate'] = '<a href="' . wp_nonce_url( 'plugins.php?action=deactivate&amp;plugin=' . $plugin_file . '&amp;plugin_status=' . $context . '&amp;paged=' . $page . '&amp;s=' . $s, 'deactivate-plugin_' . $plugin_file ) . '" aria-label="' . esc_attr( sprintf( __( 'Network deactivate %s' ), $plugin_data['Name'] ) ) . '">' . __( 'Network Deactivate' ) . '</a>';
+						$actions['deactivate'] = '<a href="' . wp_nonce_url( 'plugins.php?action=deactivate&amp;plugin=' . urlencode( $plugin_file ) . '&amp;plugin_status=' . $context . '&amp;paged=' . $page . '&amp;s=' . $s, 'deactivate-plugin_' . $plugin_file ) . '" aria-label="' . esc_attr( sprintf( __( 'Network Deactivate %s' ), $plugin_data['Name'] ) ) . '">' . __( 'Network Deactivate' ) . '</a>';
 						}
 				} else {
 					if ( current_user_can( 'manage_network_plugins' ) ) {
 						/* translators: %s: plugin name */
-						$actions['activate'] = '<a href="' . wp_nonce_url( 'plugins.php?action=activate&amp;plugin=' . $plugin_file . '&amp;plugin_status=' . $context . '&amp;paged=' . $page . '&amp;s=' . $s, 'activate-plugin_' . $plugin_file ) . '" class="edit" aria-label="' . esc_attr( sprintf( __( 'Network Activate %s' ), $plugin_data['Name'] ) ) . '">' . __( 'Network Activate' ) . '</a>';
+						$actions['activate'] = '<a href="' . wp_nonce_url( 'plugins.php?action=activate&amp;plugin=' . urlencode( $plugin_file ) . '&amp;plugin_status=' . $context . '&amp;paged=' . $page . '&amp;s=' . $s, 'activate-plugin_' . $plugin_file ) . '" class="edit" aria-label="' . esc_attr( sprintf( __( 'Network Activate %s' ), $plugin_data['Name'] ) ) . '">' . __( 'Network Activate' ) . '</a>';
 					}
 					if ( current_user_can( 'delete_plugins' ) && ! is_plugin_active( $plugin_file ) ) {
 						/* translators: %s: plugin name */
-						$actions['delete'] = '<a href="' . wp_nonce_url( 'plugins.php?action=delete-selected&amp;checked[]=' . $plugin_file . '&amp;plugin_status=' . $context . '&amp;paged=' . $page . '&amp;s=' . $s, 'bulk-plugins' ) . '" class="delete" aria-label="' . esc_attr( sprintf( __( 'Delete %s' ), $plugin_data['Name'] ) ) . '">' . __( 'Delete' ) . '</a>';
+						$actions['delete'] = '<a href="' . wp_nonce_url( 'plugins.php?action=delete-selected&amp;checked[]=' . urlencode( $plugin_file ) . '&amp;plugin_status=' . $context . '&amp;paged=' . $page . '&amp;s=' . $s, 'bulk-plugins' ) . '" class="delete" aria-label="' . esc_attr( sprintf( __( 'Delete %s' ), $plugin_data['Name'] ) ) . '">' . __( 'Delete' ) . '</a>';
 					}
 				}
 			} else {
@@ -573,14 +573,14 @@ class WP_Plugins_List_Table extends WP_List_Table {
 					);
 				} elseif ( $is_active ) {
 					/* translators: %s: plugin name */
-					$actions['deactivate'] = '<a href="' . wp_nonce_url( 'plugins.php?action=deactivate&amp;plugin=' . $plugin_file . '&amp;plugin_status=' . $context . '&amp;paged=' . $page . '&amp;s=' . $s, 'deactivate-plugin_' . $plugin_file ) . '" aria-label="' . esc_attr( sprintf( __( 'Deactivate %s' ), $plugin_data['Name'] ) ) . '">' . __( 'Deactivate' ) . '</a>';
+					$actions['deactivate'] = '<a href="' . wp_nonce_url( 'plugins.php?action=deactivate&amp;plugin=' . urlencode( $plugin_file ) . '&amp;plugin_status=' . $context . '&amp;paged=' . $page . '&amp;s=' . $s, 'deactivate-plugin_' . $plugin_file ) . '" aria-label="' . esc_attr( sprintf( __( 'Deactivate %s' ), $plugin_data['Name'] ) ) . '">' . __( 'Deactivate' ) . '</a>';
 				} else {
 					/* translators: %s: plugin name */
-					$actions['activate'] = '<a href="' . wp_nonce_url( 'plugins.php?action=activate&amp;plugin=' . $plugin_file . '&amp;plugin_status=' . $context . '&amp;paged=' . $page . '&amp;s=' . $s, 'activate-plugin_' . $plugin_file ) . '" class="edit" aria-label="' . esc_attr( sprintf( __( 'Activate %s' ), $plugin_data['Name'] ) ) . '">' . __( 'Activate' ) . '</a>';
+					$actions['activate'] = '<a href="' . wp_nonce_url( 'plugins.php?action=activate&amp;plugin=' . urlencode( $plugin_file ) . '&amp;plugin_status=' . $context . '&amp;paged=' . $page . '&amp;s=' . $s, 'activate-plugin_' . $plugin_file ) . '" class="edit" aria-label="' . esc_attr( sprintf( __( 'Activate %s' ), $plugin_data['Name'] ) ) . '">' . __( 'Activate' ) . '</a>';
 
