From the announcement post:
WordPress 3.4.2, now available for download, is a maintenance and security release for all previous versions.
After nearly 15 million downloads since 3.4 was released not three months ago, we’ve <a href=”http://core.trac.wordpress.org/query?status=closed&resolution=fixed&milestone=3.4.2&group=resolution&order=severity&desc=1″>identified and fixed a number of nagging bugs</a>, including:
<li>Fix some issues with older browsers in the administration area.</li>
<li>Fix an issue where a theme may not preview correctly, or its screenshot may not be displayed.</li>
<li>Improve plugin compatibility with the visual editor.</li>
<li>Address pagination problems with some category permalink structures.</li>
<li>Avoid errors with both oEmbed providers and trackbacks.</li>
<li>Prevent improperly sized header images from being uploaded.</li>
Version 3.4.2 also fixes a few security issues and contains some security hardening. The vulnerabilities included potential privilege escalation and a bug that affects multisite installs with untrusted users. These issues were discovered and fixed by the WordPress security team.
<a href=”http://wordpress.org/download/”><strong>Download 3.4.2</strong></a><strong> now or visit Dashboard → Updates in your site admin to update now.</strong>