If you're one of the people who still has a Drupal 6 site hanging on for dear life, you'll need to apply the security patches released today by the Drupal Security Team to your Drupal 6 site. They didn't release a D6 patch, but it's an easy enough patch to convert from Drupal 7 backwards to Drupal 6.
If you have a standard Drupal 6 install, you can just download the attached zip archive and drop the files into the /includes folder.
I also made a patch available if you prefer that instead.