Maintenance release of the Drupal 7 series. Includes fixes for incompatibilities introduced in the Drupal 7.20 security release only.
No security fixes are included in this release; however, sites which were unable to upgrade to Drupal 7.20 (or upgraded but made modifications to disable the security fixes included within it) should upgrade to Drupal 7.21 to obtain additional security protection.
No changes have been made to the .htaccess, robots.txt or settings.php files in this release, so upgrading custom versions of those files is not necessary.
If you have already upgraded to Drupal 7.20 with no problems this release does not provide any new functionality. You can upgrade to Drupal 7.21 at your leisure, without reading the notes below.
Important update notes:
Drupal 7.20 fixed a fundamental security flaw in the Drupal core Image module and therefore introduced incompatibilities with a number of contributed modules and sites (see the Drupal 7.20 release notes). To help mitigate the effect of these changes, an optional 'image_allow_insecure_derivatives' variable was provided, which sites could use to turn off the security fix.