 					if ( ! is_multisite() && current_user_can( 'delete_plugins' ) ) {
 						/* translators: %s: plugin name */
-						$actions['delete'] = '<a href="' . wp_nonce_url( 'plugins.php?action=delete-selected&amp;checked[]=' . $plugin_file . '&amp;plugin_status=' . $context . '&amp;paged=' . $page . '&amp;s=' . $s, 'bulk-plugins' ) . '" class="delete" aria-label="' . esc_attr( sprintf( __( 'Delete %s' ), $plugin_data['Name'] ) ) . '">' . __( 'Delete' ) . '</a>';
+						$actions['delete'] = '<a href="' . wp_nonce_url( 'plugins.php?action=delete-selected&amp;checked[]=' . urlencode( $plugin_file ) . '&amp;plugin_status=' . $context . '&amp;paged=' . $page . '&amp;s=' . $s, 'bulk-plugins' ) . '" class="delete" aria-label="' . esc_attr( sprintf( __( 'Delete %s' ), $plugin_data['Name'] ) ) . '">' . __( 'Delete' ) . '</a>';
 					}
 				} // end if $is_active
 
@@ -588,7 +588,7 @@ class WP_Plugins_List_Table extends WP_List_Table {
 
 			if ( ( ! is_multisite() || $screen->in_admin( 'network' ) ) && current_user_can( 'edit_plugins' ) && is_writable( WP_PLUGIN_DIR . '/' . $plugin_file ) ) {
 				/* translators: %s: plugin name */
-				$actions['edit'] = '<a href="plugin-editor.php?file=' . $plugin_file . '" class="edit" aria-label="' . esc_attr( sprintf( __( 'Edit %s' ), $plugin_data['Name'] ) ) . '">' . __( 'Edit' ) . '</a>';
+				$actions['edit'] = '<a href="plugin-editor.php?file=' . urlencode( $plugin_file ) . '" class="edit" aria-label="' . esc_attr( sprintf( __( 'Edit %s' ), $plugin_data['Name'] ) ) . '">' . __( 'Edit' ) . '</a>';
 			}
 		} // end if $context
 
diff --git a/wp-admin/includes/template.php b/wp-admin/includes/template.php
index 42a57eef16..890df25abb 100644
--- a/wp-admin/includes/template.php
+++ b/wp-admin/includes/template.php
@@ -769,7 +769,7 @@ function page_template_dropdown( $default = '' ) {
 	ksort( $templates );
 	foreach ( array_keys( $templates ) as $template ) {
 		$selected = selected( $default, $templates[ $template ], false );
-		echo "\n\t<option value='" . $templates[ $template ] . "' $selected>$template</option>";
+		echo "\n\t<option value='" . esc_attr( $templates[ $template ] ) . "' $selected>" . esc_html( $template ) . "</option>";
 	}
 }
 
diff --git a/wp-admin/plugin-editor.php b/wp-admin/plugin-editor.php
index 1558164096..3a7cc25008 100644
--- a/wp-admin/plugin-editor.php
+++ b/wp-admin/plugin-editor.php
@@ -96,9 +96,9 @@ default:
 			wp_die( $error );
 
 		if ( ( ! empty( $_GET['networkwide'] ) && ! is_plugin_active_for_network($file) ) || ! is_plugin_active($file) )
-			activate_plugin($file, "plugin-editor.php?file=$file&phperror=1", ! empty( $_GET['networkwide'] ) ); // we'll override this later if the plugin can be included without fatal error
+			activate_plugin($file, "plugin-editor.php?file=" . urlencode( $file ) . "&phperror=1", ! empty( $_GET['networkwide'] ) ); // we'll override this later if the plugin can be included without fatal error
 
