--- wp-admin/includes/class-wp-upgrader.php.orig	2017-10-12 12:24:56.053035100 +0200
+++ wp-admin/includes/class-wp-upgrader.php	2017-10-12 12:40:22.709071850 +0200
@@ -300,21 +300,10 @@
 
 		$this->skin->feedback('unpack_package');
 
-		$upgrade_folder = $wp_filesystem->wp_content_dir() . 'upgrade/';
-
-		//Clean up contents of upgrade directory beforehand.
-		$upgrade_files = $wp_filesystem->dirlist($upgrade_folder);
-		if ( !empty($upgrade_files) ) {
-			foreach ( $upgrade_files as $file )
-				$wp_filesystem->delete($upgrade_folder . $file['name'], true);
-		}
-
 		// We need a working directory - Strip off any .tmp or .zip suffixes
-		$working_dir = $upgrade_folder . basename( basename( $package, '.tmp' ), '.zip' );
-
-		// Clean up working directory
-		if ( $wp_filesystem->is_dir($working_dir) )
-			$wp_filesystem->delete($working_dir, true);
+		$working_dir = wp_tempnam(basename( basename( $package, '.tmp' ), '.zip' ));
+		$wp_filesystem->delete($working_dir, true);
+		wp_mkdir_p($working_dir);
 
 		// Unzip package to working directory
 		$result = unzip_file( $package, $working_dir );