diff --git a/wp-includes/version.php b/wp-includes/version.php
index 5ccd672dd8..6ccef7883a 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.0-alpha-42183';
+$wp_version = '5.0-alpha-42185';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.
diff --git a/wp-includes/widgets/class-wp-widget-custom-html.php b/wp-includes/widgets/class-wp-widget-custom-html.php
index 855c3c5895..c01419c351 100644
--- a/wp-includes/widgets/class-wp-widget-custom-html.php
+++ b/wp-includes/widgets/class-wp-widget-custom-html.php
@@ -80,16 +80,49 @@ class WP_Widget_Custom_HTML extends WP_Widget {
 		add_action( 'admin_head-widgets.php', array( 'WP_Widget_Custom_HTML', 'add_help_text' ) );
 	}
 
+	/**
+	 * Filter gallery shortcode attributes.
+	 *
+	 * Prevents all of a site's attachments from being shown in a gallery displayed on a
+	 * non-singular template where a $post context is not available.
+	 *
+	 * @since 4.9.0
+	 *
+	 * @param array $attrs Attributes.
+	 * @return array Attributes.
+	 */
+	public function _filter_gallery_shortcode_attrs( $attrs ) {
+		if ( ! is_singular() && empty( $attrs['id'] ) && empty( $attrs['include'] ) ) {
+			$attrs['id'] = -1;
+		}
+		return $attrs;
+	}
+
 	/**
 	 * Outputs the content for the current Custom HTML widget instance.
 	 *
 	 * @since 4.8.1
 	 *
+	 * @global WP_Post $post
 	 * @param array $args     Display arguments including 'before_title', 'after_title',
 	 *                        'before_widget', and 'after_widget'.
 	 * @param array $instance Settings for the current Custom HTML widget instance.
 	 */
 	public function widget( $args, $instance ) {
+		global $post;
+
+		// Override global $post so filters (and shortcodes) apply in a consistent context.
+		$original_post = $post;
+		if ( is_singular() ) {
+			// Make sure post is always the queried object on singular queries (not from another sub-query that failed to clean up the global $post).
+			$post = get_queried_object();
+		} else {
+			// Nullify the $post global during widget rendering to prevent shortcodes from running with the unexpected context on archive queries.
+			$post = null;
+		}
+
+		// Prevent dumping out all attachments from the media library.
+		add_filter( 'shortcode_atts_gallery', array( $this, '_filter_gallery_shortcode_attrs' ) );
 
 		$instance = array_merge( $this->default_instance, $instance );
 
@@ -118,6 +151,10 @@ class WP_Widget_Custom_HTML extends WP_Widget {
 		 */
 		$content = apply_filters( 'widget_custom_html_content', $content, $instance, $this );
 
+		// Restore post global.
+		$post = $original_post;
+		remove_filter( 'shortcode_atts_gallery', array( $this, '_filter_gallery_shortcode_attrs' ) );
+
 		// Inject the Text widget's container class name alongside this widget's class name for theme styling compatibility.
 		$args['before_widget'] = preg_replace( '/(?<=\sclass=["\'])/', 'widget_text ', $args['before_widget'] );
 
diff --git a/wp-includes/widgets/class-wp-widget-text.php b/wp-includes/widgets/class-wp-widget-text.php
index ffcfe0cd09..b62885dabb 100644
--- a/wp-includes/widgets/class-wp-widget-text.php
+++ b/wp-includes/widgets/class-wp-widget-text.php
@@ -178,6 +178,24 @@ class WP_Widget_Text extends WP_Widget {
 		return false;
 	}
 
+	/**
+	 * Filter gallery shortcode attributes.
+	 *
+	 * Prevents all of a site's attachments from being shown in a gallery displayed on a
+	 * non-singular template where a $post context is not available.
+	 *
+	 * @since 4.9.0
+	 *
+	 * @param array $attrs Attributes.
+	 * @return array Attributes.
+	 */
+	public function _filter_gallery_shortcode_attrs( $attrs ) {
+		if ( ! is_singular() && empty( $attrs['id'] ) && empty( $attrs['include'] ) ) {
+			$attrs['id'] = -1;
+		}
+		return $attrs;
+	}
+
 	/**
 	 * Outputs the content for the current Text widget instance.
 	 *
@@ -221,13 +239,19 @@ class WP_Widget_Text extends WP_Widget {
 			remove_filter( 'widget_text', 'do_shortcode', $widget_text_do_shortcode_priority );
 		}
 
-		// Nullify the $post global during widget rendering to prevent shortcodes from running with the unexpected context.
-		$suspended_post = null;
-		if ( isset( $post ) ) {
-			$suspended_post = $post;
+		// Override global $post so filters (and shortcodes) apply in a consistent context.
+		$original_post = $post;
+		if ( is_singular() ) {
+			// Make sure post is always the queried object on singular queries (not from another sub-query that failed to clean up the global $post).
+			$post = get_queried_object();
+		} else {
+			// Nullify the $post global during widget rendering to prevent shortcodes from running with the unexpected context on archive queries.
 			$post = null;
 		}
 
+		// Prevent dumping out all attachments from the media library.
+		add_filter( 'shortcode_atts_gallery', array( $this, '_filter_gallery_shortcode_attrs' ) );
+
 		/**
 		 * Filters the content of the Text widget.
 		 *
@@ -278,9 +302,8 @@ class WP_Widget_Text extends WP_Widget {
 		}
 
 		// Restore post global.
-		if ( isset( $suspended_post ) ) {
-			$post = $suspended_post;
-		}
+		$post = $original_post;
+		remove_filter( 'shortcode_atts_gallery', array( $this, '_filter_gallery_shortcode_attrs' ) );
 
 		// Undo suspension of legacy plugin-supplied shortcode handling.
 		if ( $should_suspend_legacy_shortcode_support ) {