-		wp_redirect( self_admin_url("plugin-editor.php?file=$file&a=te&scrollto=$scrollto") );
+		wp_redirect( self_admin_url("plugin-editor.php?file=" . urlencode( $file ) . "&a=te&scrollto=$scrollto") );
 		exit;
 	}
 
@@ -191,18 +191,18 @@ default:
 	if ( is_plugin_active( $plugin ) ) {
 		if ( is_writeable( $real_file ) ) {
 			/* translators: %s: plugin file name */
-			echo sprintf( __( 'Editing %s (active)' ), '<strong>' . $file . '</strong>' );
+			echo sprintf( __( 'Editing %s (active)' ), '<strong>' . esc_html( $file ) . '</strong>' );
 		} else {
 			/* translators: %s: plugin file name */
-			echo sprintf( __( 'Browsing %s (active)' ), '<strong>' . $file . '</strong>' );
+			echo sprintf( __( 'Browsing %s (active)' ), '<strong>' . esc_html( $file ) . '</strong>' );
 		}
 	} else {
 		if ( is_writeable( $real_file ) ) {
 			/* translators: %s: plugin file name */
-			echo sprintf( __( 'Editing %s (inactive)' ), '<strong>' . $file . '</strong>' );
+			echo sprintf( __( 'Editing %s (inactive)' ), '<strong>' . esc_html( $file ) . '</strong>' );
 		} else {
 			/* translators: %s: plugin file name */
-			echo sprintf( __( 'Browsing %s (inactive)' ), '<strong>' . $file . '</strong>' );
+			echo sprintf( __( 'Browsing %s (inactive)' ), '<strong>' . esc_html( $file ) . '</strong>' );
 		}
 	}
 	?></big>
@@ -247,7 +247,7 @@ foreach ( $plugin_files as $plugin_file ) :
 		continue;
 	}
 ?>
-		<li<?php echo $file == $plugin_file ? ' class="highlight"' : ''; ?>><a href="plugin-editor.php?file=<?php echo urlencode( $plugin_file ) ?>&amp;plugin=<?php echo urlencode( $plugin ) ?>"><?php echo $plugin_file ?></a></li>
+		<li<?php echo $file == $plugin_file ? ' class="highlight"' : ''; ?>><a href="plugin-editor.php?file=<?php echo urlencode( $plugin_file ) ?>&amp;plugin=<?php echo urlencode( $plugin ) ?>"><?php echo esc_html( $plugin_file ); ?></a></li>
 <?php endforeach; ?>
 	</ul>
 </div>
diff --git a/wp-admin/plugins.php b/wp-admin/plugins.php
index c079266dc1..67973854d4 100644
--- a/wp-admin/plugins.php
+++ b/wp-admin/plugins.php
@@ -17,7 +17,7 @@ $pagenum = $wp_list_table->get_pagenum();
 
 $action = $wp_list_table->current_action();
 
-$plugin = isset($_REQUEST['plugin']) ? $_REQUEST['plugin'] : '';
+$plugin = isset($_REQUEST['plugin']) ? wp_unslash( $_REQUEST['plugin'] ) : '';
 $s = isset($_REQUEST['s']) ? urlencode($_REQUEST['s']) : '';
 
 // Clean up request URI from temporary args for screen options/paging uri's to work as expected.
@@ -39,10 +39,10 @@ if ( $action ) {
 
 			check_admin_referer('activate-plugin_' . $plugin);
 
-			$result = activate_plugin($plugin, self_admin_url('plugins.php?error=true&plugin=' . $plugin), is_network_admin() );
+			$result = activate_plugin($plugin, self_admin_url('plugins.php?error=true&plugin=' . urlencode( $plugin ) ), is_network_admin() );
 			if ( is_wp_error( $result ) ) {
 				if ( 'unexpected_output' == $result->get_error_code() ) {
-					$redirect = self_admin_url('plugins.php?error=true&charsout=' . strlen($result->get_error_data()) . '&plugin=' . $plugin . "&plugin_status=$status&paged=$page&s=$s");
+					$redirect = self_admin_url('plugins.php?error=true&charsout=' . strlen($result->get_error_data()) . '&plugin=' . urlencode( $plugin ) . "&plugin_status=$status&paged=$page&s=$s");
 					wp_redirect(add_query_arg('_error_nonce', wp_create_nonce('plugin-activation-error_' . $plugin), $redirect));
 					exit;
 				} else {
@@ -73,7 +73,7 @@ if ( $action ) {
 
 			check_admin_referer('bulk-plugins');
 
