On April 27, 2015, WordPress 4.0.4 was released to the public, along with WordPress 4.2.1. This is a security update for all previous WordPress versions.
To download WordPress 4.0.4, update automatically from the Dashboard > Updates menu in your site's admin area or visit https://wordpress.org/download/release-archive/.
For step-by-step instructions on installing and updating WordPress:
If you are new to WordPress, we recommend that you begin with the following:
From the announcement post, this release fixes a critical cross-site scripting (XSS) vulnerability, which could enable commenters to compromise a site.
readme.html wp-admin/about.php wp-admin/includes/upgrade.php wp-includes/version.php wp-includes/wp-db.php