diff --git a/wp-admin/includes/class-file-upload-upgrader.php b/wp-admin/includes/class-file-upload-upgrader.php
index 2b76e41ea6..e62561518d 100644
--- a/wp-admin/includes/class-file-upload-upgrader.php
+++ b/wp-admin/includes/class-file-upload-upgrader.php
@@ -69,6 +69,30 @@ class File_Upload_Upgrader {
 				wp_die( $file['error'] );
 			}
 
+			if ( 'pluginzip' === $form || 'themezip' === $form ) {
+				$archive_is_valid = false;
+
+				/** This filter is documented in wp-admin/includes/file.php */
+				if ( class_exists( 'ZipArchive', false ) && apply_filters( 'unzip_file_use_ziparchive', true ) ) {
+					$archive          = new ZipArchive();
+					$archive_is_valid = $archive->open( $file['file'], ZIPARCHIVE::CHECKCONS );
+
+					if ( true === $archive_is_valid ) {
+						$archive->close();
+					}
+				} else {
+					require_once ABSPATH . 'wp-admin/includes/class-pclzip.php';
+
+					$archive          = new PclZip( $file['file'] );
+					$archive_is_valid = is_array( $archive->properties() );
+				}
+
+				if ( true !== $archive_is_valid ) {
+					wp_delete_file( $file['file'] );
+					wp_die( __( 'Incompatible Archive.' ) );
+				}
+			}
+
 			$this->filename = $_FILES[ $form ]['name'];
 			$this->package  = $file['file'];
 
diff --git a/wp-admin/update.php b/wp-admin/update.php
index 29480c2840..090c37cfc4 100644
--- a/wp-admin/update.php
+++ b/wp-admin/update.php
@@ -154,6 +154,10 @@ if ( isset( $_GET['action'] ) ) {
 
 		check_admin_referer( 'plugin-upload' );
 
+		if ( isset( $_FILES['pluginzip']['name'] ) && ! str_ends_with( strtolower( $_FILES['pluginzip']['name'] ), '.zip' ) ) {
+			wp_die( __( 'Only .zip archives may be uploaded.' ) );
+		}
+
 		$file_upload = new File_Upload_Upgrader( 'pluginzip', 'package' );
 
 		// Used in the HTML title tag.
@@ -302,6 +306,10 @@ if ( isset( $_GET['action'] ) ) {
 
 		check_admin_referer( 'theme-upload' );
 
+		if ( isset( $_FILES['themezip']['name'] ) && ! str_ends_with( strtolower( $_FILES['themezip']['name'] ), '.zip' ) ) {
+			wp_die( __( 'Only .zip archives may be uploaded.' ) );
+		}
+
 		$file_upload = new File_Upload_Upgrader( 'themezip', 'package' );
 
 		// Used in the HTML title tag.
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 354e16e629..528a2ec443 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.5-alpha-57387';
+$wp_version = '6.5-alpha-57388';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.