Upon updating to WordPress 3.4.1, The "Cookie" header is no longer being added to the vary header which means a user cannot log in, comment or use any feature that requires cookies (unless of course if it's already set).
I have thoroughly checked web server configurations, checked all .htaccess files which revealed nothing to suspect. I've also tried cloned and subsequently installed new WordPress installations on blank databases (empty) on DreamHost, MediaTemple and an EC2 instance I was in the middle of deploying. I have checked through a few previous wp-login.php files from the archive that do not show any indication that it has been modified - this would obviously indicate it would be a problem my end unless I've missed something.
I can use .htaccess to do 'Header append Vary Cookie', but that would also mean updating several hundred non multi-site websites, manually. Modifying wp-login.php to set the same header results in success but would also mean updating several hundred websites.
Can anyone please shed some light on what's going on or what else I can do to diagnose this issue? I must have missed something.
Here's one sites wp-config.php:
$table_prefix = '*wp_';
/** Absolute path to the WordPress directory. */
if ( !defined('ABSPATH') )
define('ABSPATH', dirname(__FILE__) . '/');
/** Sets up WordPress vars and included files. */
require_once(ABSPATH . 'wp-settings.php');