-			$plugins = isset( $_POST['checked'] ) ? (array) $_POST['checked'] : array();
+			$plugins = isset( $_POST['checked'] ) ? (array) wp_unslash( $_POST['checked'] ) : array();
 
 			if ( is_network_admin() ) {
 				foreach ( $plugins as $i => $plugin ) {
@@ -122,9 +122,9 @@ if ( $action ) {
 			check_admin_referer( 'bulk-plugins' );
 
 			if ( isset( $_GET['plugins'] ) )
-				$plugins = explode( ',', $_GET['plugins'] );
+				$plugins = explode( ',', wp_unslash( $_GET['plugins'] ) );
 			elseif ( isset( $_POST['checked'] ) )
-				$plugins = (array) $_POST['checked'];
+				$plugins = (array) wp_unslash( $_POST['checked'] );
 			else
 				$plugins = array();
 
@@ -197,7 +197,7 @@ if ( $action ) {
 
 			check_admin_referer('bulk-plugins');
 
-			$plugins = isset( $_POST['checked'] ) ? (array) $_POST['checked'] : array();
+			$plugins = isset( $_POST['checked'] ) ? (array) wp_unslash( $_POST['checked'] ) : array();
 			// Do not deactivate plugins which are already deactivated.
 			if ( is_network_admin() ) {
 				$plugins = array_filter( $plugins, 'is_plugin_active_for_network' );
@@ -234,7 +234,7 @@ if ( $action ) {
 			check_admin_referer('bulk-plugins');
 
 			//$_POST = from the plugin form; $_GET = from the FTP details screen.
-			$plugins = isset( $_REQUEST['checked'] ) ? (array) $_REQUEST['checked'] : array();
+			$plugins = isset( $_REQUEST['checked'] ) ? (array) wp_unslash( $_REQUEST['checked'] ) : array();
 			if ( empty( $plugins ) ) {
 				wp_redirect( self_admin_url("plugins.php?plugin_status=$status&paged=$page&s=$s") );
 				exit;
diff --git a/wp-admin/theme-editor.php b/wp-admin/theme-editor.php
index 6fae2b7535..53be4bc555 100644
--- a/wp-admin/theme-editor.php
+++ b/wp-admin/theme-editor.php
@@ -99,7 +99,7 @@ if ( empty( $file ) ) {
 	$relative_file = 'style.css';
 	$file = $allowed_files['style.css'];
 } else {
-	$relative_file = $file;
+	$relative_file = wp_unslash( $file );
 	$file = $theme->get_stylesheet_directory() . '/' . $relative_file;
 }
 
@@ -156,10 +156,12 @@ default:
  <div id="message" class="updated notice is-dismissible"><p><?php _e( 'File edited successfully.' ) ?></p></div>
 <?php endif;
 
-$description = get_file_description( $relative_file );
+$file_description = get_file_description( $relative_file );
 $file_show = array_search( $file, array_filter( $allowed_files ) );
-if ( $description != $file_show )
-	$description .= ' <span>(' . $file_show . ')</span>';
+$description = esc_html( $file_description );
+if ( $file_description != $file_show ) {
+	$description .= ' <span>(' . esc_html( $file_show ) . ')</span>';
+}
 ?>
 <div class="wrap">
 <h1><?php echo esc_html( $title ); ?></h1>
@@ -230,9 +232,9 @@ if ( $allowed_files ) :
 			echo "\t<ul>\n";
 		}
 		
-		$file_description = get_file_description( $filename );
+		$file_description = esc_html( get_file_description( $filename ) );
 		if ( $filename !== basename( $absolute_filename ) || $file_description !== $filename ) {
-			$file_description .= '<br /><span class="nonessential">(' . $filename . ')</span>';
+			$file_description .= '<br /><span class="nonessential">(' . esc_html( $filename ) . ')</span>';
 		}
 
 		if ( $absolute_filename === $file ) {